Video for Linux Two API Specification

Draft 0.12

Michael H Schimek

            
          

Bill Dirks

Hans Verkuil

This document is copyrighted © 1999-2006 by Bill Dirks, Michael H. Schimek and Hans Verkuil.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the appendix entitled "GNU Free Documentation License".

Programming examples can be used and distributed without restrictions.


Table of Contents
Introduction
1. Common API Elements
1.1. Opening and Closing Devices
1.1.1. Device Naming
1.1.2. Related Devices
1.1.3. Multiple Opens
1.1.4. Shared Data Streams
1.1.5. Functions
1.2. Querying Capabilities
1.3. Application Priority
1.4. Video Inputs and Outputs
1.5. Audio Inputs and Outputs
1.6. Tuners and Modulators
1.6.1. Tuners
1.6.2. Modulators
1.6.3. Radio Frequency
1.6.4. Satellite Receivers
1.7. Video Standards
1.8. Controls
1.9. Data Formats
1.9.1. Data Format Negotiation
1.9.2. Image Format Enumeration
1.10. Cropping and Scaling
1.11. Streaming Parameters
2. Image Formats
2.1. Standard Image Formats
2.2. Colorspaces
2.3. RGB Formats
2.4. YUV Formats
V4L2_PIX_FMT_GREY ('GREY') -- Grey-scale image.
V4L2_PIX_FMT_YUYV ('YUYV') -- Packed format with ½ horizontal chroma resolution, also known as YUV 4:2:2.
V4L2_PIX_FMT_UYVY ('UYVY') -- Variation of V4L2_PIX_FMT_YUYV with different order of samples in memory.
V4L2_PIX_FMT_Y41P ('Y41P') -- Packed format with ¼ horizontal chroma resolution, also known as YUV 4:1:1.
V4L2_PIX_FMT_YVU420 ('YV12'), V4L2_PIX_FMT_YUV420 ('YU12') -- Planar formats with ½ horizontal and vertical chroma resolution, also known as YUV 4:2:0.
V4L2_PIX_FMT_YVU410 ('YVU9'), V4L2_PIX_FMT_YUV410 ('YUV9') -- Planar formats with ¼ horizontal and vertical chroma resolution, also known as YUV 4:1:0.
V4L2_PIX_FMT_YUV422P ('422P') -- Format with ½ horizontal chroma resolution, also known as YUV 4:2:2. Planar layout as opposed to V4L2_PIX_FMT_YUYV.
V4L2_PIX_FMT_YUV411P ('411P') -- Format with ¼ horizontal chroma resolution, also known as YUV 4:1:1. Planar layout as opposed to V4L2_PIX_FMT_Y41P.
V4L2_PIX_FMT_NV12 ('NV12'), V4L2_PIX_FMT_NV21 ('NV21') -- Formats with ½ horizontal and vertical chroma resolution, also known as YUV 4:2:0. One luminance and one chrominance plane with alternating chroma samples as opposed to V4L2_PIX_FMT_YVU420.
2.5. Compressed Formats
2.6. Reserved Format Identifiers
3. Input/Output
3.1. Read/Write
3.2. Streaming I/O (Memory Mapping)
3.3. Streaming I/O (User Pointers)
3.4. Asynchronous I/O
3.5. Buffers
3.5.1. Timecodes
3.6. Field Order
4. Device Types
4.1. Video Capture Interface
4.1.1. Querying Capabilities
4.1.2. Supplemental Functions
4.1.3. Image Format Negotiation
4.1.4. Reading Images
4.2. Video Overlay Interface
4.2.1. Querying Capabilities
4.2.2. Supplemental Functions
4.2.3. Setup
4.2.4. Overlay Window
4.2.5. Enabling Overlay
4.3. Video Output Interface
4.3.1. Querying Capabilities
4.3.2. Supplemental Functions
4.3.3. Image Format Negotiation
4.3.4. Writing Images
4.4. Codec Interface
4.5. Effect Devices Interface
4.6. Raw VBI Data Interface
4.6.1. Querying Capabilities
4.6.2. Supplemental Functions
4.6.3. Raw VBI Format Negotiation
4.6.4. Reading and writing VBI images
4.7. Sliced VBI Data Interface
4.7.1. Querying Capabilities
4.7.2. Supplemental Functions
4.7.3. Sliced VBI Format Negotiation
4.7.4. Reading and writing sliced VBI data
4.8. Teletext Interface
4.9. Radio Interface
4.9.1. Querying Capabilities
4.9.2. Supplemental Functions
4.9.3. Programming
4.10. RDS Interface
I. Function Reference
V4L2 close() -- Close a V4L2 device
V4L2 ioctl() -- Program a V4L2 device
ioctl VIDIOC_CROPCAP -- Information about the video cropping and scaling abilities.
ioctl VIDIOC_ENUMAUDIO -- Enumerate audio inputs
ioctl VIDIOC_ENUMAUDOUT -- Enumerate audio outputs
ioctl VIDIOC_ENUM_FMT -- Enumerate image formats
ioctl VIDIOC_ENUMINPUT -- Enumerate video inputs
ioctl VIDIOC_ENUMOUTPUT -- Enumerate video outputs
ioctl VIDIOC_ENUMSTD -- Enumerate supported video standards
ioctl VIDIOC_G_AUDIO, VIDIOC_S_AUDIO -- Query or select the current audio input and its attributes
ioctl VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT -- Query or select the current audio output
ioctl VIDIOC_G_MPEGCOMP, VIDIOC_S_MPEGCOMP -- Get or set compression parameters
ioctl VIDIOC_G_CROP, VIDIOC_S_CROP -- Get or set the current cropping rectangle
ioctl VIDIOC_G_CTRL, VIDIOC_S_CTRL -- Get or set the value of a control
ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF -- Get or set frame buffer overlay parameters.
ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT -- Get or set the data format, try a format.
ioctl VIDIOC_G_FREQUENCY, VIDIOC_S_FREQUENCY -- Get or set tuner or modulator radio frequency
ioctl VIDIOC_G_INPUT, VIDIOC_S_INPUT -- Query or select the current video input
ioctl VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP -- 
ioctl VIDIOC_G_MODULATOR, VIDIOC_S_MODULATOR -- Get or set modulator attributes
ioctl VIDIOC_G_OUTPUT, VIDIOC_S_OUTPUT -- Query or select the current video output
ioctl VIDIOC_G_PARM, VIDIOC_S_PARM -- Get or set streaming parameters
ioctl VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY -- Query or request the access priority associated with a file descriptor
ioctl VIDIOC_G_SLICED_VBI_CAP -- Query sliced VBI capabilities
ioctl VIDIOC_G_STD, VIDIOC_S_STD -- Query or select the video standard of the current input
ioctl VIDIOC_G_TUNER, VIDIOC_S_TUNER -- Get or set tuner attributes
ioctl VIDIOC_LOG_STATUS -- Log driver status information
ioctl VIDIOC_OVERLAY -- Start or stop video overlay
ioctl VIDIOC_QBUF, VIDIOC_DQBUF -- Exchange a buffer with the driver
ioctl VIDIOC_QUERYBUF -- Query the status of a buffer
ioctl VIDIOC_QUERYCAP -- Query device capabilities
ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU -- Enumerate controls and menu control items
ioctl VIDIOC_QUERYSTD -- Sense the video standard received by the current input
ioctl VIDIOC_REQBUFS -- Initiate Memory Mapping or User Pointer I/O
ioctl VIDIOC_STREAMON, VIDIOC_STREAMOFF -- Start or stop streaming I/O
V4L2 mmap() -- Map device memory into application address space
V4L2 munmap() -- Unmap device memory
V4L2 open() -- Open a V4L2 device
V4L2 poll() -- Wait for some event on a file descriptor
V4L2 read() -- Read from a V4L2 device
V4L2 select() -- Synchronous I/O multiplexing
V4L2 write() -- Write to a V4L2 device
5. V4L2 Driver Programming
6. History
6.1. Differences between V4L and V4L2
6.1.1. Opening and Closing Devices
6.1.2. Querying Capabilities
6.1.3. Video Sources
6.1.4. Tuning
6.1.5. Image Properties
6.1.6. Audio
6.1.7. Frame Buffer Overlay
6.1.8. Cropping
6.1.9. Reading Images, Memory Mapping
6.1.10. Reading Raw VBI Data
6.1.11. Miscellaneous
6.2. History of the V4L2 API
6.2.1. Early Versions
6.2.2. V4L2 Version 0.16 1999-01-31
6.2.3. V4L2 Version 0.18 1999-03-16
6.2.4. V4L2 Version 0.19 1999-06-05
6.2.5. V4L2 Version 0.20 1999-09-10
6.2.6. V4L2 Version 0.20 incremental changes
6.2.7. V4L2 Version 0.20 2000-11-23
6.2.8. V4L2 Version 0.20 2002-07-25
6.2.9. V4L2 in Linux 2.5.46, 2002-10
6.2.10. V4L2 2003-06-19
6.2.11. V4L2 2003-11-05
6.2.12. V4L2 in Linux 2.6.6, 2004-05-09
6.2.13. V4L2 in Linux 2.6.8
6.2.14. V4L2 spec erratum 2004-08-01
6.2.15. V4L2 in Linux 2.6.14
6.2.16. V4L2 in Linux 2.6.15
6.2.17. V4L2 spec erratum 2005-11-27
6.2.18. V4L2 spec erratum 2006-01-10
6.2.19. V4L2 spec erratum 2006-02-03
6.3. Relation of V4L2 to other Linux multimedia APIs
6.3.1. X Video Extension
6.3.2. Digital Video
6.3.3. Audio Interfaces
A. Video For Linux Two Header File
B. Video Capture Example
C. GNU Free Documentation License
C.1. 0. PREAMBLE
C.2. 1. APPLICABILITY AND DEFINITIONS
C.3. 2. VERBATIM COPYING
C.4. 3. COPYING IN QUANTITY
C.5. 4. MODIFICATIONS
C.6. 5. COMBINING DOCUMENTS
C.7. 6. COLLECTIONS OF DOCUMENTS
C.8. 7. AGGREGATION WITH INDEPENDENT WORKS
C.9. 8. TRANSLATION
C.10. 9. TERMINATION
C.11. 10. FUTURE REVISIONS OF THIS LICENSE
C.12. Addendum
Bibliography
List of Tables
1-1. Control IDs
2-1. struct v4l2_pix_format
2-2. enum v4l2_colorspace
2-3. Packed RGB Image Formats
2-4. Reserved Image Formats
3-1. struct v4l2_buffer
3-2. enum v4l2_buf_type
3-3. Buffer Flags
3-4. enum v4l2_memory
3-5. struct v4l2_timecode
3-6. Timecode Types
3-7. Timecode Flags
3-8. enum v4l2_field
4-1. struct v4l2_window
4-2. struct v4l2_clip[22]
4-3. struct v4l2_rect
4-4. struct v4l2_vbi_format
4-5. Raw VBI Format Flags
4-6. struct v4l2_sliced_vbi_format
4-7. Sliced VBI services
4-8. struct v4l2_sliced_vbi_data
1. struct v4l2_cropcap
2. struct v4l2_rect
1. struct v4l2_fmtdesc
2. Image Format Description Flags
1. struct v4l2_input
2. Input Types
3. Input Status Flags
1. struct v4l2_output
2. Output Type
1. struct v4l2_standard
2. struct v4l2_fract
3. typedef v4l2_std_id
4. Video Standards (based on [ITU470>])
1. struct v4l2_audio
2. Audio Capability Flags
3. Audio Modes
1. struct v4l2_audioout
1. struct v4l2_mpeg_compression
1. struct v4l2_crop
1. struct v4l2_control
1. struct v4l2_framebuffer
2. Frame Buffer Capability Flags
3. Frame Buffer Flags
1. struct v4l2_format
1. struct v4l2_frequency
1. struct v4l2_jpegcompression
2. JPEG Markers Flags
1. struct v4l2_modulator
2. Modulator Audio Transmission Flags
1. struct v4l2_streamparm
2. struct v4l2_captureparm
3. struct v4l2_outputparm
4. Streaming Parameters Capabilites
5. Capture Parameters Flags
1. enum v4l2_priority
1. struct v4l2_sliced_vbi_cap
2. Sliced VBI services
1. struct v4l2_tuner
2. enum v4l2_tuner_type
3. Tuner and Modulator Capability Flags
4. Tuner Audio Reception Flags
5. Tuner Audio Modes
6. Tuner Audio Matrix
1. struct v4l2_capability
2. Device Capabilities Flags
1. struct v4l2_queryctrl
2. struct v4l2_querymenu
3. enum v4l2_ctrl_type
4. Control Flags
1. struct v4l2_requestbuffers
6-1. V4L Device Types, Names and Numbers
List of Figures
1-1. Cropping and Scaling
3-1. Field Order, Top Field First Transmitted
3-2. Field Order, Bottom Field First Transmitted
4-1. Line synchronization
4-2. ITU-R 525 line numbering (M/NTSC and M/PAL)
4-3. ITU-R 625 line numbering
List of Examples
1-1. Information about the current video input
1-2. Switching to the first video input
1-3. Information about the current audio input
1-4. Switching to the first audio input
1-5. Information about the current video standard
1-6. Listing the video standards supported by the current input
1-7. Selecting a new video standard
1-8. Enumerating all controls
1-9. Changing controls
1-10. Resetting the cropping parameters
1-11. Simple downscaling
1-12. Current scaling factor and pixel aspect
2-1. ITU-R Rec. BT.601 color conversion
2-2. V4L2_PIX_FMT_BGR24 4 × 4 pixel image
2-1. V4L2_PIX_FMT_GREY 4 × 4 pixel image
2-1. V4L2_PIX_FMT_YUYV 4 × 4 pixel image
2-1. V4L2_PIX_FMT_UYVY 4 × 4 pixel image
2-1. V4L2_PIX_FMT_Y41P 8 × 4 pixel image
2-1. V4L2_PIX_FMT_YVU420 4 × 4 pixel image
2-1. V4L2_PIX_FMT_YVU410 4 × 4 pixel image
2-1. V4L2_PIX_FMT_YUV422P 4 × 4 pixel image
2-1. V4L2_PIX_FMT_YUV411P 4 × 4 pixel image
2-1. V4L2_PIX_FMT_NV12 4 × 4 pixel image
3-1. Mapping buffers
3-2. Initiating streaming I/O with user pointers

Introduction

[to do]

If you have questions or ideas regarding the API, please try the Video4Linux mailing list: https://listman.redhat.com/mailman/listinfo/video4linux-list

For documentation related requests contact the maintainer at mschimek@gmx.at.

The latest version of this document and the DocBook SGML sources is currently hosted at http://v4l2spec.bytesex.org, and http://linuxtv.org/downloads/video4linux/API/V4L2_API.


Chapter 1. Common API Elements

Programming a V4L2 device consists of these steps:

In practice most steps are optional and can be executed out of order. It depends on the V4L2 device type, you can read about the details in Chapter 4>. In this chapter we will discuss the basic concepts applicable to all devices.


1.1. Opening and Closing Devices

1.1.1. Device Naming

V4L2 drivers are implemented as kernel modules, loaded manually by the system administrator or automatically when a device is first opened. The driver modules plug into the "videodev" kernel module. It provides helper functions and a common application interface specified in this document.

Each driver thus loaded registers one or more device nodes with major number 81 and a minor number between 0 and 255. Assigning minor numbers to V4L2 devices is entirely up to the system administrator, this is primarily intended to solve conflicts between devices.[1] The module options to select minor numbers are named after the device special file with a "_nr" suffix. For example "video_nr" for /dev/video video capture devices. The number is an offset to the base minor number associated with the device type. [2] When the driver supports multiple devices of the same type more than one minor number can be assigned, separated by commas:

> insmod mydriver.o video_nr=0,1 radio_nr=0,1

In /etc/modules.conf this may be written as:

alias char-major-81-0 mydriver
alias char-major-81-1 mydriver
alias char-major-81-64 mydriver (1)>
options mydriver video_nr=0,1 radio_nr=0,1 (2)>
(1)
When an application attempts to open a device special file with major number 81 and minor number 0, 1, or 64, load "mydriver" (and the "videodev" module it depends upon).
(2)
Register the first two video capture devices with minor number 0 and 1 (base number is 0), the first two radio device with minor number 64 and 65 (base 64).

When no minor number is given as module option the driver supplies a default. Chapter 4> recommends the base minor numbers to be used for the various device types. Obviously minor numbers must be unique. When the number is already in use the offending device will not be registered.

By convention system administrators create various character device special files with these major and minor numbers in the /dev directory. The names recomended for the different V4L2 device types are listed in Chapter 4>.

The creation of character special files (with mknod) is a privileged operation and devices cannot be opened by major and minor number. That means applications cannot reliable scan for loaded or installed drivers. The user must enter a device name, or the application can try the conventional device names.

Under the device filesystem (devfs) the minor number options are ignored. V4L2 drivers (or by proxy the "videodev" module) automatically create the required device files in the /dev/v4l directory using the conventional device names above.


1.1.2. Related Devices

Devices can support several related functions. For example video capturing, video overlay and VBI capturing are related because these functions share, amongst other, the same video input and tuner frequency. V4L and earlier versions of V4L2 used the same device name and minor number for video capturing and overlay, but different ones for VBI. Experience showed this approach has several problems[3], and to make things worse the V4L videodev module used to prohibit multiple opens of a device.

As a remedy the present version of the V4L2 API relaxed the concept of device types with specific names and minor numbers. For compatibility with old applications drivers must still register different minor numbers to assign a default function to the device. But if related functions are supported by the driver they must be available under all registered minor numbers. The desired function can be selected after opening the device as described in Chapter 4>.

Imagine a driver supporting video capturing, video overlay, raw VBI capturing, and FM radio reception. It registers three devices with minor number 0, 64 and 224 (this numbering scheme is inherited from the V4L API). Regardless if /dev/video (81, 0) or /dev/vbi (81, 224) is opened the application can select any one of the video capturing, overlay or VBI capturing functions. Without programming (e. g. reading from the device with dd or cat) /dev/video captures video images, while /dev/vbi captures raw VBI data. /dev/radio (81, 64) is invariable a radio device, unrelated to the video functions. Being unrelated does not imply the devices can be used at the same time, however. The open() function may very well return an EBUSY error code.

Besides video input or output the hardware may also support audio sampling or playback. If so, these functions are implemented as OSS or ALSA PCM devices and eventually OSS or ALSA audio mixer. The V4L2 API makes no provisions yet to find these related devices. If you have an idea please write to the Video4Linux mailing list: https://listman.redhat.com/mailman/listinfo/video4linux-list.


1.1.3. Multiple Opens

In general, V4L2 devices can be opened more than once. When this is supported by the driver, users can for example start a "panel" application to change controls like brightness or audio volume, while another application captures video and audio. In other words, panel applications are comparable to an OSS or ALSA audio mixer application. When a device supports multiple functions like capturing and overlay simultaneously, multiple opens allow concurrent use of the device by forked processes or specialized applications.

Multiple opens are optional, although drivers should permit at least concurrent accesses without data exchange, i. e. panel applications. This implies open() can return an EBUSY error code when the device is already in use, as well as ioctl() functions initiating data exchange (namely the VIDIOC_S_FMT ioctl), and the read() and write() functions.

Mere opening a V4L2 device does not grant exclusive access.[4] Initiating data exchange however assigns the right to read or write the requested type of data, and to change related properties, to this file descriptor. Applications can request additional access privileges using the priority mechanism described in Section 1.3>.


1.1.4. Shared Data Streams

V4L2 drivers should not support multiple applications reading or writing the same data stream on a device by copying buffers, time multiplexing or similar means. This is better handled by a proxy application in user space. When the driver supports stream sharing anyway it must be implemented transparently. The V4L2 API does not specify how conflicts are solved.


1.1.5. Functions

To open and close V4L2 devices applications use the open() and close() function, respectively. Devices are programmed using the ioctl() function as explained in the following sections.


1.2. Querying Capabilities

Because V4L2 covers a wide variety of devices not all aspects of the API are equally applicable to all types of devices. Furthermore devices of the same type have different capabilities and this specification permits the omission of a few complicated and less important parts of the API.

The VIDIOC_QUERYCAP ioctl is available to check if the kernel device is compatible with this specification, and to query the functions and I/O methods supported by the device. Other features can be queried by calling the respective ioctl, for example VIDIOC_ENUMINPUT to learn about the number, types and names of video connectors on the device. Although abstraction is a major objective of this API, the ioctl also allows driver specific applications to reliable identify the driver.

All V4L2 drivers must support VIDIOC_QUERYCAP. Applications should always call this ioctl after opening the device.


1.3. Application Priority

When multiple applications share a device it may be desirable to assign them different priorities. Contrary to the traditional "rm -rf /" school of thought a video recording application could for example block other applications from changing video controls or switching the current TV channel. Another objective is to permit low priority applications working in background, which can be preempted by user controlled applications and automatically regain control of the device at a later time.

Since these features cannot be implemented entirely in user space V4L2 defines the VIDIOC_G_PRIORITY and VIDIOC_S_PRIORITY ioctls to request and query the access priority associate with a file descriptor. Opening a device assigns a medium priority, compatible with earlier versions of V4L2 and drivers not supporting these ioctls. Applications requiring a different priority will usually call VIDIOC_S_PRIORITY after verifying the device with the VIDIOC_QUERYCAP ioctl.

Ioctls changing driver properties, such as VIDIOC_S_INPUT, return an EBUSY error code after another application obtained higher priority. An event mechanism to notify applications about asynchronous property changes has been proposed but not added yet.


1.4. Video Inputs and Outputs

Video inputs and outputs are physical connectors of a device. These can be for example RF connectors (antenna/cable), CVBS a.k.a. Composite Video, S-Video or RGB connectors. Only video and VBI capture devices have inputs, output devices have outputs, at least one each. Radio devices have no video inputs or outputs.

To learn about the number and attributes of the available inputs and outputs applications can enumerate them with the VIDIOC_ENUMINPUT and VIDIOC_ENUMOUTPUT ioctl, respectively. The struct v4l2_input returned by the VIDIOC_ENUMINPUT ioctl also contains signal status information applicable when the current video input is queried.

The VIDIOC_G_INPUT and VIDIOC_G_OUTPUT ioctl return the index of the current video input or output. To select a different input or output applications call the VIDIOC_S_INPUT and VIDIOC_S_OUTPUT ioctl. Drivers must implement all the input ioctls when the device has one or more inputs, all the output ioctls when the device has one or more outputs.

Example 1-1. Information about the current video input

struct v4l2_input input;
int index;
if (-1 == ioctl (fd, VIDIOC_G_INPUT, &index)) {
perror ("VIDIOC_G_INPUT");
exit (EXIT_FAILURE);
}
memset (&input, 0, sizeof (input));
input.index = index;
if (-1 == ioctl (fd, VIDIOC_ENUMINPUT, &input)) {
perror ("VIDIOC_ENUMINPUT");
exit (EXIT_FAILURE);
}
printf ("Current input: %s\n", input.name);

Example 1-2. Switching to the first video input

int index;
index = 0;
if (-1 == ioctl (fd, VIDIOC_S_INPUT, &index)) {
perror ("VIDIOC_S_INPUT");
exit (EXIT_FAILURE);
}

1.5. Audio Inputs and Outputs

Audio inputs and outputs are physical connectors of a device. Video capture devices have inputs, output devices have outputs, zero or more each. Radio devices have no audio inputs or outputs. They have exactly one tuner which in fact is an audio source, but this API associates tuners with video inputs or outputs only, and radio devices have none of these.[5] A connector on a TV card to loop back the received audio signal to a sound card is not considered an audio output.

Audio and video inputs and outputs are associated. Selecting a video source also selects an audio source. This is most evident when the video and audio source is a tuner. Further audio connectors can combine with more than one video input or output. Assumed two composite video inputs and two audio inputs exist, there may be up to four valid combinations. The relation of video and audio connectors is defined in the audioset field of the respective struct v4l2_input or struct v4l2_output, where each bit represents the index number, starting at zero, of one audio input or output.

To learn about the number and attributes of the available inputs and outputs applications can enumerate them with the VIDIOC_ENUMAUDIO and VIDIOC_ENUMAUDOUT ioctl, respectively. The struct v4l2_audio returned by the VIDIOC_ENUMAUDIO ioctl also contains signal status information applicable when the current audio input is queried.

The VIDIOC_G_AUDIO and VIDIOC_G_AUDOUT ioctl report the current audio input and output, respectively. Note that, unlike VIDIOC_G_INPUT and VIDIOC_G_OUTPUT these ioctls return a structure as VIDIOC_ENUMAUDIO and VIDIOC_ENUMAUDOUT do, not just an index.

To select an audio input and change its properties applications call the VIDIOC_S_AUDIO ioctl. To select an audio output (which presently has no changeable properties) applications call the VIDIOC_S_AUDOUT ioctl.

Drivers must implement all input ioctls when the device has one or more inputs, all output ioctls when the device has one or more outputs. When the device has any audio inputs or outputs the driver must set the V4L2_CAP_AUDIO flag in the struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl.

Example 1-3. Information about the current audio input

struct v4l2_audio audio;
memset (&audio, 0, sizeof (audio));
if (-1 == ioctl (fd, VIDIOC_G_AUDIO, &audio)) {
perror ("VIDIOC_G_AUDIO");
exit (EXIT_FAILURE);
}
printf ("Current input: %s\n", audio.name);

Example 1-4. Switching to the first audio input

struct v4l2_audio audio;
memset (&audio, 0, sizeof (audio)); /* clear audio.mode, audio.reserved */
audio.index = 0;
if (-1 == ioctl (fd, VIDIOC_S_AUDIO, &audio)) {
perror ("VIDIOC_S_AUDIO");
exit (EXIT_FAILURE);
}

1.6. Tuners and Modulators

1.6.1. Tuners

Video input devices can have one or more tuners demodulating a RF signal. Each tuner is associated with one or more video inputs, depending on the number of RF connectors on the tuner. The type field of the respective struct v4l2_input returned by the VIDIOC_ENUMINPUT ioctl is set to V4L2_INPUT_TYPE_TUNER and its tuner field contains the index number of the tuner.

Radio devices have exactly one tuner with index zero, no video inputs.

To query and change tuner properties applications use the VIDIOC_G_TUNER and VIDIOC_S_TUNER ioctl, respectively. The struct v4l2_tuner returned by VIDIOC_G_TUNER also contains signal status information applicable when the tuner of the current video input, or a radio tuner is queried. Note that VIDIOC_S_TUNER does not switch the current tuner, when there is more than one at all. The tuner is solely determined by the current video input. Drivers must support both ioctls and set the V4L2_CAP_TUNER flag in the struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl when the device has one or more tuners.


1.6.2. Modulators

Video output devices can have one or more modulators, uh, modulating a video signal for radiation or connection to the antenna input of a TV set or video recorder. Each modulator is associated with one or more video outputs, depending on the number of RF connectors on the modulator. The type field of the respective struct v4l2_output returned by the VIDIOC_ENUMOUTPUT is set to V4L2_OUTPUT_TYPE_MODULATOR and its modulator field contains the index number of the modulator. This specification does not define radio output devices.

To query and change modulator properties applications use the VIDIOC_G_MODULATOR and VIDIOC_S_MODULATOR ioctl. Note that VIDIOC_S_MODULATOR does not switch the current modulator, when there is more than one at all. The modulator is solely determined by the current video output. Drivers must support both ioctls and set the V4L2_CAP_TUNER (sic) flag in the struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl when the device has one or more modulators.


1.6.3. Radio Frequency

To get and set the tuner or modulator radio frequency applications use the VIDIOC_G_FREQUENCY and VIDIOC_S_FREQUENCY ioctl which both take a pointer to a struct v4l2_frequency. These ioctls are used for TV and radio devices alike. Drivers must support both ioctls when the tuner or modulator ioctls are supported, or when the device is a radio device.


1.6.4. Satellite Receivers

To be discussed. See also proposals by Peter Schlaf, video4linux-list@redhat.com on 23 Oct 2002, subject: "Re: [V4L] Re: v4l2 api".


1.7. Video Standards

Video devices typically support one or more different video standards or variations of standards. Each video input and output may support another set of standards. This set is reported by the std field of struct v4l2_input and struct v4l2_output returned by the VIDIOC_ENUMINPUT and VIDIOC_ENUMOUTPUT ioctl, respectively.

V4L2 defines one bit for each analog video standard currently in use worldwide, and sets aside bits for driver defined standards, e. g. hybrid standards to watch NTSC video tapes on PAL TVs and vice versa. Applications can use the predefined bits to select a particular standard, although presenting the user a menu of supported standards is preferred. To enumerate and query the attributes of the supported standards applications use the VIDIOC_ENUMSTD ioctl.

Many of the defined standards are actually just variations of a few major standards. The hardware may in fact not distinguish between them, or do so internal and switch automatically. Therefore enumerated standards also contain sets of one or more standard bits.

Assume a hypothetic tuner capable of demodulating B/PAL, G/PAL and I/PAL signals. The first enumerated standard is a set of B and G/PAL, switched automatically depending on the selected radio frequency in UHF or VHF band. Enumeration gives a "PAL-B/G" or "PAL-I" choice. Similar a Composite input may collapse standards, enumerating "PAL-B/G/H/I", "NTSC-M" and "SECAM-D/K".[6]

To query and select the standard used by the current video input or output applications call the VIDIOC_G_STD and VIDIOC_S_STD ioctl, respectively. The received standard can be sensed with the VIDIOC_QUERYSTD ioctl. Note parameter of all these ioctls is a pointer to a v4l2_std_id type (a standard set), not an index into the standard enumeration.[7] Drivers must implement all video standard ioctls when the device has one or more video inputs or outputs.

Special rules apply to USB cameras where the notion of video standards makes little sense. More generally any capture device, output devices accordingly, which is

  • incapable of capturing fields or frames at the nominal rate of the video standard, or

  • where timestamps refer to the instant the field or frame was received by the driver, not the capture time, or

  • where sequence numbers refer to the frames received by the driver, not the captured frames.

Here the driver shall set the std field of struct v4l2_input and struct v4l2_output to zero, the VIDIOC_G_STD, VIDIOC_S_STD, VIDIOC_QUERYSTD and VIDIOC_ENUMSTD ioctls shall return the EINVAL error code.[8]

Example 1-5. Information about the current video standard

v4l2_std_id std_id;
struct v4l2_standard standard;
if (-1 == ioctl (fd, VIDIOC_G_STD, &std_id)) {
/* Note when VIDIOC_ENUMSTD always returns EINVAL this
is no video device or it falls under the USB exception,
and VIDIOC_G_STD returning EINVAL is no error. */
perror ("VIDIOC_G_STD");
exit (EXIT_FAILURE);
}
memset (&standard, 0, sizeof (standard));
standard.index = 0;
while (0 == ioctl (fd, VIDIOC_ENUMSTD, &standard)) {
if (standard.id & std_id) {
printf ("Current video standard: %s\n", standard.name);
exit (EXIT_SUCCESS);
}
standard.index++;
}
/* EINVAL indicates the end of the enumeration, which cannot be
empty unless this device falls under the USB exception. */
if (errno == EINVAL || standard.index == 0) {
perror ("VIDIOC_ENUMSTD");
exit (EXIT_FAILURE);
}

Example 1-6. Listing the video standards supported by the current input

struct v4l2_input input;
struct v4l2_standard standard;
memset (&input, 0, sizeof (input));
if (-1 == ioctl (fd, VIDIOC_G_INPUT, &input.index)) {
perror ("VIDIOC_G_INPUT");
exit (EXIT_FAILURE);
}
if (-1 == ioctl (fd, VIDIOC_ENUMINPUT, &input)) {
perror ("VIDIOC_ENUM_INPUT");
exit (EXIT_FAILURE);
}
printf ("Current input %s supports:\n", input.name);
memset (&standard, 0, sizeof (standard));
standard.index = 0;
while (0 == ioctl (fd, VIDIOC_ENUMSTD, &standard)) {
if (standard.id & input.std)
printf ("%s\n", standard.name);
standard.index++;
}
/* EINVAL indicates the end of the enumeration, which cannot be
empty unless this device falls under the USB exception. */
if (errno != EINVAL || standard.index == 0) {
perror ("VIDIOC_ENUMSTD");
exit (EXIT_FAILURE);
}

Example 1-7. Selecting a new video standard

struct v4l2_input input;
v4l2_std_id std_id;
memset (&input, 0, sizeof (input));
if (-1 == ioctl (fd, VIDIOC_G_INPUT, &input.index)) {
perror ("VIDIOC_G_INPUT");
exit (EXIT_FAILURE);
}
if (-1 == ioctl (fd, VIDIOC_ENUMINPUT, &input)) {
perror ("VIDIOC_ENUM_INPUT");
exit (EXIT_FAILURE);
}
if (0 == (input.std & V4L2_STD_PAL_BG)) {
fprintf (stderr, "Oops. B/G PAL is not supported.\n");
exit (EXIT_FAILURE);
}
/* Note this is also supposed to work when only B
or G/PAL is supported. */
std_id = V4L2_STD_PAL_BG;
if (-1 == ioctl (fd, VIDIOC_S_STD, &std_id)) {
perror ("VIDIOC_S_STD");
exit (EXIT_FAILURE);
}

1.8. Controls

Devices typically have a number of user-settable controls such as brightness, saturation and so on, which would be presented to the user on a graphical user interface. But, different devices will have different controls available, and furthermore, the range of possible values, and the default value will vary from device to device. The control ioctls provide the information and a mechanism to create a nice user interface for these controls that will work correctly with any device.

All controls are accessed using an ID value. V4L2 defines several IDs for specific purposes. Drivers can also implement their own custom controls using V4L2_CID_PRIVATE_BASE and higher values. The pre-defined control IDs have the prefix V4L2_CID_, and are listed in Table 1-1>. The ID is used when querying the attributes of a control, and when getting or setting the current value.

Generally applications should present controls to the user without assumptions about their purpose. Each control comes with a name string the user is supposed to understand. When the purpose is non-intuitive the driver writer should provide a user manual, a user interface plug-in or a driver specific panel application. Predefined IDs were introduced to change a few controls programmatically, for example to mute a device during a channel switch.

Drivers may enumerate different controls after switching the current video input or output, tuner or modulator, or audio input or output. Different in the sense of other bounds, another default and current value, step size or other menu items. A control with a certain custom ID can also change name and type.[9] Control values are stored globally, they do not change when switching except to stay within the reported bounds. They also do not change e. g. when the device is opened or closed, when the tuner radio frequency is changed or generally never without application request. Since V4L2 specifies no event mechanism, panel applications intended to cooperate with other panel applications (be they built into a larger application, as a TV viewer) may need to regularly poll control values to update their user interface.[10]

Table 1-1. Control IDs

IDTypeDescription
V4L2_CID_BASE First predefined ID, equal to V4L2_CID_BRIGHTNESS.
V4L2_CID_BRIGHTNESSintegerPicture brightness, or more precisely, the black level. Will not turn up the intelligence of the program you're watching.
V4L2_CID_CONTRASTintegerPicture contrast or luma gain.
V4L2_CID_SATURATIONintegerPicture color saturation or chroma gain.
V4L2_CID_HUEintegerHue or color balance.
V4L2_CID_AUDIO_VOLUMEintegerOverall audio volume. Note some drivers also provide an OSS or ALSA mixer interface.
V4L2_CID_AUDIO_BALANCEintegerAudio stereo balance. Minimum corresponds to all the way left, maximum to right.
V4L2_CID_AUDIO_BASSintegerAudio bass adjustment.
V4L2_CID_AUDIO_TREBLEintegerAudio treble adjustment.
V4L2_CID_AUDIO_MUTEbooleanMute audio, i. e. set the volume to zero, however without affecting V4L2_CID_AUDIO_VOLUME. Like ALSA drivers, V4L2 drivers must mute at load time to avoid excessive noise. Actually the entire device should be reset to a low power consumption state.
V4L2_CID_AUDIO_LOUDNESSbooleanLoudness mode (bass boost).
V4L2_CID_BLACK_LEVELintegerAnother name for brightness (not a synonym of V4L2_CID_BRIGHTNESS). [?]
V4L2_CID_AUTO_WHITE_BALANCEbooleanAutomatic white balance (cameras).
V4L2_CID_DO_WHITE_BALANCEbuttonThis is an action control. When set (the value is ignored), the device will do a white balance and then hold the current setting. Contrast this with the boolean V4L2_CID_AUTO_WHITE_BALANCE, which, when activated, keeps adjusting the white balance.
V4L2_CID_RED_BALANCEintegerRed chroma balance.
V4L2_CID_BLUE_BALANCEintegerBlue chroma balance.
V4L2_CID_GAMMAintegerGamma adjust.
V4L2_CID_WHITENESSintegerWhiteness for grey-scale devices. This is a synonym for V4L2_CID_GAMMA.
V4L2_CID_EXPOSUREintegerExposure (cameras). [Unit?]
V4L2_CID_AUTOGAINbooleanAutomatic gain/exposure control.
V4L2_CID_GAINintegerGain control.
V4L2_CID_HFLIPbooleanMirror the picture horizontally.
V4L2_CID_VFLIPbooleanMirror the picture vertically.
V4L2_CID_HCENTERintegerHorizontal image centering.
V4L2_CID_VCENTERintegerVertical image centering. Centering is intended to physically adjust cameras. For image cropping see Section 1.10>, for clipping Section 4.2>.
V4L2_CID_LASTP1 End of the predefined control IDs (currently V4L2_CID_VCENTER + 1).
V4L2_CID_PRIVATE_BASE ID of the first custom (driver specific) control. Applications depending on particular custom controls should check the driver name and version, see Section 1.2>.

Applications can enumerate the available controls with the VIDIOC_QUERYCTRL and VIDIOC_QUERYMENU ioctls, get and set a control value with the VIDIOC_G_CTRL and VIDIOC_S_CTRL ioctls. Drivers must implement VIDIOC_QUERYCTRL, VIDIOC_G_CTRL and VIDIOC_S_CTRL when the device has one or more controls, VIDIOC_QUERYMENU when it has one or more menu type controls.

Example 1-8. Enumerating all controls

struct v4l2_queryctrl queryctrl;
struct v4l2_querymenu querymenu;
static void
enumerate_menu (void)
{
printf (" Menu items:\n");
memset (&querymenu, 0, sizeof (querymenu));
querymenu.id = queryctrl.id;
for (querymenu.index = queryctrl.minimum;
querymenu.index <= queryctrl.maximum;
querymenu.index++) {
if (0 == ioctl (fd, VIDIOC_QUERYMENU, &querymenu)) {
printf (" %s\n", querymenu.name);
} else {
perror ("VIDIOC_QUERYMENU");
exit (EXIT_FAILURE);
}
}
}
memset (&queryctrl, 0, sizeof (queryctrl));
for (queryctrl.id = V4L2_CID_BASE;
queryctrl.id < V4L2_CID_LASTP1;
queryctrl.id++) {
if (0 == ioctl (fd, VIDIOC_QUERYCTRL, &queryctrl)) {
if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
continue;
printf ("Control %s\n", queryctrl.name);
if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
enumerate_menu ();
} else {
if (errno == EINVAL)
continue;
perror ("VIDIOC_QUERYCTRL");
exit (EXIT_FAILURE);
}
}
for (queryctrl.id = V4L2_CID_PRIVATE_BASE;;
queryctrl.id++) {
if (0 == ioctl (fd, VIDIOC_QUERYCTRL, &queryctrl)) {
if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED)
continue;
printf ("Control %s\n", queryctrl.name);
if (queryctrl.type == V4L2_CTRL_TYPE_MENU)
enumerate_menu ();
} else {
if (errno == EINVAL)
break;
perror ("VIDIOC_QUERYCTRL");
exit (EXIT_FAILURE);
}
}

Example 1-9. Changing controls

struct v4l2_queryctrl queryctrl;
struct v4l2_control control;
memset (&queryctrl, 0, sizeof (queryctrl));
queryctrl.id = V4L2_CID_BRIGHTNESS;
if (-1 == ioctl (fd, VIDIOC_QUERYCTRL, &queryctrl)) {
if (errno != EINVAL) {
perror ("VIDIOC_QUERYCTRL");
exit (EXIT_FAILURE);
} else {
printf ("V4L2_CID_BRIGHTNESS is not supported\n");
}
} else if (queryctrl.flags & V4L2_CTRL_FLAG_DISABLED) {
printf ("V4L2_CID_BRIGHTNESS is not supported\n");
} else {
memset (&control, 0, sizeof (control));
control.id = V4L2_CID_BRIGHTNESS;
control.value = queryctrl.default_value;
if (-1 == ioctl (fd, VIDIOC_S_CTRL, &control)) {
perror ("VIDIOC_S_CTRL");
exit (EXIT_FAILURE);
}
}
memset (&control, 0, sizeof (control));
control.id = V4L2_CID_CONTRAST;
if (0 == ioctl (fd, VIDIOC_G_CTRL, &control)) {
control.value += 1;
/* The driver may clamp the value or return ERANGE, ignored here */
if (-1 == ioctl (fd, VIDIOC_S_CTRL, &control)
&& errno != ERANGE) {
perror ("VIDIOC_S_CTRL");
exit (EXIT_FAILURE);
}
/* Ignore if V4L2_CID_CONTRAST is unsupported */
} else if (errno != EINVAL) {
perror ("VIDIOC_G_CTRL");
exit (EXIT_FAILURE);
}
control.id = V4L2_CID_AUDIO_MUTE;
control.value = TRUE; /* silence */
/* Errors ignored */
ioctl (fd, VIDIOC_S_CTRL, &control);

1.9. Data Formats

1.9.1. Data Format Negotiation

Different devices exchange different kinds of data with applications, for example video images, raw or sliced VBI data, RDS datagrams. Even within one kind many different formats are possible, in particular an abundance of image formats. Although drivers must provide a default and the selection persists across closing and reopening a device, applications should always negotiate a data format before engaging in data exchange. Negotiation means the application asks for a particular format and the driver selects and reports the best the hardware can do to satisfy the request. Of course applications can also just query the current selection.

A single mechanism exists to negotiate all data formats using the aggregate struct v4l2_format and the VIDIOC_G_FMT and VIDIOC_S_FMT ioctls. Additionally the VIDIOC_TRY_FMT ioctl can be used to examine what the hardware could do, without actually selecting a new data format. The data formats supported by the V4L2 API are covered in the respective device section in Chapter 4>. For a closer look at image formats see Chapter 2>.

The VIDIOC_S_FMT ioctl is a major turning-point in the initialization sequence. Prior to this point multiple panel applications can access the same device concurrently to select the current input, change controls or modify other properties. The first VIDIOC_S_FMT assigns a logical stream (video data, VBI data etc.) exclusively to one file descriptor.

Exclusive means no other application, more precisely no other file descriptor, can grab this stream or change device properties inconsistent with the negotiated parameters. A video standard change for example, when the new standard uses a different number of scan lines, can invalidate the selected image format. Therefore only the file descriptor owning the stream can make invalidating changes. Accordingly multiple file descriptors which grabbed different logical streams prevent each other from interfering with their settings. When for example video overlay is about to start or already in progress, simultaneous video capturing may be restricted to the same cropping and image size.

When applications omit the VIDIOC_S_FMT ioctl its locking side effects are implied by the next step, the selection of an I/O method with the VIDIOC_REQBUFS ioctl or implicit with the first read() or write() call.

Generally only one logical stream can be assigned to a file descriptor, the exception being drivers permitting simultaneous video capturing and overlay using the same file descriptor for compatibility with V4L and earlier versions of V4L2. Switching the logical stream or returning into "panel mode" is possible by closing and reopening the device. Drivers may support a switch using VIDIOC_S_FMT.

All drivers exchanging data with applications must support the VIDIOC_G_FMT and VIDIOC_S_FMT ioctl. Implementation of the VIDIOC_TRY_FMT is highly recommended but optional.


1.9.2. Image Format Enumeration

Apart of the generic format negotiation functions a special ioctl to enumerate all image formats supported by video capture, overlay or output devices is available.[11]

The VIDIOC_ENUM_FMT ioctl must be supported by all drivers exchanging image data with applications.

Important: Drivers are not supposed to convert image formats in kernel space. They must enumerate only formats directly supported by the hardware. If necessary driver writers should publish an example conversion routine or library for integration into applications.


1.10. Cropping and Scaling

Some video capture devices can take a subsection of the complete picture and shrink or enlarge to an image of arbitrary size. We call these abilities cropping and scaling. Not quite correct "cropping" shall also refer to the inverse process, output devices showing an image in only a region of the picture, and/or scaled from a source image of different size.

To crop and scale this API defines a source and target rectangle. On a video capture and overlay device the source is the received video picture, the target is the captured or overlaid image. On a video output device the source is the image passed by the application and the target is the generated video picture. The remainder of this section refers only to video capture drivers, the definitions apply to output drivers accordingly.

Figure 1-1. Cropping and Scaling

It is assumed the driver can capture a subsection of the picture within an arbitrary capture window. Its bounds are defined by struct v4l2_cropcap, giving the coordinates of the top, left corner and width and height of the window in pixels. Origin and units of the coordinate system in the analog domain are arbitrarily chosen by the driver writer.[12]

The source rectangle is defined by struct v4l2_crop, giving the coordinates of its top, left corner, width and height using the same coordinate system as struct v4l2_cropcap. The source rectangle must lie completely within the capture window. Further each driver defines a default source rectangle. The center of this rectangle shall align with the center of the active picture area of the video signal, and cover what the driver writer considers the complete picture. The source rectangle is set to the default when the driver is first loaded, but not later.

The target rectangle is given either by the width and height fields of struct v4l2_pix_format or the width and height fields of the struct v4l2_rect w substructure of struct v4l2_window.

In principle cropping and scaling always happens. When the device supports scaling but not cropping, applications will be unable to change the cropping rectangle. It remains at the defaults all the time. When the device supports cropping but not scaling, changing the image size will also affect the cropping size in order to maintain a constant scaling factor. The position of the cropping rectangle is only adjusted to move the rectangle completely inside the capture window.

When cropping and scaling is supported applications can change both the source and target rectangle. Various hardware limitations must be expected, for example discrete scaling factors, different scaling abilities in horizontal and vertical direction, limitations of the image size or the cropping alignment. Therefore as usual drivers adjust the requested parameters against hardware capabilities and return the actual values selected. An important difference, because two rectangles are defined, is that the last rectangle changed shall take priority, and the driver may also adjust the opposite rectangle.

Suppose scaling is restricted to a factor 1:1 or 2:1 in either direction and the image size must be a multiple of 16 × 16 pixels. The cropping rectangle be set to the upper limit, 640 × 400 pixels at offset 0, 0. Let a video capture application request an image size of 300 × 225 pixels, assuming video will be scaled down from the "full picture" accordingly. The driver will set the image size to the closest possible values 304 × 224, then choose the cropping rectangle closest to the requested size, that is 608 × 224 (224 × 2:1 would exceed the limit 400). The offset 0, 0 is still valid, thus unmodified. Given the default cropping rectangle reported by VIDIOC_CROPCAP the application can easily propose another offset to center the cropping rectangle. Now the application may insist on covering an area using an aspect closer to the original request. Sheepish it asks for a cropping rectangle of 608 × 456 pixels. The present scaling factors limit cropping to 640 × 384, so the driver returns the cropping size 608 × 384 and accordingly adjusts the image size to 304 × 192.

Eventually some crop or scale parameters are locked, for example when the driver supports simultaneous video capturing and overlay, another application already started overlay and the cropping parameters cannot be changed anymore. Also VIDIOC_TRY_FMT cannot change the cropping rectangle. In these cases the driver has to approach the closest values possible without adjusting the opposite rectangle.

The struct v4l2_cropcap, which also reports the pixel aspect ratio, can be obtained with the VIDIOC_CROPCAP ioctl. To get or set the current cropping rectangle applications call the VIDIOC_G_CROP or VIDIOC_S_CROP ioctl, respectively. All video capture and output devices must support the VIDIOC_CROPCAP ioctl. The VIDIOC_G_CROP and VIDIOC_S_CROP ioctls only when the cropping rectangle can be changed.

Note as usual the cropping parameters remain unchanged across closing and reopening a device. Applications should ensure the parameters are suitable before starting I/O.

Example 1-10. Resetting the cropping parameters

(A video capture device is assumed.)

struct v4l2_cropcap cropcap;
struct v4l2_crop crop;
memset (&cropcap, 0, sizeof (cropcap));
cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (-1 == ioctl (fd, VIDIOC_CROPCAP, &cropcap)) {
perror ("VIDIOC_CROPCAP");
exit (EXIT_FAILURE);
}
memset (&crop, 0, sizeof (crop));
crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
crop.c = cropcap.defrect;
/* Ignore if cropping is not supported (EINVAL) */
if (-1 == ioctl (fd, VIDIOC_S_CROP, &crop)
&& errno != EINVAL) {
perror ("VIDIOC_S_CROP");
exit (EXIT_FAILURE);
}

Example 1-11. Simple downscaling

(A video capture device is assumed.)

struct v4l2_cropcap cropcap;
struct v4l2_format format;
reset_cropping_parameters ();
/* Scale down to 1/4 size of full picture */
memset (&format, 0, sizeof (format)); /* defaults */
format.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
format.fmt.pix.width = cropcap.defrect.width >> 1;
format.fmt.pix.height = cropcap.defrect.height >> 1;
format.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
if (-1 == ioctl (fd, VIDIOC_S_FMT, &format)) {
perror ("VIDIOC_S_FORMAT");
exit (EXIT_FAILURE);
}
/* We could check now what we got, the exact scaling factor
or if the driver can scale at all. At mere 2:1 the cropping
rectangle was probably not changed. */

Example 1-12. Current scaling factor and pixel aspect

(A video capture device is assumed.)

struct v4l2_cropcap cropcap;
struct v4l2_crop crop;
struct v4l2_format format;
double hscale, vscale;
double aspect;
int dwidth, dheight;
memset (&cropcap, 0, sizeof (cropcap));
cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (-1 == ioctl (fd, VIDIOC_CROPCAP, &cropcap)) {
perror ("VIDIOC_CROPCAP");
exit (EXIT_FAILURE);
}
memset (&crop, 0, sizeof (crop));
crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (-1 == ioctl (fd, VIDIOC_G_CROP, &crop)) {
if (errno != EINVAL) {
perror ("VIDIOC_G_CROP");
exit (EXIT_FAILURE);
}
/* Cropping not supported */
crop.c = cropcap.defrect;
}
memset (&format, 0, sizeof (format));
format.fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (-1 == ioctl (fd, VIDIOC_G_FMT, &format)) {
perror ("VIDIOC_G_FMT");
exit (EXIT_FAILURE);
}
hscale = format.fmt.pix.width / (double) crop.c.width;
vscale = format.fmt.pix.height / (double) crop.c.height;
aspect = cropcap.pixelaspect.numerator /
(double) cropcap.pixelaspect.denominator;
aspect = aspect * hscale / vscale;
/* Aspect corrected display size */
dwidth = format.fmt.pix.width / aspect;
dheight = format.fmt.pix.height;

1.11. Streaming Parameters

Streaming parameters are intended to optimize the video capture process as well as I/O. Presently applications can request a high quality capture mode with the VIDIOC_S_PARM ioctl.

The current video standard determines a nominal number of frames per second. If less than this number of frames is to be captured or output, applications can request frame skipping or duplicating on the driver side. This is especially useful when using the read() or write(), which are not augmented by timestamps or sequence counters, and to avoid unneccessary data copying.

Finally these ioctls can be used to determine the number of buffers used internally by a driver in read/write mode. For implications see the section discussing the read() function.

To get and set the streaming parameters applications call the VIDIOC_G_PARM and VIDIOC_S_PARM ioctl, respectively. They take a pointer to a struct v4l2_streamparm, which contains a union holding separate parameters for input and output devices.

These ioctls are optional, drivers need not implement them. If so, they return the EINVAL error code.


Chapter 2. Image Formats

The V4L2 API was primarily designed for devices exchanging image data with applications. The v4l2_pix_format structure defines the format and layout of an image in memory. Image formats are negotiated with the VIDIOC_S_FMT ioctl. (The explanations here focus on video capturing and output, for overlay frame buffer formats see also VIDIOC_G_FBUF.)

Table 2-1. struct v4l2_pix_format

__u32widthImage width in pixels.
__u32heightImage height in pixels.
Applications set these fields to request an image size, drivers return the closest possible values. In case of planar formats the width and height applies to the largest plane. To avoid ambiguities drivers must return values rounded up to a multiple of the scale factor of any smaller planes. For example when the image format is YUV 4:2:0, width and height must be multiples of two.
__u32pixelformatThe pixel format or type of compression, set by the application. This is a little endian four character code. V4L2 defines standard RGB formats in Table 2-3>, YUV formats in Section 2.4>, and reserved codes in Table 2-4>
enum v4l2_fieldfieldVideo images are typically interlaced. Applications can request to capture or output only the top or bottom field, or both fields interlaced or sequentially stored in one buffer or alternating in separate buffers. Drivers return the actual field order selected. For details see Section 3.6>.
__u32bytesperlineDistance in bytes between the leftmost pixels in two adjacent lines.

Both applications and drivers can set this field to request padding bytes at the end of each line. Drivers however may ignore the value requested by the application, returning width times bytes per pixel or a larger value required by the hardware. That implies applications can just set this field to zero to get a reasonable default.

Video hardware may access padding bytes, therefore they must reside in accessible memory. Consider cases where padding bytes after the last line of an image cross a system page boundary. Input devices may write padding bytes, the value is undefined. Output devices ignore the contents of padding bytes.

When the image format is planar the bytesperline value applies to the largest plane and is divided by the same factor as the width field for any smaller planes. For example the Cb and Cr planes of a YUV 4:2:0 image have half as many padding bytes following each line as the Y plane. To avoid ambiguities drivers must return a bytesperline value rounded up to a multiple of the scale factor.

__u32sizeimageSize in bytes of the buffer to hold a complete image, set by the driver. Usually this is bytesperline times height. When the image consists of variable length compressed data this is the maximum number of bytes required to hold an image.
enum v4l2_colorspacecolorspaceThis information supplements the pixelformat and must be set by the driver, see Section 2.2>.
__u32privReserved for custom (driver defined) additional information about formats. When not used drivers and applications must set this field to zero.

2.1. Standard Image Formats

In order to exchange images between drivers and applications, it is necessary to have standard image data formats which both sides will interpret the same way. V4L2 includes several such formats, and this section is intended to be an unambiguous specification of the standard image data formats in V4L2.

V4L2 drivers are not limited to these formats, however. Driver-specific formats are possible. In that case the application may depend on a codec to convert images to one of the standard formats when needed. But the data can still be stored and retrieved in the proprietary format. For example, a device may support a proprietary compressed format. Applications can still capture and save the data in the compressed format, saving much disk space, and later use a codec to convert the images to the X Windows screen format when the video is to be displayed.

Even so, ultimately, some standard formats are needed, so the V4L2 specification would not be complete without well-defined standard formats.

The V4L2 standard formats are mainly uncompressed formats. The pixels are always arranged in memory from left to right, and from top to bottom. The first byte of data in the image buffer is always for the leftmost pixel of the topmost row. Following that is the pixel immediately to its right, and so on until the end of the top row of pixels. Following the rightmost pixel of the row there may be zero or more bytes of padding to guarantee that each row of pixel data has a certain alignment. Following the pad bytes, if any, is data for the leftmost pixel of the second row from the top, and so on. The last row has just as many pad bytes after it as the other rows.

In V4L2 each format has an identifier which looks like PIX_FMT_XXX, defined in the videodev.h header file. These identifiers represent four character codes which are also listed below, however they are not the same as those used in the Windows world.


2.2. Colorspaces

[intro]

Gamma Correction

[to do]

E'R = f(R)

E'G = f(G)

E'B = f(B)

Construction of luminance and color-difference signals

[to do]

E'Y = CoeffR E'R + CoeffG E'G + CoeffB E'B

(E'R - E'Y) = E'R - CoeffR E'R - CoeffG E'G - CoeffB E'B

(E'B - E'Y) = E'B - CoeffR E'R - CoeffG E'G - CoeffB E'B

Re-normalized color-difference signals

The color-difference signals are scaled back to unity range [-0.5;+0.5]:

KB = 0.5 / (1 - CoeffB)

KR = 0.5 / (1 - CoeffR)

PB = KB (E'B - E'Y) = 0.5 (CoeffR / CoeffB) E'R + 0.5 (CoeffG / CoeffB) E'G + 0.5 E'B

PR = KR (E'R - E'Y) = 0.5 E'R + 0.5 (CoeffG / CoeffR) E'G + 0.5 (CoeffB / CoeffR) E'B

Quantization

[to do]

Y' = (Lum. Levels - 1) · E'Y + Lum. Offset

CB = (Chrom. Levels - 1) · PB + Chrom. Offset

CR = (Chrom. Levels - 1) · PR + Chrom. Offset

Rounding to the nearest integer and clamping to the range [0;255] finally yields the digital color components Y'CbCr stored in YUV images.

Example 2-1. ITU-R Rec. BT.601 color conversion

Forward Transformation

int ER, EG, EB; /* gamma corrected RGB input [0;255] */
int Y1, Cb, Cr; /* output [0;255] */
double r, g, b; /* temporaries */
double y1, pb, pr;
int
clamp (double x)
{
int r = x; /* round to nearest */
if (r < 0) return 0;
else if (r > 255) return 255;
else return r;
}
r = ER / 255.0;
g = EG / 255.0;
b = EB / 255.0;
y1 = 0.299 * r + 0.587 * g + 0.114 * b;
pb = -0.169 * r - 0.331 * g + 0.5 * b;
pr = 0.5 * r - 0.419 * g - 0.081 * b;
Y1 = clamp (219 * y1 + 16);
Cb = clamp (224 * pb + 128);
Cr = clamp (224 * pr + 128);
/* or shorter */
y1 = 0.299 * ER + 0.587 * EG + 0.114 * EB;
Y1 = clamp ( (219 / 255.0) * y1 + 16);
Cb = clamp (((224 / 255.0) / (2 - 2 * 0.114)) * (EB - y1) + 128);
Cr = clamp (((224 / 255.0) / (2 - 2 * 0.299)) * (ER - y1) + 128);

Inverse Transformation

int Y1, Cb, Cr; /* gamma pre-corrected input [0;255] */
int ER, EG, EB; /* output [0;255] */
double r, g, b; /* temporaries */
double y1, pb, pr;
int
clamp (double x)
{
int r = x; /* round to nearest */
if (r < 0) return 0;
else if (r > 255) return 255;
else return r;
}
y1 = (255 / 219.0) * (Y1 - 16);
pb = (255 / 224.0) * (Cb - 128);
pr = (255 / 224.0) * (Cr - 128);
r = 1.0 * y1 + 0 * pb + 1.402 * pr;
g = 1.0 * y1 - 0.344 * pb - 0.714 * pr;
b = 1.0 * y1 + 1.772 * pb + 0 * pr;
ER = clamp (r * 255); /* [ok? one should prob. limit y1,pb,pr] */
EG = clamp (g * 255);
EB = clamp (b * 255);

Table 2-2. enum v4l2_colorspace

IdentifierValueDescriptionChromaticities[a]White PointGamma CorrectionLuminance E'YQuantization
RedGreenBlueY'Cb, Cr
V4L2_COLORSPACE_SMPTE170M1NTSC/PAL according to SMPTE170M>, ITU601>x = 0.630, y = 0.340x = 0.310, y = 0.595x = 0.155, y = 0.070x = 0.3127, y = 0.3290, Illuminant D65E' = 4.5 I for I ≤0.018, 1.099 I0.45 - 0.099 for 0.018 < I0.299 E'R + 0.587 E'G + 0.114 E'B219 E'Y + 16224 PB,R + 128
V4L2_COLORSPACE_SMPTE240M21125-Line (US) HDTV, see SMPTE240M>x = 0.630, y = 0.340x = 0.310, y = 0.595x = 0.155, y = 0.070x = 0.3127, y = 0.3290, Illuminant D65E' = 4 I for I ≤0.0228, 1.1115 I0.45 - 0.1115 for 0.0228 < I0.212 E'R + 0.701 E'G + 0.087 E'B219 E'Y + 16224 PB,R + 128
V4L2_COLORSPACE_REC7093HDTV and modern devices, see ITU709>x = 0.640, y = 0.330x = 0.300, y = 0.600x = 0.150, y = 0.060x = 0.3127, y = 0.3290, Illuminant D65E' = 4.5 I for I ≤0.018, 1.099 I0.45 - 0.099 for 0.018 < I0.2125 E'R + 0.7154 E'G + 0.0721 E'B219 E'Y + 16224 PB,R + 128
V4L2_COLORSPACE_BT8784Broken Bt878 extents[b], ITU601>?????0.299 E'R + 0.587 E'G + 0.114 E'B237 E'Y + 16224 PB,R + 128 (probably)
V4L2_COLORSPACE_470_SYSTEM_M5M/NTSC[c] according to ITU470>, ITU601>x = 0.67, y = 0.33x = 0.21, y = 0.71x = 0.14, y = 0.08x = 0.310, y = 0.316, Illuminant C?0.299 E'R + 0.587 E'G + 0.114 E'B219 E'Y + 16224 PB,R + 128
V4L2_COLORSPACE_470_SYSTEM_BG6625-line PAL and SECAM systems according to ITU470>, ITU601>x = 0.64, y = 0.33x = 0.29, y = 0.60x = 0.15, y = 0.06x = 0.313, y = 0.329, Illuminant D65?0.299 E'R + 0.587 E'G + 0.114 E'B219 E'Y + 16224 PB,R + 128
V4L2_COLORSPACE_JPEG7JPEG Y'CbCr, see JFIF>, ITU601>?????0.299 E'R + 0.587 E'G + 0.114 E'B256 E'Y + 16[d]256 PB,R + 128
V4L2_COLORSPACE_SRGB8[?]x = 0.640, y = 0.330x = 0.300, y = 0.600x = 0.150, y = 0.060x = 0.3127, y = 0.3290, Illuminant D65E' = 4.5 I for I ≤0.018, 1.099 I0.45 - 0.099 for 0.018 < In/a
Notes:
a. The coordinates of the color primaries are given in the CIE system (1931)
b. The ubiquitous Bt878 video capture chip quantizes E'Y to 238 levels, yielding a range of Y' = 16 … 253, unlike Rec. 601 Y' = 16 … 235. This is not a typo in the Bt878 documentation, it has been implemented in silicon. The chroma extents are unclear.
c. No identifier exists for M/PAL which uses the chromaticities of M/NTSC, the remaining parameters are equal to B and G/PAL.
d. Note JFIF quantizes Y'PBPR in range [0;+1] and [-0.5;+0.5] to 257 levels, however Y'CbCr signals are still clamped to [0;255].

2.3. RGB Formats

These formats are designed to match the pixel formats of typical PC graphics frame buffers. They occupy 8, 16, 24 or 32 bits per pixel. These are all packed-pixel formats, meaning all the data for a pixel lie next to each other in memory.

When one of these formats is used, drivers shall report the colorspace V4L2_COLORSPACE_SRGB.

Table 2-3. Packed RGB Image Formats

IdentifierCode Byte 0 Byte 1 Byte 2 Byte 3
Bit76543210 76543210 76543210 76543210
V4L2_PIX_FMT_RGB332'RGB1' b1b0g2g1g0r2r1r0                          
V4L2_PIX_FMT_RGB555'RGBO' g2g1g0r4r3r2r1r0 ?b4b3b2b1b0g4g3                 
V4L2_PIX_FMT_RGB565'RGBP' g2g1g0r4r3r2r1r0 b4b3b2b1b0g5g4g3                 
V4L2_PIX_FMT_RGB555X'RGBQ' ?b4b3b2b1b0g4g3 g2g1g0r4r3r2r1r0                 
V4L2_PIX_FMT_RGB565X'RGBR' b4b3b2b1b0g5g4g3 g2g1g0r4r3r2r1r0                 
V4L2_PIX_FMT_BGR24'BGR3' b7b6b5b4b3b2b1b0 g7g6g5g4g3g2g1g0 r7r6r5r4r3r2r1r0        
V4L2_PIX_FMT_RGB24'RGB3' r7r6r5r4r3r2r1r0 g7g6g5g4g3g2g1g0 b7b6b5b4b3b2b1b0        
V4L2_PIX_FMT_BGR32'BGR4' b7b6b5b4b3b2b1b0 g7g6g5g4g3g2g1g0 r7r6r5r4r3r2r1r0 ????????
V4L2_PIX_FMT_RGB32'RGB4' r7r6r5r4r3r2r1r0 g7g6g5g4g3g2g1g0 b7b6b5b4b3b2b1b0 ????????

Bit 7 is the most significant bit. ? = undefined bit, ignored on output, random value on input.

Example 2-2. V4L2_PIX_FMT_BGR24 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:B00G00R00B01G01R01B02G02R02B03G03R03
start + 12:B10G10R10B11G11R11B12G12R12B13G13R13
start + 24:B20G20R20B21G21R21B22G22R22B23G23R23
start + 36:B30G30R30B31G31R31B32G32R32B33G33R33

Important: Drivers may interpret these formats differently.

The V4L2_PIX_FMT_RGB555, V4L2_PIX_FMT_RGB565, V4L2_PIX_FMT_RGB555X and V4L2_PIX_FMT_RGB565X formats are uncommon. Video and display hardware typically supports variants with reversed order of color components, i. e. blue towards the least, red towards the most significant bit. Although presumably the original authors had the common formats in mind, the definitions were always very clear and cannot be simply regarded as erroneous.

If V4L2_PIX_FMT_RGB332 has been chosen in accordance with the 15 and 16 bit formats, this format might as well be interpreted differently, as "rrrgggbb" rather than "bbgggrrr".

Finally some drivers, most prominently the BTTV driver, might interpret V4L2_PIX_FMT_RGB32 as the big-endian variant of V4L2_PIX_FMT_BGR32, consisting of bytes "?RGB" in memory. V4L2 never defined such a format, lack of a X suffix to the symbol suggests it was intended this way, and a new symbol and four character code should have been used instead.

Until these issues are solved, application writers are advised that drivers might interpret these formats either way.


2.4. YUV Formats

Table of Contents
V4L2_PIX_FMT_GREY ('GREY') -- Grey-scale image.
V4L2_PIX_FMT_YUYV ('YUYV') -- Packed format with ½ horizontal chroma resolution, also known as YUV 4:2:2.
V4L2_PIX_FMT_UYVY ('UYVY') -- Variation of V4L2_PIX_FMT_YUYV with different order of samples in memory.
V4L2_PIX_FMT_Y41P ('Y41P') -- Packed format with ¼ horizontal chroma resolution, also known as YUV 4:1:1.
V4L2_PIX_FMT_YVU420 ('YV12'), V4L2_PIX_FMT_YUV420 ('YU12') -- Planar formats with ½ horizontal and vertical chroma resolution, also known as YUV 4:2:0.
V4L2_PIX_FMT_YVU410 ('YVU9'), V4L2_PIX_FMT_YUV410 ('YUV9') -- Planar formats with ¼ horizontal and vertical chroma resolution, also known as YUV 4:1:0.
V4L2_PIX_FMT_YUV422P ('422P') -- Format with ½ horizontal chroma resolution, also known as YUV 4:2:2. Planar layout as opposed to V4L2_PIX_FMT_YUYV.
V4L2_PIX_FMT_YUV411P ('411P') -- Format with ¼ horizontal chroma resolution, also known as YUV 4:1:1. Planar layout as opposed to V4L2_PIX_FMT_Y41P.
V4L2_PIX_FMT_NV12 ('NV12'), V4L2_PIX_FMT_NV21 ('NV21') -- Formats with ½ horizontal and vertical chroma resolution, also known as YUV 4:2:0. One luminance and one chrominance plane with alternating chroma samples as opposed to V4L2_PIX_FMT_YVU420.

YUV is the format native to TV broadcast and composite video signals. It separates the brightness information (Y) from the color information (U and V or Cb and Cr). The color information consists of red and blue color difference signals, this way the green component can be reconstructed by subtracting from the brightness component. See Section 2.2> for conversion examples. YUV was chosen because early television would only transmit brightness information. To add color in a way compatible with existing receivers a new signal carrier was added to transmit the color difference signals. Secondary in the YUV format the U and V components usually have lower resolution than the Y component. This is an analog video compression technique taking advantage of a property of the human visual system, being more sensitive to brightness information.

V4L2_PIX_FMT_GREY ('GREY')

Name

V4L2_PIX_FMT_GREY -- Grey-scale image.

Description

This is a grey-scale image. It is really a degenerate Y'CbCr format which simply contains no Cb or Cr data.

Example 2-1. V4L2_PIX_FMT_GREY 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Y'00Y'01Y'02Y'03
start + 4:Y'10Y'11Y'12Y'13
start + 8:Y'20Y'21Y'22Y'23
start + 12:Y'30Y'31Y'32Y'33

V4L2_PIX_FMT_YUYV ('YUYV')

Name

V4L2_PIX_FMT_YUYV -- Packed format with ½ horizontal chroma resolution, also known as YUV 4:2:2.

Description

In this format each four bytes is two pixels. Each four bytes is two Y's, a Cb and a Cr. Each Y goes to one of the pixels, and the Cb and Cr belong to both pixels. As you can see, the Cr and Cb components have half the horizontal resolution of the Y component. V4L2_PIX_FMT_YUYV is known in the Windows environment as YUY2.

Example 2-1. V4L2_PIX_FMT_YUYV 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Y'00Cb00Y'01Cr00Y'02Cb01Y'03Cr01
start + 8:Y'10Cb10Y'11Cr10Y'12Cb11Y'13Cr11
start + 16:Y'20Cb20Y'21Cr20Y'22Cb21Y'23Cr21
start + 24:Y'30Cb30Y'31Cr30Y'32Cb31Y'33Cr31

Color Sample Location.

 0 1 2 3
0YCY YCY
1YCY YCY
2YCY YCY
3YCY YCY

V4L2_PIX_FMT_UYVY ('UYVY')

Name

V4L2_PIX_FMT_UYVY -- Variation of V4L2_PIX_FMT_YUYV with different order of samples in memory.

Description

In this format each four bytes is two pixels. Each four bytes is two Y's, a Cb and a Cr. Each Y goes to one of the pixels, and the Cb and Cr belong to both pixels. As you can see, the Cr and Cb components have half the horizontal resolution of the Y component.

Example 2-1. V4L2_PIX_FMT_UYVY 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Cb00Y'00Cr00Y'01Cb01Y'02Cr01Y'03
start + 8:Cb10Y'10Cr10Y'11Cb11Y'12Cr11Y'13
start + 16:Cb20Y'20Cr20Y'21Cb21Y'22Cr21Y'23
start + 24:Cb30Y'30Cr30Y'31Cb31Y'32Cr31Y'33

Color Sample Location.

 0 1 2 3
0YCY YCY
1YCY YCY
2YCY YCY
3YCY YCY

V4L2_PIX_FMT_Y41P ('Y41P')

Name

V4L2_PIX_FMT_Y41P -- Packed format with ¼ horizontal chroma resolution, also known as YUV 4:1:1.

Description

In this format each 12 bytes is eight pixels. In the twelve bytes are two CbCr pairs and eight Y's. The first CbCr pair goes with the first four Y's, and the second CbCr pair goes with the other four Y's. The Cb and Cr components have one fourth the horizontal resolution of the Y component.

Do not confuse this format with V4L2_PIX_FMT_YUV411P. Y41P is derived from "YUV 4:1:1 packed", possibly in reference to a Windows FOURCC, while YUV411P stands for "YUV 4:1:1 planar".

Example 2-1. V4L2_PIX_FMT_Y41P 8 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Cb00Y'00Cr00Y'01Cb01Y'02Cr01Y'03Y'04Y'05Y'06Y'07
start + 12:Cb10Y'10Cr10Y'11Cb11Y'12Cr11Y'13Y'14Y'15Y'16Y'17
start + 24:Cb20Y'20Cr20Y'21Cb21Y'22Cr21Y'23Y'24Y'25Y'26Y'27
start + 36:Cb30Y'30Cr30Y'31Cb31Y'32Cr31Y'33Y'34Y'35Y'36Y'37

Color Sample Location.

 0 1 2 3 4 5 6 7
0Y YCY Y Y YCY Y
1Y YCY Y Y YCY Y
2Y YCY Y Y YCY Y
3Y YCY Y Y YCY Y

V4L2_PIX_FMT_YVU420 ('YV12'), V4L2_PIX_FMT_YUV420 ('YU12')

Name

V4L2_PIX_FMT_YVU420, V4L2_PIX_FMT_YUV420 -- Planar formats with ½ horizontal and vertical chroma resolution, also known as YUV 4:2:0.

Description

These are planar formats, as opposed to a packed format. The three components are separated into three sub- images or planes. The Y plane is first. The Y plane has one byte per pixel. For V4L2_PIX_FMT_YVU420, the Cr plane immediately follows the Y plane in memory. The Cr plane is half the width and half the height of the Y plane (and of the image). Each Cr belongs to four pixels, a two-by-two square of the image. For example, Cr0 belongs to Y'00, Y'01, Y'10, and Y'11. Following the Cr plane is the Cb plane, just like the Cr plane. V4L2_PIX_FMT_YUV420 is the same except the Cb plane comes first, then the Cr plane.

If the Y plane has pad bytes after each row, then the Cr and Cb planes have half as many pad bytes after their rows. In other words, two Cx rows (including padding) is exactly as long as one Y row (including padding).

Example 2-1. V4L2_PIX_FMT_YVU420 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Y'00Y'01Y'02Y'03
start + 4:Y'10Y'11Y'12Y'13
start + 8:Y'20Y'21Y'22Y'23
start + 12:Y'30Y'31Y'32Y'33
start + 16:Cr00Cr01  
start + 18:Cr10Cr11  
start + 20:Cb00Cb01  
start + 22:Cb10Cb11  

Color Sample Location.

 0 1 2 3
0Y Y Y Y
  C   C 
1Y Y Y Y
       
2Y Y Y Y
  C   C 
3Y Y Y Y

V4L2_PIX_FMT_YVU410 ('YVU9'), V4L2_PIX_FMT_YUV410 ('YUV9')

Name

V4L2_PIX_FMT_YVU410, V4L2_PIX_FMT_YUV410 -- Planar formats with ¼ horizontal and vertical chroma resolution, also known as YUV 4:1:0.

Description

These are planar formats, as opposed to a packed format. The three components are separated into three sub-images or planes. The Y plane is first. The Y plane has one byte per pixel. For V4L2_PIX_FMT_YVU410, the Cr plane immediately follows the Y plane in memory. The Cr plane is ¼ the width and ¼ the height of the Y plane (and of the image). Each Cr belongs to 16 pixels, a four-by-four square of the image. Following the Cr plane is the Cb plane, just like the Cr plane. V4L2_PIX_FMT_YUV410 is the same, except the Cb plane comes first, then the Cr plane.

If the Y plane has pad bytes after each row, then the Cr and Cb planes have ¼ as many pad bytes after their rows. In other words, four Cx rows (including padding) are exactly as long as one Y row (including padding).

Example 2-1. V4L2_PIX_FMT_YVU410 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Y'00Y'01Y'02Y'03
start + 4:Y'10Y'11Y'12Y'13
start + 8:Y'20Y'21Y'22Y'23
start + 12:Y'30Y'31Y'32Y'33
start + 16:Cr00   
start + 17:Cb00   

Color Sample Location.

 0 1 2 3
0Y Y Y Y
       
1Y Y Y Y
    C   
2Y Y Y Y
       
3Y Y Y Y

V4L2_PIX_FMT_YUV422P ('422P')

Name

V4L2_PIX_FMT_YUV422P -- Format with ½ horizontal chroma resolution, also known as YUV 4:2:2. Planar layout as opposed to V4L2_PIX_FMT_YUYV.

Description

This format is not commonly used. This is a planar version of the YUYV format. The three components are separated into three sub-images or planes. The Y plane is first. The Y plane has one byte per pixel. The Cb plane immediately follows the Y plane in memory. The Cb plane is half the width of the Y plane (and of the image). Each Cb belongs to two pixels. For example, Cb0 belongs to Y'00, Y'01. Following the Cb plane is the Cr plane, just like the Cb plane.

If the Y plane has pad bytes after each row, then the Cr and Cb planes have half as many pad bytes after their rows. In other words, two Cx rows (including padding) is exactly as long as one Y row (including padding).

Example 2-1. V4L2_PIX_FMT_YUV422P 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Y'00Y'01Y'02Y'03
start + 4:Y'10Y'11Y'12Y'13
start + 8:Y'20Y'21Y'22Y'23
start + 12:Y'30Y'31Y'32Y'33
start + 16:Cb00Cb01  
start + 18:Cb10Cb11  
start + 20:Cb20Cb21  
start + 22:Cb30Cb31  
start + 24:Cr00Cr01  
start + 26:Cr10Cr11  
start + 28:Cr20Cr21  
start + 30:Cr30Cr31  

Color Sample Location.

 0 1 2 3
0YCY YCY
1YCY YCY
2YCY YCY
3YCY YCY

V4L2_PIX_FMT_YUV411P ('411P')

Name

V4L2_PIX_FMT_YUV411P -- Format with ¼ horizontal chroma resolution, also known as YUV 4:1:1. Planar layout as opposed to V4L2_PIX_FMT_Y41P.

Description

This format is not commonly used. This is a planar format similar to the 4:2:2 planar format except with half as many chroma. The three components are separated into three sub-images or planes. The Y plane is first. The Y plane has one byte per pixel. The Cb plane immediately follows the Y plane in memory. The Cb plane is ¼ the width of the Y plane (and of the image). Each Cb belongs to 4 pixels all on the same row. For example, Cb0 belongs to Y'00, Y'01, Y'02 and Y'03. Following the Cb plane is the Cr plane, just like the Cb plane.

If the Y plane has pad bytes after each row, then the Cr and Cb planes have ¼ as many pad bytes after their rows. In other words, four C x rows (including padding) is exactly as long as one Y row (including padding).

Example 2-1. V4L2_PIX_FMT_YUV411P 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Y'00Y'01Y'02Y'03
start + 4:Y'10Y'11Y'12Y'13
start + 8:Y'20Y'21Y'22Y'23
start + 12:Y'30Y'31Y'32Y'33
start + 16:Cb00   
start + 17:Cb10   
start + 18:Cb20   
start + 19:Cb30   
start + 20:Cr00   
start + 21:Cr10   
start + 22:Cr20   
start + 23:Cr30   

Color Sample Location.

 0 1 2 3
0Y YCY Y
1Y YCY Y
2Y YCY Y
3Y YCY Y

V4L2_PIX_FMT_NV12 ('NV12'), V4L2_PIX_FMT_NV21 ('NV21')

Name

V4L2_PIX_FMT_NV12, V4L2_PIX_FMT_NV21 -- Formats with ½ horizontal and vertical chroma resolution, also known as YUV 4:2:0. One luminance and one chrominance plane with alternating chroma samples as opposed to V4L2_PIX_FMT_YVU420.

Description

These are two-plane versions of the YUV 4:2:0 format. The three components are separated into two sub-images or planes. The Y plane is first. The Y plane has one byte per pixel. For V4L2_PIX_FMT_NV12, a combined CbCr plane immediately follows the Y plane in memory. The CbCr plane is the same width, in bytes, as the Y plane (and of the image), but is half as tall in pixels. Each CbCr pair belongs to four pixels. For example, Cb0/Cr0 belongs to Y'00, Y'01, Y'10, Y'11. V4L2_PIX_FMT_NV21 is the same except the Cb and Cr bytes are swapped, the CrCb plane starts with a Cr byte.

If the Y plane has pad bytes after each row, then the CbCr plane has as many pad bytes after its rows.

Example 2-1. V4L2_PIX_FMT_NV12 4 × 4 pixel image

Byte Order. Each cell is one byte.

start + 0:Y'00Y'01Y'02Y'03
start + 4:Y'10Y'11Y'12Y'13
start + 8:Y'20Y'21Y'22Y'23
start + 12:Y'30Y'31Y'32Y'33
start + 16:Cb00Cr00Cb01Cr01
start + 20:Cb10Cr10Cb11Cr11

Color Sample Location.

 0 1 2 3
0Y Y Y Y
  C   C 
1Y Y Y Y
       
2Y Y Y Y
  C   C 
3Y Y Y Y


2.5. Compressed Formats

[to do, see also VIDIOC_G_MPEGCOMP, VIDIOC_S_MPEGCOMP, VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP. The only compressed standard format should be [M]JPEG.]


2.6. Reserved Format Identifiers

These formats are not defined by this specification, they are just listed for reference and to avoid naming conflicts. If you want to register your own format, send an e-mail to the V4L mailing list https://listman.redhat.com/mailman/listinfo/video4linux-list for inclusion in the videodev.h file. If you want to share your format with other developers add a link to your documentation and send a copy to the maintainer of this document, Michael Schimek , for inclusion in this section. If you think your format should be listed in a standard format section please make a proposal on the V4L mailing list.

Table 2-4. Reserved Image Formats

IdentifierCodeDetails
V4L2_PIX_FMT_YYUV 'YYUV'unknown
V4L2_PIX_FMT_HI240 'HI24' Used by the BTTV driver, http://bytesex.org/bttv/
V4L2_PIX_FMT_MJPEG 'MJPG'Used by the Zoran driver
V4L2_PIX_FMT_JPEG 'JPEG'unknown [?]
V4L2_PIX_FMT_DV 'dvsd'unknown
V4L2_PIX_FMT_MPEG 'MPEG'unknown
V4L2_PIX_FMT_WNVA 'WNVA' Used by the Winnov Videum driver, http://www.thedirks.org/winnov/

Chapter 3. Input/Output

The V4L2 API defines several different methods to read from or write to a device. All drivers exchanging data with applications must support at least one of them.

The classic I/O method using the read() and write() function is automatically selected after opening a V4L2 device. When the driver does not support this method attempts to read or write will fail at any time.

Other methods must be negotiated. To select the streaming I/O method with memory mapped or user buffers applications call the VIDIOC_REQBUFS ioctl. The asynchronous I/O method is not defined yet.

Video overlay can be considered another I/O method, although the application does not directly receive the image data. It is selected by initiating video overlay with the VIDIOC_S_FMT ioctl. For more information see Section 4.2>.

Generally exactly one I/O method, including overlay, is associated with each file descriptor. The only exceptions are applications not exchanging data with a driver ("panel applications", see Section 1.1>) and drivers permitting simultaneous video capturing and overlay using the same file descriptor, for compatibility with V4L and earlier versions of V4L2.

VIDIOC_S_FMT and VIDIOC_REQBUFS would permit this to some degree, but for simplicity drivers need not support switching the I/O method (after first switching away from read/write) other than by closing and reopening the device.

The following sections describe the various I/O methods in more detail.


3.1. Read/Write

Input and output devices support the read() and write() function, respectively, when the V4L2_CAP_READWRITE flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl is set.

Drivers may need the CPU to copy the data, but they may also support DMA to or from user memory, so this I/O method is not necessarily less efficient than other methods merely exchanging buffer pointers. It is considered inferior though because no meta-information like frame counters or timestamps are passed. This information is necessary to recognize frame dropping and to synchronize with other data streams. However this is also the simplest I/O method, requiring little or no setup to exchange data. It permits command line stunts like this (the vidctrl tool is fictitious):

> vidctrl /dev/video --input=0 --format=YUYV --size=352x288
> dd if=/dev/video of=myimage.422 bs=202752 count=1

To read from the device applications use the read() function, to write the write() function. Drivers must implement one I/O method if they exchange data with applications, but it need not be this.[13] When reading or writing is supported, the driver must also support the select() and poll() function.[14]


3.2. Streaming I/O (Memory Mapping)

Input and output devices support this I/O method when the V4L2_CAP_STREAMING flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl is set. There are two streaming methods, to determine if the memory mapping flavor is supported applications must call the VIDIOC_REQBUFS ioctl.

Streaming is an I/O method where only pointers to buffers are exchanged between application and driver, the data itself is not copied. Memory mapping is primarily intended to map buffers in device memory into the application's address space. Device memory can be for example the video memory on a graphics card with a video capture add-on. However, being the most efficient I/O method available for a long time, many other drivers support streaming as well, allocating buffers in DMA-able main memory.

A driver can support many sets of buffers. Each set is identified by a unique buffer type value. The sets are independent and each set can hold a different type of data. To access different sets at the same time different file descriptors must be used.[15]

To allocate device buffers applications call the VIDIOC_REQBUFS ioctl with the desired number of buffers and buffer type, for example V4L2_BUF_TYPE_VIDEO_CAPTURE. This ioctl can also be used to change the number of buffers or to free the allocated memory, provided none of the buffers are still mapped.

Before applications can access the buffers they must map them into their address space with the mmap() function. The location of the buffers in device memory can be determined with the VIDIOC_QUERYBUF ioctl. The m.offset and length returned in a struct v4l2_buffer are passed as sixth and second parameter to the mmap() function. The offset and length values must not be modified. Remember the buffers are allocated in physical memory, as opposed to virtual memory which can be swapped out to disk. Applications should free the buffers as soon as possible with the munmap() function.

Example 3-1. Mapping buffers

struct v4l2_requestbuffers reqbuf;
struct {
void *start;
size_t length;
} *buffers;
unsigned int i;
memset (&reqbuf, 0, sizeof (reqbuf));
reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
reqbuf.memory = V4L2_MEMORY_MMAP;
reqbuf.count = 20;
if (-1 == ioctl (fd, VIDIOC_REQBUFS, &reqbuf)) {
if (errno == EINVAL)
printf ("Video capturing or mmap-streaming is not supported\n");
else
perror ("VIDIOC_REQBUFS");
exit (EXIT_FAILURE);
}
/* We want at least five buffers. */
if (reqbuf.count < 5) {
/* You may need to free the buffers here. */
printf ("Not enough buffer memory\n");
exit (EXIT_FAILURE);
}
buffers = calloc (reqbuf.count, sizeof (*buffers));
assert (buffers != NULL);
for (i = 0; i < reqbuf.count; i++) {
struct v4l2_buffer buffer;
memset (&buffer, 0, sizeof (buffer));
buffer.type = reqbuf.type;
buffer.memory = V4L2_MEMORY_MMAP;
buffer.index = i;
if (-1 == ioctl (fd, VIDIOC_QUERYBUF, &buffer)) {
perror ("VIDIOC_QUERYBUF");
exit (EXIT_FAILURE);
}
buffers[i].length = buffer.length; /* remember for munmap() */
buffers[i].start = mmap (NULL, buffer.length,
PROT_READ | PROT_WRITE, /* required */
MAP_SHARED, /* recommended */
fd, buffer.m.offset);
if (buffers[i].start == MAP_FAILED) {
/* You may need to unmap and free the so far
mapped buffers here. */
perror ("mmap");
exit (EXIT_FAILURE);
}
}
/* Cleanup. */
for (i = 0; i < reqbuf.count; i++)
munmap (buffers[i].start, buffers[i].length);

Streaming drivers maintain two buffer queues, an incoming and an outgoing queue. They separate the synchronous capture or output operation locked to a video clock from the application which is subject to random disk or network delays and preemption by other processes, thereby reducing the probability of data loss. The queues are organized as FIFOs, buffers will be output in the order enqueued in the incoming FIFO, and were captured in the order dequeued from the outgoing FIFO.

The driver may require a minimum number of buffers enqueued at all times to function, apart of this no limit exists on the number of buffers applications can enqueue in advance, or dequeue and process. They can also enqueue in a different order than buffers have been dequeued, and the driver can fill enqueued empty buffers in any order. [16] The index number of a buffer (struct v4l2_buffer index) plays no role here, it only identifies the buffer.

Initially all mapped buffers are in dequeued state, inaccessible by the driver. For capturing applications it is customary to first enqueue all mapped buffers, then to start capturing and enter the read loop. Here the application waits until a filled buffer can be dequeued, and re-enqueues the buffer when the data is no longer needed. Output applications fill and enqueue buffers, when enough buffers are stacked up the output is started with VIDIOC_STREAMON. In the write loop, when the application runs out of free buffers, it must wait until an empty buffer can be dequeued and reused.

To enqueue and dequeue a buffer applications use the VIDIOC_QBUF and VIDIOC_DQBUF ioctl. The status of a buffer being mapped, enqueued, full or empty can be determined at any time using the VIDIOC_QUERYBUF ioctl. Two methods exist to suspend execution of the application until one or more buffers can be dequeued. By default VIDIOC_DQBUF blocks when no buffer is in the outgoing queue. When the O_NONBLOCK flag was given to the open() function, VIDIOC_DQBUF returns immediately with an EAGAIN error code when no buffer is available. The select() or poll() function are always available.

To start and stop capturing or output applications call the VIDIOC_STREAMON and VIDIOC_STREAMOFF ioctl. Note VIDIOC_STREAMOFF removes all buffers from both queues as a side effect. Since there is no notion of doing anything "now" on a multitasking system, if an application needs to synchronize with another event it should examine the struct v4l2_buffer timestamp of captured buffers, or set the field before enqueuing buffers for output.

Drivers implementing memory mapping I/O must support the VIDIOC_REQBUFS, VIDIOC_QUERYBUF, VIDIOC_QBUF, VIDIOC_DQBUF, VIDIOC_STREAMON and VIDIOC_STREAMOFF ioctl, the mmap(), munmap(), select() and poll() function.[17]

[capture example]


3.3. Streaming I/O (User Pointers)

Input and output devices support this I/O method when the V4L2_CAP_STREAMING flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl is set. If the particular user pointer method (not only memory mapping) is supported must be determined by calling the VIDIOC_REQBUFS ioctl.

This I/O method combines advantages of the read/write and memory mapping methods. Buffers are allocated by the application itself, and can reside for example in virtual or shared memory. Only pointers to data are exchanged, these pointers and meta-information are passed in struct v4l2_buffer. The driver must be switched into user pointer I/O mode by calling the VIDIOC_REQBUFS with the desired buffer type. No buffers are allocated beforehands, consequently they are not indexed and cannot be queried like mapped buffers with the VIDIOC_QUERYBUF ioctl.

Example 3-2. Initiating streaming I/O with user pointers

struct v4l2_requestbuffers reqbuf;
memset (&reqbuf, 0, sizeof (reqbuf));
reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
reqbuf.memory = V4L2_MEMORY_USERPTR;
if (ioctl (fd, VIDIOC_REQBUFS, &reqbuf) == -1) {
if (errno == EINVAL)
printf ("Video capturing or user pointer streaming is not supported\n");
else
perror ("VIDIOC_REQBUFS");
exit (EXIT_FAILURE);
}

Buffer addresses and sizes are passed on the fly with the VIDIOC_QBUF ioctl. Although buffers are commonly cycled, applications can pass different addresses and sizes at each VIDIOC_QBUF call. If required by the hardware the driver swaps memory pages within physical memory to create a continuous area of memory. This happens transparently to the application in the virtual memory subsystem of the kernel. When buffer pages have been swapped out to disk they are brought back and finally locked in physical memory for DMA.[18]

Filled or displayed buffers are dequeued with the VIDIOC_DQBUF ioctl. The driver can unlock the memory pages at any time between the completion of the DMA and this ioctl. The memory is also unlocked when VIDIOC_STREAMOFF is called, VIDIOC_REQBUFS, or when the device is closed. Applications must take care not to free buffers without dequeuing. For once, the buffers remain locked until further, wasting physical memory. Second the driver will not be notified when the memory is returned to the application's free list and subsequently reused for other purposes, possibly completing the requested DMA and overwriting valuable data.

For capturing applications it is customary to enqueue a number of empty buffers, to start capturing and enter the read loop. Here the application waits until a filled buffer can be dequeued, and re-enqueues the buffer when the data is no longer needed. Output applications fill and enqueue buffers, when enough buffers are stacked up output is started. In the write loop, when the application runs out of free buffers it must wait until an empty buffer can be dequeued and reused. Two methods exist to suspend execution of the application until one or more buffers can be dequeued. By default VIDIOC_DQBUF blocks when no buffer is in the outgoing queue. When the O_NONBLOCK flag was given to the open() function, VIDIOC_DQBUF returns immediately with an EAGAIN error code when no buffer is available. The select() or poll() function are always available.

To start and stop capturing or output applications call the VIDIOC_STREAMON and VIDIOC_STREAMOFF ioctl. Note VIDIOC_STREAMOFF removes all buffers from both queues and unlocks all buffers as a side effect. Since there is no notion of doing anything "now" on a multitasking system, if an application needs to synchronize with another event it should examine the struct v4l2_buffer timestamp of captured buffers, or set the field before enqueuing buffers for output.

Drivers implementing user pointer I/O must support the VIDIOC_REQBUFS, VIDIOC_QBUF, VIDIOC_DQBUF, VIDIOC_STREAMON and VIDIOC_STREAMOFF ioctl, the select() and poll() function.[19]


3.4. Asynchronous I/O

This method is not defined yet.


3.5. Buffers

A buffer contains data exchanged by application and driver using one of the Streaming I/O methods. Only pointers to buffers are exchanged, the data itself is not copied. These pointers, together with meta-information like timestamps or field parity, are stored in a struct v4l2_buffer, argument to the VIDIOC_QUERYBUF, VIDIOC_QBUF and VIDIOC_DQBUF ioctl.

Nominally timestamps refer to the first data byte transmitted. In practice however the wide range of hardware covered by the V4L2 API limits timestamp accuracy. Often an interrupt routine will sample the system clock shortly after the field or frame was stored completely in memory. So applications must expect a constant difference up to one field or frame period plus a small (few scan lines) random error. The delay and error can be much larger due to compression or transmission over an external bus when the frames are not properly stamped by the sender. This is frequently the case with USB cameras. Here timestamps refer to the instant the field or frame was received by the driver, not the capture time. These devices identify by not enumerating any video standards, see Section 1.7>.

Similar limitations apply to output timestamps. Typically the video hardware locks to a clock controlling the video timing, the horizontal and vertical synchronization pulses. At some point in the line sequence, possibly the vertical blanking, an interrupt routine samples the system clock, compares against the timestamp and programs the hardware to repeat the previous field or frame, or to display the buffer contents.

Apart of limitations of the video device and natural inaccuracies of all clocks, it should be noted system time itself is not perfectly stable. It can be affected by power saving cycles, warped to insert leap seconds, or even turned back or forth by the system administrator affecting long term measurements. [20]

Table 3-1. struct v4l2_buffer

__u32index Number of the buffer, set by the application. This field is only used for memory mapping I/O and can range from zero to the number of buffers allocated with the VIDIOC_REQBUFS ioctl (struct v4l2_requestbuffers count) minus one.
enum v4l2_buf_typetype Type of the buffer, same as struct v4l2_format type or struct v4l2_requestbuffers type, set by the application.
__u32bytesused The number of bytes occupied by the data in the buffer. It depends on the negotiated data format and may change with each buffer for compressed variable size data like JPEG images. Drivers must set this field when type refers to an input stream, applications when an output stream.
__u32flags Flags set by the application or driver, see Table 3-3>.
enum v4l2_fieldfield Indicates the field order of the image in the buffer, see Table 3-8>. This field is not used when the buffer contains VBI data. Drivers must set it when type refers to an input stream, applications when an output stream.
struct timevaltimestamp 

For input streams this is the system time (as returned by the gettimeofday() function) when the first data byte was captured. For output streams the data will not be displayed before this time, secondary to the nominal frame rate determined by the current video standard in enqueued order. Applications can for example zero this field to display frames as soon as possible. The driver stores the time at which the first data byte was actually sent out in the timestamp field. This permits applications to monitor the drift between the video and system clock.

struct v4l2_timecodetimecode When type is V4L2_BUF_TYPE_VIDEO_CAPTURE and the V4L2_BUF_FLAG_TIMECODE flag is set in flags, this structure contains a frame timecode. In V4L2_FIELD_ALTERNATE mode the top and bottom field contain the same timecode. Timecodes are intended to help video editing and are typically recorded on video tapes, but also embedded in compressed formats like MPEG. This field is independent of the timestamp and sequence fields.
__u32sequence Set by the driver, counting the frames in the sequence.

In V4L2_FIELD_ALTERNATE mode the top and bottom field have the same sequence number. The count starts at zero and includes dropped or repeated frames. A dropped frame was received by an input device but could not be stored due to lack of free buffer space. A repeated frame was displayed again by an output device because the application did not pass new data in time.

Note this may count the frames received e.g. over USB, without taking into account the frames dropped by the remote hardware due to limited compression throughput or bus bandwidth. These devices identify by not enumerating any video standards, see Section 1.7>.

enum v4l2_memorymemory This field must be set by applications and/or drivers in accordance with the selected I/O method.
unionm  
 __u32offsetWhen memory is V4L2_MEMORY_MMAP this is the offset of the buffer from the start of the device memory. The value is returned by the driver and apart of serving as parameter to the mmap() function not useful for applications. See Section 3.2> for details.
 unsigned longuserptrWhen memory is V4L2_MEMORY_USERPTR this is a pointer to the buffer (casted to unsigned long type) in virtual memory, set by the application. See Section 3.3> for details.
__u32length Size of the buffer (not the payload) in bytes.
__u32input Some video capture drivers support rapid and synchronous video input changes, a function useful for example in video surveillance applications. For this purpose applications set the V4L2_BUF_FLAG_INPUT flag, and this field to the number of a video input as in struct v4l2_input field index.
__u32reserved A place holder for future extensions and custom (driver defined) buffer types V4L2_BUF_TYPE_PRIVATE and higher.

Table 3-2. enum v4l2_buf_type

V4L2_BUF_TYPE_VIDEO_CAPTURE1Buffer of a video capture stream, see Section 4.1>.
V4L2_BUF_TYPE_VIDEO_OUTPUT2Buffer of a video output stream, see Section 4.3>.
V4L2_BUF_TYPE_VIDEO_OVERLAY3Buffer for video overlay, see Section 4.2>.
V4L2_BUF_TYPE_VBI_CAPTURE4Buffer of a raw VBI capture stream, see Section 4.6>.
V4L2_BUF_TYPE_VBI_OUTPUT5Buffer of a raw VBI output stream, see Section 4.6>.
V4L2_BUF_TYPE_PRIVATE0x80This and higher values are reserved for custom (driver defined) buffer types.

Table 3-3. Buffer Flags

V4L2_BUF_FLAG_MAPPED0x0001The buffer resides in device memory and has been mapped into the application's address space, see Section 3.2> for details. Drivers set or clear this flag when the VIDIOC_QUERYBUF, VIDIOC_QBUF or VIDIOC_DQBUF ioctl is called. Set by the driver.
V4L2_BUF_FLAG_QUEUED0x0002Internally drivers maintain two buffer queues, an incoming and outgoing queue. When this flag is set, the buffer is currently on the incoming queue. It automatically moves to the outgoing queue after the buffer has been filled (capture devices) or displayed (output devices). Drivers set or clear this flag when the VIDIOC_QUERYBUF ioctl is called. After (successful) calling the VIDIOC_QBUF ioctl it is always set and after VIDIOC_DQBUF always cleared.
V4L2_BUF_FLAG_DONE0x0004When this flag is set, the buffer is currently on the outgoing queue, ready to be dequeued from the driver. Drivers set or clear this flag when the VIDIOC_QUERYBUF ioctl is called. After calling the VIDIOC_QBUF or VIDIOC_DQBUF it is always cleared. Of course a buffer cannot be on both queues at the same time, the V4L2_BUF_FLAG_QUEUED and V4L2_BUF_FLAG_DONE flag are mutually exclusive. They can be both cleared however, then the buffer is in "dequeued" state, in the application domain to say so.
V4L2_BUF_FLAG_KEYFRAME0x0008Drivers set or clear this flag when calling the VIDIOC_DQBUF ioctl. It may be set by video capture devices when the buffer contains a compressed image which is a key frame (or field), i.e. can be decompressed on its own.
V4L2_BUF_FLAG_PFRAME0x0010Similar to V4L2_BUF_FLAG_KEYFRAME this flags predicted frames or fields which contain only differences to a previous key frame.
V4L2_BUF_FLAG_BFRAME0x0020Similar to V4L2_BUF_FLAG_PFRAME this is a bidirectional predicted frame or field. [ooc tbd]
V4L2_BUF_FLAG_TIMECODE0x0100The timecode field is valid. Drivers set or clear this flag when the VIDIOC_DQBUF ioctl is called.
V4L2_BUF_FLAG_INPUT0x0200The input field is valid. Applications set or clear this flag before calling the VIDIOC_QBUF ioctl.

Table 3-4. enum v4l2_memory

V4L2_MEMORY_MMAP1The buffer is used for memory mapping I/O.
V4L2_MEMORY_USERPTR2The buffer is used for user pointer I/O.
V4L2_MEMORY_OVERLAY3[to do]

3.5.1. Timecodes

The v4l2_timecode structure is designed to hold a SMPTE12M> or similar timecode. (struct timeval timestamps are stored in struct v4l2_buffer field timestamp.)

Table 3-5. struct v4l2_timecode

__u32typeFrame rate the timecodes are based on, see Table 3-6>.
__u32flagsTimecode flags, see Table 3-7>.
__u8framesFrame count, 0 ... 23/24/29/49/59, depending on the type of timecode.
__u8secondsSeconds count, 0 ... 59. This is a binary, not BCD number.
__u8minutesMinutes count, 0 ... 59. This is a binary, not BCD number.
__u8hoursHours count, 0 ... 29. This is a binary, not BCD number.
__u8userbits[4]The "user group" bits from the timecode.

Table 3-6. Timecode Types

V4L2_TC_TYPE_24FPS124 frames per second, i. e. film.
V4L2_TC_TYPE_25FPS225 frames per second, i.e. PAL or SECAM video.
V4L2_TC_TYPE_30FPS330 frames per second, i.e. NTSC video.
V4L2_TC_TYPE_50FPS4 
V4L2_TC_TYPE_60FPS5 

Table 3-7. Timecode Flags

V4L2_TC_FLAG_DROPFRAME0x0001Indicates "drop frame" semantics for counting frames in 29.97 fps material. When set, frame numbers 0 and 1 at the start of each minute, except minutes 0, 10, 20, 30, 40, 50 are omitted from the count.
V4L2_TC_FLAG_COLORFRAME0x0002The "color frame" flag.
V4L2_TC_USERBITS_field0x000CField mask for the "binary group flags".
V4L2_TC_USERBITS_USERDEFINED0x0000Unspecified format.
V4L2_TC_USERBITS_8BITCHARS0x00088-bit ISO characters.

3.6. Field Order

We have to distinguish between progressive and interlaced video. Progressive video transmits all lines of a video image sequentially. Interlaced video divides an image into two fields, containing only the odd and even lines of the image, respectively. Alternating the so called odd and even field are transmitted, and due to a small delay between fields a cathode ray TV displays the lines interleaved, yielding the original frame. This curious technique was invented because at refresh rates similar to film the image would fade out too quickly. Transmitting fields reduces the flicker without the necessity of doubling the frame rate and with it the bandwidth required for each channel.

It is important to understand a video camera does not expose one frame at a time, merely transmitting the frames separated into fields. The fields are in fact captured at two different instances in time. An object on screen may well move between one field and the next. For applications analysing motion it is of paramount importance to recognize which field of a frame is older, the temporal order.

When the driver provides or accepts images field by field rather than interleaved, it is also important applications understand how the fields combine to frames. We distinguish between top and bottom fields, the spatial order: The first line of the top field is the first line of an interlaced frame, the first line of the bottom field is the second line of that frame.

However because fields were captured one after the other, arguing whether a frame commences with the top or bottom field is pointless. Any two successive top and bottom, or bottom and top fields yield a valid frame. Only when the source was progressive to begin with, e. g. when transferring film to video, two fields may come from the same frame, creating a natural order.

Counter to intuition the top field is not necessarily the older field. Whether the older field contains the top or bottom lines is a convention determined by the video standard. Hence the distinction between temporal and spatial order of fields. The diagrams below should make this clearer.

All video capture and output devices must report the current field order. Some drivers may permit the selection of a different order, to this end applications initialize the field field of struct v4l2_pix_format before calling the VIDIOC_S_FMT ioctl. If this is not desired it should have the value V4L2_FIELD_ANY (0).

Table 3-8. enum v4l2_field

V4L2_FIELD_ANY0Applications request this field order when any one of the V4L2_FIELD_NONE, V4L2_FIELD_TOP, V4L2_FIELD_BOTTOM, or V4L2_FIELD_INTERLACED formats is acceptable. Drivers choose depending on hardware capabilities or e. g. the requested image size, and return the actual field order. struct v4l2_buffer field can never be V4L2_FIELD_ANY.
V4L2_FIELD_NONE1Images are in progressive format, not interlaced. The driver may also indicate this order when it cannot distinguish between V4L2_FIELD_TOP and V4L2_FIELD_BOTTOM.
V4L2_FIELD_TOP2Images consist of the top field only.
V4L2_FIELD_BOTTOM3Images consist of the bottom field only. Applications may wish to prevent a device from capturing interlaced images because they will have "comb" or "feathering" artefacts around moving objects.
V4L2_FIELD_INTERLACED4Images contain both fields, interleaved line by line. The temporal order of the fields (whether the top or bottom field is first transmitted) depends on the current video standard. M/NTSC transmits the bottom field first, all other standards the top field first.
V4L2_FIELD_SEQ_TB5Images contain both fields, the top field lines are stored first in memory, immediately followed by the bottom field lines. Fields are always stored in temporal order, the older one first in memory. Image sizes refer to the frame, not fields.
V4L2_FIELD_SEQ_BT6Images contain both fields, the bottom field lines are stored first in memory, immediately followed by the top field lines. Fields are always stored in temporal order, the older one first in memory. Image sizes refer to the frame, not fields.
V4L2_FIELD_ALTERNATE7The two fields of a frame are passed in separate buffers, in temporal order, i. e. the older one first. To indicate the field parity (whether the current field is a top or bottom field) the driver or application, depending on data direction, must set struct v4l2_buffer field to V4L2_FIELD_TOP or V4L2_FIELD_BOTTOM. Any two successive fields pair to build a frame. If fields are successive, without any dropped fields between them (fields can drop individually), can be determined from the struct v4l2_buffer sequence field. Image sizes refer to the frame, not fields. This format cannot be selected when using the read/write I/O method.

Figure 3-1. Field Order, Top Field First Transmitted

Figure 3-2. Field Order, Bottom Field First Transmitted


Chapter 4. Device Types

4.1. Video Capture Interface

Video capture devices sample an analog video signal and store the digitized images in memory. Today nearly all devices can capture at full 25 or 30 frames/second. With this interface applications can control the capture process and move images from the driver into user space.

Conventionally V4L2 video capture devices are accessed through character device special files named /dev/video and /dev/video0 to /dev/video63 with major number 81 and minor numbers 0 to 63. /dev/video is typically a symbolic link to the preferred video capture device.


4.1.1. Querying Capabilities

Devices supporting the video capture interface set the V4L2_CAP_VIDEO_CAPTURE flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP. As secondary device functions they may also support the video overlay (V4L2_CAP_VIDEO_OVERLAY) and the raw VBI capture (V4L2_CAP_VBI_CAPTURE) interface. At least one of the read/write or streaming I/O methods must be supported. Tuners and audio inputs are optional.


4.1.2. Supplemental Functions

Video capture devices shall support audio input, tuner, controls, cropping and scaling and streaming parameter ioctls as needed. The video input and video standard ioctls must be supported by all video capture devices.


4.1.3. Image Format Negotiation

The result of a capture operation is determined by cropping and image format parameters. The former select an area of the video picture to capture, the latter how images are stored in memory, i. e. in RGB or YUV format, the number of bits per pixel or width and height. Together they also define how images are scaled in the process.

As usual these parameters are not reset at open() time to permit Unix tool chains, programming a device and then reading from it as if it was a plain file. Well written V4L2 applications ensure they really get what they want, including cropping and scaling.

Cropping initialization at minimum requires to reset the parameters to defaults. An example is given in Section 1.10>.

To query the current image format applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_VIDEO_CAPTURE and call the VIDIOC_G_FMT ioctl with a pointer to this structure. Drivers fill the struct v4l2_pix_format pix member of the fmt union.

To request different parameters applications set the type field of a struct v4l2_format as above and initialize all fields of the struct v4l2_pix_format vbi member of the fmt union, or better just modify the results of VIDIOC_G_FMT, and call the VIDIOC_S_FMT ioctl with a pointer to this structure. Drivers may adjust the parameters and finally return the actual parameters as VIDIOC_G_FMT does.

Like VIDIOC_S_FMT the VIDIOC_TRY_FMT ioctl can be used to learn about hardware limitations without disabling I/O or possibly time consuming hardware preparations.

The contents of struct v4l2_pix_format are discussed in Chapter 2>. See also the specification of the VIDIOC_G_FMT, VIDIOC_S_FMT and VIDIOC_TRY_FMT ioctls for details. Video capture devices must implement both the VIDIOC_G_FMT and VIDIOC_S_FMT ioctl, even if VIDIOC_S_FMT ignores all requests and always returns default parameters as VIDIOC_G_FMT does. VIDIOC_TRY_FMT is optional.


4.1.4. Reading Images

A video capture device may support the read() function and/or streaming (memory mapping or user pointer) I/O. See Chapter 3> for details.


4.2. Video Overlay Interface

Also known as Frame Buffer Overlay or Previewing

Video overlay devices have the ability to genlock (TV-)video into the (VGA-)video signal of a graphics card, or to store captured images directly in video memory of a graphics card, typically with clipping. This can be considerable more efficient than capturing images and displaying them by other means. In the old days when only nuclear power plants needed cooling towers this used to be the only way to put live video into a window.

Video overlay devices are accessed through the same character special files as video capture devices. Note the default function of a /dev/video device is video capturing. The overlay function is only available after calling the VIDIOC_S_FMT ioctl.

The driver may support simultaneous overlay and capturing using the read/write and streaming I/O methods. If so, operation at the nominal frame rate of the video standard is not guaranteed. Frames may be directed away from overlay to capture, or one field may be used for overlay and the other for capture if the capture parameters permit this.

Applications should use different file descriptors for capturing and overlay. This must be supported by all drivers capable of simultaneous capturing and overlay. Optionally these drivers may also permit capturing and overlay with a single file descriptor for compatibility with V4L and earlier versions of V4L2.[21]


4.2.1. Querying Capabilities

Devices supporting the video overlay interface set the V4L2_CAP_VIDEO_OVERLAY flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP. The overlay I/O method specified below must be supported. Tuners and audio inputs are optional.


4.2.2. Supplemental Functions

Video overlay devices shall support audio input, tuner, controls, cropping and scaling and streaming parameter ioctls as needed. The video input and video standard ioctls must be supported by all video overlay devices.


4.2.3. Setup

Before overlay can commence applications must program the driver with frame buffer parameters, namely the address and size of the frame buffer and the image format, for example RGB 5:6:5. The VIDIOC_G_FBUF and VIDIOC_S_FBUF ioctls are available to get and set these parameters, respectively. The VIDIOC_S_FBUF ioctl is privileged because it allows to set up DMA into physical memory, bypassing the memory protection mechanisms of the kernel. Only the superuser can change the frame buffer address and size. Users are not supposed to run TV applications as root or with SUID bit set. A small helper application with suitable privileges should query the graphics system and program the V4L2 driver at the appropriate time.

Some devices add the video overlay to the output signal of the graphics card. In this case the frame buffer is not modified by the video device, and the frame buffer address and pixel format are not needed by the driver. The VIDIOC_S_FBUF ioctl is not privileged. An application can check for this type of device by calling the VIDIOC_G_FBUF ioctl.

A driver may support any (or none) of three clipping methods:

  1. Chroma-keying displays the overlaid image only where pixels in the primary graphics surface assume a certain color.

  2. A bitmap can be specified where each bit corresponds to a pixel in the overlaid image. When the bit is set, the corresponding video pixel is displayed, otherwise a pixel of the graphics surface.

  3. A list of clipping rectangles can be specified. In these regions no video is displayed, so the graphics surface can be seen here.

When simultaneous capturing and overlay is supported and the hardware prohibits different image and frame buffer formats, the format requested first takes precedence. The attempt to capture (VIDIOC_S_FMT) or overlay (VIDIOC_S_FBUF) may fail with an EBUSY error code or return accordingly modified parameters..


4.2.4. Overlay Window

The overlaid image is determined by cropping and overlay window parameters. The former select an area of the video picture to capture, the latter how images are overlaid and clipped. Cropping initialization at minimum requires to reset the parameters to defaults. An example is given in Section 1.10>.

The overlay window is described by a struct v4l2_window. It defines the size of the image, its position over the graphics surface and the clipping to be applied. To get the current parameters applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_VIDEO_OVERLAY and call the VIDIOC_G_FMT ioctl. The driver fills the v4l2_window substructure named win. Retrieving a previously programmed clipping list or bitmap is not possible.

To program the overlay window applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_VIDEO_OVERLAY, initialize the win substructure and call the VIDIOC_S_FMT ioctl. The driver adjusts the parameters against hardware limits and returns the actual parameters as VIDIOC_G_FMT does. Like VIDIOC_S_FMT, the VIDIOC_TRY_FMT ioctl can be used to learn about driver capabilities without actually changing driver state. Unlike VIDIOC_S_FMT this also works after the overlay has been enabled.

The scaling factor of the overlaid image is implied by the width and height given in struct v4l2_window and the size of the cropping rectangle. For more information see Section 1.10>.

When simultaneous capturing and overlay is supported and the hardware prohibits different image and window sizes, the size requested first takes precedence. The attempt to capture or overlay as well (VIDIOC_S_FMT) may fail with an EBUSY error code or return accordingly modified parameters.

Table 4-1. struct v4l2_window

struct v4l2_rectwSize and position of the window relative to the top, left corner of the frame buffer defined with VIDIOC_S_FBUF. The window can extend the frame buffer width and height, the x and y coordinates can be negative, and it can lie completely outside the frame buffer. The driver clips the window accordingly, or if that is not possible, modifies its size and/or position.
enum v4l2_fieldfieldApplications set this field to determine which video field shall be overlaid, typically one of V4L2_FIELD_ANY (0), V4L2_FIELD_TOP, V4L2_FIELD_BOTTOM or V4L2_FIELD_INTERLACED. Drivers may have to choose a different field order and return the actual setting here.
__u32chromakeyWhen chroma-keying has been negotiated with VIDIOC_S_FBUF applications set this field to the desired host order RGB32 value for the chroma key. [host order? alpha channel?]
struct v4l2_clip *clipsWhen chroma-keying has not been negotiated and VIDIOC_G_FBUF indicated this capability, applications can set this field to point to the first in a list of clipping rectangles.
Like the window coordinates w, clipping rectangles are defined relative to the top, left corner of the frame buffer. However clipping rectangles must not extend the frame buffer width and height, and they must not overlap. If possible applications should merge adjacent rectangles. Whether this must create x-y or y-x bands, or the order of rectangles, is not defined. When clip lists are not supported the driver ignores this field. Its contents after calling VIDIOC_S_FMT are undefined.
__u32clipcountWhen the application set the clips field, this field must contain the number of clipping rectangles in the list. When clip lists are not supported the driver ignores this field, its contents after calling VIDIOC_S_FMT are undefined. When clip lists are supported but no clipping is desired this field must be set to zero.
void *bitmapWhen chroma-keying has not been negotiated and VIDIOC_G_FBUF indicated this capability, applications can set this field to point to a clipping bit mask.

It must be of the same size as the window, w.width and w.height. Each bit corresponds to a pixel in the overlaid image, which is displayed only when the bit is set. Pixel coordinates translate to bits like:

((__u8 *) bitmap)[w.width * y + x / 8] & (1 << (x & 7))

where 0 ≤ x < w.width and 0 ≤ y <w.height.a

When a clipping bit mask is not supported the driver ignores this field, its contents after calling VIDIOC_S_FMT are undefined. When a bit mask is supported but no clipping is desired this field must be set to NULL.

Applications need not create a clip list or bit mask. When they pass both, or despite negotiating chroma-keying, the results are undefined. Regardless of the chosen method, the clipping abilities of the hardware may be limited in quantity or quality. The results when these limits are exceeded are undefined.b

Notes:
a. Should we require w.width to be a multiple of eight?
b. When the image is written into frame buffer memory it will be undesirable if the driver clips out less pixels than expected, because the application and graphics system are not aware these regions need to be refreshed. The driver should clip out more pixels or not write the image at all.

Table 4-2. struct v4l2_clip[22]

struct v4l2_rectcCoordinates of the clipping rectangle, relative to the top, left corner of the frame buffer. Only window pixels outside all clipping rectangles are displayed.
struct v4l2_clip *nextPointer to the next clipping rectangle in the list, NULL when this is the last rectangle.

Table 4-3. struct v4l2_rect

__s32leftHorizontal offset of the top, left corner of the rectangle, in pixels.
__s32topVertical offset of the top, left corner of the rectangle, in pixels. Offsets increase to the right and down.
__s32widthWidth of the rectangle, in pixels.
__s32heightHeight of the rectangle, in pixels. Width and height cannot be negative, the fields are signed for hysterical reasons.

4.2.5. Enabling Overlay

To start or stop the frame buffer overlay applications call the VIDIOC_OVERLAY ioctl.


4.3. Video Output Interface

Video output devices encode stills or image sequences as analog video signal. With this interface applications can control the encoding process and move images from user space to the driver.

Conventionally V4L2 video output devices are accessed through character device special files named /dev/vout and /dev/vout0 to /dev/voutN. No minor numbers were recommended yet. /dev/vout is typically a symbolic link to the preferred video output device.


4.3.1. Querying Capabilities

Devices supporting the video output interface set the V4L2_CAP_VIDEO_OUTPUT flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP. As secondary device functions they may also support the raw VBI output (V4L2_CAP_VBI_OUTPUT) interface. At least one of the read/write or streaming I/O methods must be supported. Modulators and audio outputs are optional.


4.3.2. Supplemental Functions

Video output devices shall support audio output, modulator, controls, cropping and scaling and streaming parameter ioctls as needed. The video output and video standard ioctls must be supported by all video output devices.


4.3.3. Image Format Negotiation

The output is determined by cropping and image format parameters. The former select an area of the video picture where the image will appear, the latter how images are stored in memory, i. e. in RGB or YUV format, the number of bits per pixel or width and height. Together they also define how images are scaled in the process.

As usual these parameters are not reset at open() time to permit Unix tool chains, programming a device and then writing to it as if it was a plain file. Well written V4L2 applications ensure they really get what they want, including cropping and scaling.

Cropping initialization at minimum requires to reset the parameters to defaults. An example is given in Section 1.10>.

To query the current image format applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_VIDEO_OUTPUT and call the VIDIOC_G_FMT ioctl with a pointer to this structure. Drivers fill the struct v4l2_pix_format pix member of the fmt union.

To request different parameters applications set the type field of a struct v4l2_format as above and initialize all fields of the struct v4l2_pix_format vbi member of the fmt union, or better just modify the results of VIDIOC_G_FMT, and call the VIDIOC_S_FMT ioctl with a pointer to this structure. Drivers may adjust the parameters and finally return the actual parameters as VIDIOC_G_FMT does.

Like VIDIOC_S_FMT the VIDIOC_TRY_FMT ioctl can be used to learn about hardware limitations without disabling I/O or possibly time consuming hardware preparations.

The contents of struct v4l2_pix_format are discussed in Chapter 2>. See also the specification of the VIDIOC_G_FMT, VIDIOC_S_FMT and VIDIOC_TRY_FMT ioctls for details. Video output devices must implement both the VIDIOC_G_FMT and VIDIOC_S_FMT ioctl, even if VIDIOC_S_FMT ignores all requests and always returns default parameters as VIDIOC_G_FMT does. VIDIOC_TRY_FMT is optional.


4.3.4. Writing Images

A video output device may support the write() function and/or streaming (memory mapping or user pointer) I/O. See Chapter 3> for details.


4.4. Codec Interface

Suspended: This interface has been be suspended from the V4L2 API implemented in Linux 2.6 until we have more experience with codec device interfaces.

A V4L2 codec can compress, decompress, transform, or otherwise convert video data from one format into another format, in memory. Applications send data to be converted to the driver through the write() call, and receive the converted data through the read() call. For efficiency, a driver may also support streaming I/O.

[to do]


4.5. Effect Devices Interface

Suspended: This interface has been be suspended from the V4L2 API implemented in Linux 2.6 until we have more experience with effect device interfaces.

A V4L2 video effect device can do image effects, filtering, or combine two or more images or image streams. For example video transitions or wipes. Applications send data to be processed and receive the result data either with read() and write() functions, or through the streaming I/O mechanism.

[to do]


4.6. Raw VBI Data Interface

VBI is an abbreviation of Vertical Blanking Interval, a gap in the sequence of lines of an analog video signal. During VBI no picture information is transmitted, allowing some time while the electron beam of a cathode ray tube TV returns to the top of the screen. Using an oscilloscope you will find here the vertical synchronization pulses and short data packages ASK modulated[23] onto the video signal. These are transmissions of services such as Teletext or Closed Caption.

Subject of this interface type is raw VBI data, as sampled off a video signal, or to be added to a signal for output. The data format is similar to uncompressed video images, a number of lines times a number of samples per line, we call this a VBI image.

Conventionally V4L2 VBI devices are accessed through character device special files named /dev/vbi and /dev/vbi0 to /dev/vbi15 with major number 81 and minor numbers 224 to 239. /dev/vbi is typically a symbolic link to the preferred VBI device. This convention applies to both input and output devices.

To address the problems of finding related video and VBI devices VBI capturing is also available as device function under /dev/video, VBI output under /dev/vout. To capture raw VBI data from these devices applications must call the VIDIOC_S_FMT ioctl. Accessed as /dev/vbi, raw VBI capturing or output is the default device function.


4.6.1. Querying Capabilities

Devices supporting the raw VBI capturing or output API set the V4L2_CAP_VBI_CAPTURE or V4L2_CAP_VBI_OUTPUT flags, respectively, in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP. At least one of the read/write, streaming or asynchronous I/O methods must be supported. VBI devices may or may not have a tuner or modulator.


4.6.2. Supplemental Functions

VBI devices shall support video input or output, tuner or modulator, and controls ioctls as needed. The video standard ioctls provide information vital to program a VBI device, therefore must be supported.


4.6.3. Raw VBI Format Negotiation

Raw VBI sampling abilities can vary, in particular the sampling frequency. To properly interpret the data V4L2 specifies an ioctl to query the sampling parameters. Moreover, to allow for some flexibility applications can also suggest different parameters.

As usual these parameters are not reset at open() time to permit Unix tool chains, programming a device and then reading from it as if it was a plain file. Well written V4L2 applications should always ensure they really get what they want, requesting reasonable parameters and then checking if the actual parameters are suitable.

To query the current raw VBI capture parameters applications set the type field of a struct v4l2_format to V4L2_BUF_TYPE_VBI_CAPTURE or V4L2_BUF_TYPE_VBI_OUTPUT, and call the VIDIOC_G_FMT ioctl with a pointer to this structure. Drivers fill the struct v4l2_vbi_format vbi member of the fmt union.

To request different parameters applications set the type field of a struct v4l2_format as above and initialize all fields of the struct v4l2_vbi_format vbi member of the fmt union, or better just modify the results of VIDIOC-G-FMT, and call the VIDIOC_S_FMT ioctl with a pointer to this structure. Drivers return an EINVAL error code only when the given parameters are ambiguous, otherwise they modify the parameters according to the hardware capabilites and return the actual parameters. When the driver allocates resources at this point, it may return an EBUSY error code to indicate the returned parameters are valid but the required resources are currently not available. That may happen for instance when the video and VBI areas to capture would overlap, or when the driver supports multiple opens and another process already requested VBI capturing or output. Anyway, applications must expect other resource allocation points which may return EBUSY, at the VIDIOC_STREAMON ioctl and the first read(), write() and select() call.

VBI devices must implement both the VIDIOC_G_FMT and VIDIOC_S_FMT ioctl, even if VIDIOC_S_FMT ignores all requests and always returns default parameters as VIDIOC_G_FMT does. VIDIOC_TRY_FMT is optional.

Table 4-4. struct v4l2_vbi_format

__u32sampling_rateSamples per second, i. e. unit 1 Hz.
__u32offset

Horizontal offset of the VBI image, relative to the leading edge of the line synchronization pulse and counted in samples: The first sample in the VBI image will be located offset / sampling_rate seconds following the leading edge. See also Figure 4-1>.

__u32samples_per_line 
__u32sample_format

Defines the sample format as in Chapter 2>, a four-character-code.a Usually this is V4L2_PIX_FMT_GREY, i. e. each sample consists of 8 bits with lower values oriented towards the black level. Do not assume any other correlation of values with the signal level. For example, the MSB does not necessarily indicate if the signal is 'high' or 'low' because 128 may not be the mean value of the signal. Drivers shall not convert the sample format by software.

__u32start[2]This is the scanning system line number associated with the first line of the VBI image, of the first and the second field respectively. See Figure 4-2> and Figure 4-3> for valid values. VBI input drivers can return start values 0 if the hardware cannot reliable identify scanning lines, VBI acquisition may not require this information.
__u32count[2]The number of lines in the first and second field image, respectively.

Drivers should be as flexibility as possible. For example, it may be possible to extend or move the VBI capture window down to the picture area, implementing a 'full field mode' to capture data service transmissions embedded in the picture.

An application can set the first or second count value to zero if no data is required from the respective field; count[1] if the scanning system is progressive, i. e. not interlaced. The corresponding start value shall be ignored by the application and driver. Anyway, drivers may not support single field capturing and return both count values non-zero.

Both count values set to zero, or line numbers outside the bounds depicted in Figure 4-2> and Figure 4-3>, or a field image covering lines of two fields, are invalid and shall not be returned by the driver.

To initialize the start and count fields, applications must first determine the current video standard selection. The v4l2_std_id or the framelines field of struct v4l2_standard can be evaluated for this purpose.

__u32flagsSee Table 4-5> below. Currently only drivers set flags, applications must set this field to zero.
__u32reserved[2]This array is reserved for future extensions. Drivers and applications must set it to zero.
Notes:
a. A few devices may be unable to sample VBI data at all but can extend the video capture window to the VBI region.

Table 4-5. Raw VBI Format Flags

V4L2_VBI_UNSYNC0x0001

This flag indicates hardware which does not properly distinguish between fields. Normally the VBI image stores the first field (lower scanning line numbers) first in memory. This may be a top or bottom field depending on the video standard. When this flag is set the first or second field may be stored first, however the fields are still in correct temporal order with the older field first in memory.a

V4L2_VBI_INTERLACED0x0002By default the two field images will be passed sequentially; all lines of the first field followed by all lines of the second field (compare Section 3.6> V4L2_FIELD_SEQ_TB and V4L2_FIELD_SEQ_BT, whether the top or bottom field is first in memory depends on the video standard). When this flag is set, the two fields are interlaced (cf. V4L2_FIELD_INTERLACED). The first line of the first field followed by the first line of the second field, then the two second lines, and so on. Such a layout may be necessary when the hardware has been programmed to capture or output interlaced video images and is unable to separate the fields for VBI capturing at the same time. For simplicity setting this flag implies that both count values are equal and non-zero.
Notes:
a. Most VBI services transmit on both fields, but some have different semantics depending on the field number. These cannot be reliable decoded or encoded when V4L2_VBI_UNSYNC is set.

Figure 4-1. Line synchronization

Figure 4-2. ITU-R 525 line numbering (M/NTSC and M/PAL)

(1) For the purpose of this specification field 2 starts in line 264 and not 263.5 because half line capturing is not supported.

Figure 4-3. ITU-R 625 line numbering

(1) For the purpose of this specification field 2 starts in line 314 and not 313.5 because half line capturing is not supported.

Remember the VBI image format depends on the selected video standard, therefore the application must choose a new standard or query the current standard first. Attempts to read or write data ahead of format negotiation, or after switching the video standard which may invalidate the negotiated VBI parameters, should be refused by the driver. A format change during active I/O is not permitted.


4.6.4. Reading and writing VBI images

To assure synchronization with the field number and easier implementation, the smallest unit of data passed at a time is one frame, consisting of two fields of VBI images immediately following in memory.

The total size of a frame computes as follows:

(count[0] + count[1]) *
samples_per_line * sample size in bytes

The sample size is most likely always one byte, applications must check the sample_format field though, to function properly with other drivers.

A VBI device may support read/write and/or streaming (memory mapping or user pointer) I/O. The latter bears the possibility of synchronizing video and VBI data by using buffer timestamps.

Remember the VIDIOC_STREAMON ioctl and the first read(), write() and select() call can be resource allocation points returning an EBUSY error code if the required hardware resources are temporarily unavailable, for example the device is already in use by another process.


4.7. Sliced VBI Data Interface

VBI stands for Vertical Blanking Interval, a gap in the sequence of lines of an analog video signal. During VBI no picture information is transmitted, allowing some time while the electron beam of a cathode ray tube TV returns to the top of the screen.

Sliced VBI devices use hardware to demodulate data transmitted in the VBI. V4L2 drivers shall not do this by software, see also the raw VBI interface. The data is passed as short packets of fixed size, covering one scan line each. The number of packets per video frame is variable.

Sliced VBI capture and output devices are accessed through the same character special files as raw VBI devices. When a driver supports both interfaces, the default function of a /dev/vbi device is raw VBI capturing or output, and the sliced VBI function is only available after calling the VIDIOC_S_FMT ioctl as defined below. Different file descriptors must be used to pass raw and sliced VBI data simultaneously, if this is supported by the driver.


4.7.1. Querying Capabilities

Devices supporting the sliced VBI capturing or output API set the V4L2_CAP_SLICED_VBI_CAPTURE or V4L2_CAP_SLICED_VBI_OUTPUT flag respectively, in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl. At least one of the read/write, streaming or asynchronous I/O methods must be supported. Sliced VBI devices may have a tuner or modulator.


4.7.2. Supplemental Functions

Sliced VBI devices shall support video input or output and tuner or modulator ioctls if they have these capabilities, and they may support control ioctls. The video standard ioctls provide information vital to program a sliced VBI device, therefore must be supported.


4.7.3. Sliced VBI Format Negotiation

To find out which data services are supported by the hardware applications can call the VIDIOC_G_SLICED_VBI_CAP ioctl. All drivers implementing the sliced VBI interface must support this ioctl. The results may differ from those of the VIDIOC_S_FMT ioctl when the number of VBI lines the hardware can capture or output per frame, or the number of services it can identify on a given line are limited. For example on PAL line 16 the hardware may be able to look for a VPS or Teletext signal, but not both at the same time.

To determine the currently selected services applications set the type field of struct v4l2_format to V4L2_BUF_TYPE_SLICED_VBI_CAPTURE or V4L2_BUF_TYPE_SLICED_VBI_OUTPUT, and the VIDIOC_G_FMT ioctl fills the fmt.sliced member, a struct v4l2_sliced_vbi_format.

Applications can request different parameters by initializing or modifying the fmt.sliced member and calling the VIDIOC_S_FMT ioctl with a pointer to the v4l2_format structure.

The sliced VBI API is more complicated than the raw VBI API because the hardware must be told which VBI service to expect on each scan line. Not all services may be supported by the hardware on all lines (this is especially true for VBI output where Teletext is often unsupported and other services can only be inserted in one specific line). In many cases, however, it is sufficient to just set the service_set field to the required services and let the driver fill the service_lines array according to hardware capabilities. Only if more precise control is needed should the programmer set the service_lines array explicitly.

The VIDIOC_S_FMT ioctl returns an EINVAL error code only when the given parameters are ambiguous, otherwise it modifies the parameters according to hardware capabilities. When the driver allocates resources at this point, it may return an EBUSY error code if the required resources are temporarily unavailable. Other resource allocation points which may return EBUSY can be the VIDIOC_STREAMON ioctl and the first read(), write() and select() call.

Table 4-6. struct v4l2_sliced_vbi_format

__u32service_set

If service_set is non-zero when passed with VIDIOC_S_FMT or VIDIOC_TRY_FMT, the service_lines array will be filled by the driver according to the services specified in this field. For example, if service_set is initialized with V4L2_SLICED_TELETEXT_B | V4L2_SLICED_WSS_625, a driver for the cx25840 video decoder sets lines 7-22 of both fieldsa to V4L2_SLICED_TELETEXT_B and line 23 of the first field to V4L2_SLICED_WSS_625. If service_set is set to zero, then the values of service_lines will be used instead.

On return the driver sets this field to the union of all elements of the returned service_lines array. It may contain less services than requested, perhaps just one, if the hardware cannot handle more services simultaneously. It may be empty (zero) if none of the requested services are supported by the hardware.

__u16service_lines[2][24]

Applications initialize this array with sets of data services the driver shall look for or insert on the respective scan line. Subject to hardware capabilities drivers return the requested set, a subset, which may be just a single service, or an empty set. When the hardware cannot handle multiple services on the same line the driver shall choose one. No assumptions can be made on which service the driver chooses.

Data services are defined in Table 4-7>. Array indices map to ITU-R line numbers (see also Figure 4-2> and Figure 4-3>) as follows:

  Element525 line systems625 line systems
  service_lines[0][1]11
  service_lines[0][23]2323
  service_lines[1][1]264314
  service_lines[1][23]286336
  Drivers must set service_lines[0][0] and service_lines[1][0] to zero.
__u32io_sizeMaximum number of bytes passed by one read() or write() call, and the buffer size in bytes for the VIDIOC_QBUF and VIDIOC_DQBUF ioctl. Drivers set this field to the size of struct v4l2_sliced_vbi_data times the number of non-zero elements in the returned service_lines array (that is the number of lines potentially carrying data).
__u32reserved[2]This array is reserved for future extensions. Applications and drivers must set it to zero.
Notes:
a. According to ETS 300 706 lines 6-22 of the first field and lines 5-22 of the second field may carry Teletext data.

Table 4-7. Sliced VBI services

SymbolValueReferenceLines, usuallyPayload
V4L2_SLICED_TELETEXT_B (Teletext System B)0x0001TELETEXT>PAL/SECAM line 7-22, 320-335 (second field 7-22)Last 42 of the 45 byte Teletext packet, that is without clock run-in and framing code, lsb first transmitted.
V4L2_SLICED_VPS0x0400VPS>PAL line 16Byte number 3 to 15 according to Figure 9 of ETS 300 231, lsb first transmitted.
V4L2_SLICED_CAPTION_5250x1000EIA608>NTSC line 21, 284 (second field 21)Two bytes in transmission order, including parity bit, lsb first transmitted.
V4L2_SLICED_WSS_6250x4000WSS>PAL/SECAM line 23
Byte 0 1
msb lsb msb lsb
Bit 7 6 5 4 3 2 1 0 x x 13 12 11 10 9
V4L2_SLICED_VBI_5250x1000Set of services applicable to 525 line systems.
V4L2_SLICED_VBI_6250x4401Set of services applicable to 625 line systems.

Drivers may return an EINVAL error code when applications attempt to read or write data without prior format negotiation, after switching the video standard (which may invalidate the negotiated VBI parameters) and after switching the video input (which may change the video standard as a side effect). The VIDIOC_S_FMT ioctl may return an EBUSY error code when applications attempt to change the format while i/o is in progress (between a VIDIOC_STREAMON and VIDIOC_STREAMOFF call, and after the first read() or write() call).


4.7.4. Reading and writing sliced VBI data

A single read() or write() call must pass all data belonging to one video frame. That is an array of v4l2_sliced_vbi_data structures with one or more elements and a total size not exceeding io_size bytes. Likewise in streaming I/O mode one buffer of io_size bytes must contain data of one video frame. The id of unused v4l2_sliced_vbi_data elements must be zero.

Table 4-8. struct v4l2_sliced_vbi_data

__u32idA flag from Table 2> identifying the type of data in this packet. Only a single bit must be set. When the id of a captured packet is zero, the packet is empty and the contents of other fields are undefined. Applications shall ignore empty packets. When the id of a packet for output is zero the contents of the data field are undefined and the driver must no longer insert data on the requested field and line.
__u32fieldThe video field number this data has been captured from, or shall be inserted at. 0 for the first field, 1 for the second field.
__u32lineThe field (as opposed to frame) line number this data has been captured from, or shall be inserted at. See Figure 4-2> and Figure 4-3> for valid values. Sliced VBI capture devices can set the line number of all packets to 0 if the hardware cannot reliably identify scan lines. The field number must always be valid.
__u32reservedThis field is reserved for future extensions. Applications and drivers must set it to zero.
__u8data[48]The packet payload. See Table 2> for the contents and number of bytes passed for each data type. The contents of padding bytes at the end of this array are undefined, drivers and applications shall ignore them.

Packets are always passed in ascending line number order, without duplicate line numbers. The write() function and the VIDIOC_QBUF ioctl must return an EINVAL error code when applications violate this rule. They must also return an EINVAL error code when applications pass an incorrect field or line number, or a combination of field, line and id which has not been negotiated with the VIDIOC_G_FMT or VIDIOC_S_FMT ioctl. When the line numbers are unknown the driver must pass the packets in transmitted order. The driver can insert empty packets with id set to zero anywhere in the packet array.

To assure synchronization and to distinguish from frame dropping, when a captured frame does not carry any of the requested data services drivers must pass one or more empty packets. When an application fails to pass VBI data in time for output, the driver must output the last VPS and WSS packet again, and disable the output of Closed Caption and Teletext data, or output data which is ignored by Closed Caption and Teletext decoders.

A sliced VBI device may support read/write and/or streaming (memory mapping and/or user pointer) I/O. The latter bears the possibility of synchronizing video and VBI data by using buffer timestamps.


4.8. Teletext Interface

This interface aims at devices receiving and demodulating Teletext data [TELETEXT>], evaluating the Teletext packages and storing formatted pages in cache memory. Such devices are usually implemented as microcontrollers with serial interface (I2C) and can be found on older TV cards, dedicated Teletext decoding cards and home-brew devices connected to the PC parallel port.

The Teletext API was designed by Martin Buck. It is defined in the kernel header file linux/videotext.h, the specification is available from http://home.pages.de/~videotext/. (Videotext is the name of the German public television Teletext service.) Conventional character device file names are /dev/vtx and /dev/vttuner, with device number 83, 0 and 83, 16 respectively. A similar interface exists for the Philips SAA5249 Teletext decoder [specification?] with character device file names /dev/tlkN, device number 102, N.

Eventually the Teletext API was integrated into the V4L API with character device file names /dev/vtx0 to /dev/vtx31, device major number 81, minor numbers 192 to 223. For reference the V4L Teletext API specification is reproduced here in full: "Teletext interfaces talk the existing VTX API." Teletext devices with major number 83 and 102 will be removed in Linux 2.6.

There are no plans to replace the Teletext API or to integrate it into V4L2. Please write to the Video4Linux mailing list: https://listman.redhat.com/mailman/listinfo/video4linux-list when the need arises.


4.9. Radio Interface

This interface is intended for AM and FM (analog) radio receivers.

Conventionally V4L2 radio devices are accessed through character device special files named /dev/radio and /dev/radio0 to /dev/radio63 with major number 81 and minor numbers 64 to 127.


4.9.1. Querying Capabilities

Devices supporting the radio interface set the V4L2_CAP_RADIO and V4L2_CAP_TUNER flag in the capabilities field of struct v4l2_capability returned by the VIDIOC_QUERYCAP ioctl. Other combinations of capability flags are reserved for future extensions.


4.9.2. Supplemental Functions

Radio devices can support controls, and must support the tuner ioctls.

They do not support the video input or output, audio input or output, video standard, cropping and scaling, compression and streaming parameter, or overlay ioctls. All other ioctls and I/O methods are reserved for future extensions.


4.9.3. Programming

Radio devices may have a couple audio controls (as discussed in Section 1.8>) such as a volume control, possibly custom controls. Further all radio devices have one tuner (these are discussed in Section 1.6>) with index number zero to select the radio frequency and to determine if a monaural or FM stereo program is received. Drivers switch automatically between AM and FM depending on the selected frequency. The VIDIOC_G_TUNER ioctl reports the supported frequency range.


4.10. RDS Interface

The Radio Data System transmits supplementary information in binary format, for example the station name or travel information, on a inaudible audio subcarrier of a radio program. This interface aims at devices capable of receiving and decoding RDS information.

The V4L API defines its RDS API as follows.

From radio devices supporting it, RDS data can be read with the read() function. The data is packed in groups of three, as follows:

  1. First Octet Least Significant Byte of RDS Block

  2. Second Octet Most Significant Byte of RDS Block

  3. Third Octet Bit 7: Error bit. Indicates that an uncorrectable error occurred during reception of this block. Bit 6: Corrected bit. Indicates that an error was corrected for this data block. Bits 5-3: Received Offset. Indicates the offset received by the sync system. Bits 2-0: Offset Name. Indicates the offset applied to this data.

It was argued the RDS API should be extended before integration into V4L2, no new API has been devised yet. Please write to the Video4Linux mailing list for discussion: https://listman.redhat.com/mailman/listinfo/video4linux-list. Meanwhile no V4L2 driver should set the V4L2_CAP_RDS_CAPTURE capability flag.

I. Function Reference

Table of Contents
V4L2 close() -- Close a V4L2 device
V4L2 ioctl() -- Program a V4L2 device
ioctl VIDIOC_CROPCAP -- Information about the video cropping and scaling abilities.
ioctl VIDIOC_ENUMAUDIO -- Enumerate audio inputs
ioctl VIDIOC_ENUMAUDOUT -- Enumerate audio outputs
ioctl VIDIOC_ENUM_FMT -- Enumerate image formats
ioctl VIDIOC_ENUMINPUT -- Enumerate video inputs
ioctl VIDIOC_ENUMOUTPUT -- Enumerate video outputs
ioctl VIDIOC_ENUMSTD -- Enumerate supported video standards
ioctl VIDIOC_G_AUDIO, VIDIOC_S_AUDIO -- Query or select the current audio input and its attributes
ioctl VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT -- Query or select the current audio output
ioctl VIDIOC_G_MPEGCOMP, VIDIOC_S_MPEGCOMP -- Get or set compression parameters
ioctl VIDIOC_G_CROP, VIDIOC_S_CROP -- Get or set the current cropping rectangle
ioctl VIDIOC_G_CTRL, VIDIOC_S_CTRL -- Get or set the value of a control
ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF -- Get or set frame buffer overlay parameters.
ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT -- Get or set the data format, try a format.
ioctl VIDIOC_G_FREQUENCY, VIDIOC_S_FREQUENCY -- Get or set tuner or modulator radio frequency
ioctl VIDIOC_G_INPUT, VIDIOC_S_INPUT -- Query or select the current video input
ioctl VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP -- 
ioctl VIDIOC_G_MODULATOR, VIDIOC_S_MODULATOR -- Get or set modulator attributes
ioctl VIDIOC_G_OUTPUT, VIDIOC_S_OUTPUT -- Query or select the current video output
ioctl VIDIOC_G_PARM, VIDIOC_S_PARM -- Get or set streaming parameters
ioctl VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY -- Query or request the access priority associated with a file descriptor
ioctl VIDIOC_G_SLICED_VBI_CAP -- Query sliced VBI capabilities
ioctl VIDIOC_G_STD, VIDIOC_S_STD -- Query or select the video standard of the current input
ioctl VIDIOC_G_TUNER, VIDIOC_S_TUNER -- Get or set tuner attributes
ioctl VIDIOC_LOG_STATUS -- Log driver status information
ioctl VIDIOC_OVERLAY -- Start or stop video overlay
ioctl VIDIOC_QBUF, VIDIOC_DQBUF -- Exchange a buffer with the driver
ioctl VIDIOC_QUERYBUF -- Query the status of a buffer
ioctl VIDIOC_QUERYCAP -- Query device capabilities
ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU -- Enumerate controls and menu control items
ioctl VIDIOC_QUERYSTD -- Sense the video standard received by the current input
ioctl VIDIOC_REQBUFS -- Initiate Memory Mapping or User Pointer I/O
ioctl VIDIOC_STREAMON, VIDIOC_STREAMOFF -- Start or stop streaming I/O
V4L2 mmap() -- Map device memory into application address space
V4L2 munmap() -- Unmap device memory
V4L2 open() -- Open a V4L2 device
V4L2 poll() -- Wait for some event on a file descriptor
V4L2 read() -- Read from a V4L2 device
V4L2 select() -- Synchronous I/O multiplexing
V4L2 write() -- Write to a V4L2 device

V4L2 close()

Name

v4l2-close -- Close a V4L2 device

Synopsis

#include <unistd.h>

int close(int fd);

Arguments

fd

File descriptor returned by open().

Description

Closes the device. Any I/O in progress is terminated and resources associated with the file descriptor are freed. However data format parameters, current input or output, control values or other properties remain unchanged.

Return Value

The function returns 0 on success, -1 on failure and the errno is set appropriately. Possible error codes:

EBADF

fd is not a valid open file descriptor.

V4L2 ioctl()

Name

v4l2-ioctl -- Program a V4L2 device

Synopsis

#include <sys/ioctl.h>

int ioctl(int fd, int request, void *argp);

Arguments

fd

File descriptor returned by open().

request

V4L2 ioctl request code as defined in the videodev.h header file, for example VIDIOC_QUERYCAP.

argp

Pointer to a function parameter, usually a structure.

Description

The ioctl() function is used to program V4L2 devices. The argument fd must be an open file descriptor. An ioctl request has encoded in it whether the argument is an input, output or read/write parameter, and the size of the argument argp in bytes. Macros and defines specifying V4L2 ioctl requests are located in the videodev.h header file. Applications should use their own copy, not include the version in the kernel sources on the system they compile on. All V4L2 ioctl requests, their respective function and parameters are specified in Reference I, Function Reference>.

Return Value

On success the ioctl() function returns 0 and does not reset the errno variable. On failure -1 is returned, when the ioctl takes an output or read/write parameter it remains unmodified, and the errno variable is set appropriately. See below for possible error codes. Generic errors like EBADF or EFAULT are not listed in the sections discussing individual ioctl requests.

Note ioctls may return undefined error codes. Since errors may have side effects such as a driver reset applications should abort on unexpected errors.

EBADF

fd is not a valid open file descriptor.

EBUSY

The property cannot be changed right now. Typically this error code is returned when I/O is in progress or the driver supports multiple opens and another process locked the property.

EFAULT

argp references an inaccessible memory area.

ENOTTY

fd is not associated with a character special device.

EINVAL

The request or the data pointed to by argp is not valid. This is a very common error code, see the individual ioctl requests listed in Reference I, Function Reference> for actual causes.

ENOMEM

Insufficient memory to complete the request.

ERANGE

The application attempted to set a control with the VIDIOC_S_CTRL ioctl to a value which is out of bounds.

ioctl VIDIOC_CROPCAP

Name

VIDIOC_CROPCAP -- Information about the video cropping and scaling abilities.

Synopsis

int ioctl(int fd, int request, struct v4l2_cropcap *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_CROPCAP

argp

Description

Applications use this function to query the cropping limits, the pixel aspect of images and to calculate scale factors. They set the type field of a v4l2_cropcap structure to the respective buffer (stream) type and call the VIDIOC_CROPCAP ioctl with a pointer to this structure. Drivers fill the rest of the structure. The results are constant except when switching the video standard. Remember this switch can occur implicit when switching the video input or output.

Table 1. struct v4l2_cropcap

enum v4l2_buf_typetypeType of the data stream, set by the application. Only these types are valid here: V4L2_BUF_TYPE_VIDEO_CAPTURE, V4L2_BUF_TYPE_VIDEO_OUTPUT, V4L2_BUF_TYPE_VIDEO_OVERLAY, and custom (driver defined) types with code V4L2_BUF_TYPE_PRIVATE and higher.
struct v4l2_rectboundsDefines the window within capturing or output is possible, this may exclude for example the horizontal and vertical blanking areas. The cropping rectangle cannot exceed these limits. Width and height are defined in pixels, the driver writer is free to choose origin and units of the coordinate system in the analog domain.
struct v4l2_rectdefrectDefault cropping rectangle, it shall cover the "whole picture". Assuming pixel aspect 1/1 this could be for example a 640 × 480 rectangle for NTSC, a 768 × 576 rectangle for PAL and SECAM centered over the active picture area. The same co-ordinate system as for bounds is used.
struct v4l2_fractpixelaspect

This is the pixel aspect (y / x) when no scaling is applied, the ratio of the actual sampling frequency and the frequency required to get square pixels.

When cropping coordinates refer to square pixels, the driver sets pixelaspect to 1/1. Other common values are 54/59 for PAL and SECAM, 11/10 for NTSC sampled according to [ITU601>].

Table 2. struct v4l2_rect

__s32leftHorizontal offset of the top, left corner of the rectangle, in pixels.
__s32topVertical offset of the top, left corner of the rectangle, in pixels.
__s32widthWidth of the rectangle, in pixels.
__s32heightHeight of the rectangle, in pixels. Width and height cannot be negative, the fields are signed for hysterical reasons.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_cropcap type is invalid or the ioctl is not supported. This is not permitted for video capture, output and overlay devices, which must support VIDIOC_CROPCAP.

ioctl VIDIOC_ENUMAUDIO

Name

VIDIOC_ENUMAUDIO -- Enumerate audio inputs

Synopsis

int ioctl(int fd, int request, struct v4l2_audio *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUMAUDIO

argp

Description

To query the attributes of an audio input applications initialize the index field and zero out the reserved array of a struct v4l2_audio and call the VIDIOC_ENUMAUDIO ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the index is out of bounds. To enumerate all audio inputs applications shall begin at index zero, incrementing by one until the driver returns EINVAL.

See ioctl VIDIOC_G_AUDIO, VIDIOC_S_AUDIO(2)> for a description of struct v4l2_audio.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The number of the audio input is out of bounds, or there are no audio inputs at all and this ioctl is not supported.

ioctl VIDIOC_ENUMAUDOUT

Name

VIDIOC_ENUMAUDOUT -- Enumerate audio outputs

Synopsis

int ioctl(int fd, int request, struct v4l2_audioout *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUMAUDOUT

argp

Description

To query the attributes of an audio output applications initialize the index field and zero out the reserved array of a struct v4l2_audioout and call the VIDIOC_G_AUDOUT ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the index is out of bounds. To enumerate all audio outputs applications shall begin at index zero, incrementing by one until the driver returns EINVAL.

Note connectors on a TV card to loop back the received audio signal to a sound card are not audio outputs in this sense.

See ioctl VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT(2)> for a description of struct v4l2_audioout.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The number of the audio output is out of bounds, or there are no audio outputs at all and this ioctl is not supported.

ioctl VIDIOC_ENUM_FMT

Name

VIDIOC_ENUM_FMT -- Enumerate image formats

Synopsis

int ioctl(int fd, int request, struct v4l2_fmtdesc *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUM_FMT

argp

Description

To enumerate image formats applications initialize the type and index field of struct v4l2_fmtdesc and call the VIDIOC_ENUM_FMT ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code. All formats are enumerable by beginning at index zero and incrementing by one until EINVAL is returned.

Table 1. struct v4l2_fmtdesc

__u32indexNumber of the format in the enumeration, set by the application. This is in no way related to the pixelformat field.
enum v4l2_buf_typetypeType of the data stream, set by the application. Only these types are valid here: V4L2_BUF_TYPE_VIDEO_CAPTURE, V4L2_BUF_TYPE_VIDEO_OUTPUT, V4L2_BUF_TYPE_VIDEO_OVERLAY, and custom (driver defined) types with code V4L2_BUF_TYPE_PRIVATE and higher.
__u32flagsSee Table 2>
__u8description[32]Description of the format, a NUL-terminated ASCII string. This information is intended for the user, for example: "YUV 4:2:2".
__u32pixelformatThe image format identifier. This is a four character code as computed by the v4l2_fourcc() macro:

#define v4l2_fourcc(a,b,c,d) (((__u32)(a)<<0)|((__u32)(b)<<8)|((__u32)(c)<<16)|((__u32)(d)<<24))

Several image formats are already defined by this specification in Chapter 2>. Note these codes are not the same as those used in the Windows world.

__u32reserved[4]Reserved for future extensions. Drivers must set the array to zero.

Table 2. Image Format Description Flags

V4L2_FMT_FLAG_COMPRESSED0x0001This is a compressed format.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_fmtdesc type is not supported or the index is out of bounds.

ioctl VIDIOC_ENUMINPUT

Name

VIDIOC_ENUMINPUT -- Enumerate video inputs

Synopsis

int ioctl(int fd, int request, struct v4l2_input *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUMINPUT

argp

Description

To query the attributes of a video input applications initialize the index field of struct v4l2_input and call the VIDIOC_ENUMINPUT ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the index is out of bounds. To enumerate all inputs applications shall begin at index zero, incrementing by one until the driver returns EINVAL.

Table 1. struct v4l2_input

__u32indexIdentifies the input, set by the application.
__u8name[32]Name of the video input, a NUL-terminated ASCII string, for example: "Vin (Composite 2)". This information is intended for the user, preferably the connector label on the device itself.
__u32typeType of the input, see Table 2>.
__u32audiosetVideo inputs combine with zero or more audio inputs. For example one composite video connectors may exist, but two audio connectors. On the other hand, video from a tuner will likely combine only with audio from the same tuner. Devices with N audio inputs number them 0 … N-1 with N ≤ 32. Each bit position of audioset represents one audio input. For details on audio inputs and how to switch see Section 1.5>.
__u32tunerCapture devices can have zero or more tuners (RF demodulators). When the type is set to V4L2_INPUT_TYPE_TUNER this is an RF connector and this field identifies the tuner. It corresponds to struct v4l2_tuner field index. For details on tuners see Section 1.6>.
v4l2_std_idstdEvery video input supports one or more different video standards. This field is a set of all supported standards. For details on video standards and how to switch see Section 1.7>.
__u32statusThis field provides status information about the input. See Table 3> for flags. status is only valid when this is the current input.
__u32reserved[4]Reserved for future extensions. Drivers must set the array to zero.

Table 2. Input Types

V4L2_INPUT_TYPE_TUNER1This input uses a tuner (RF demodulator).
V4L2_INPUT_TYPE_CAMERA2Analog baseband input, for example CVBS / Composite Video, S-Video, RGB.

Table 3. Input Status Flags

General
V4L2_IN_ST_NO_POWER0x00000001Attached device is off.
V4L2_IN_ST_NO_SIGNAL0x00000002 
V4L2_IN_ST_NO_COLOR0x00000004The hardware supports color decoding, but does not detect color modulation in the signal.
Analog Video
V4L2_IN_ST_NO_H_LOCK0x00000100No horizontal sync lock.
V4L2_IN_ST_COLOR_KILL0x00000200A color killer circuit automatically disables color decoding when it detects no color modulation. When this flag is set the color killer is enabled and has shut off color decoding.
Digital Video
V4L2_IN_ST_NO_SYNC0x00010000No synchronization lock.
V4L2_IN_ST_NO_EQU0x00020000No equalizer lock.
V4L2_IN_ST_NO_CARRIER0x00040000Carrier recovery failed.
VCR and Set-Top Box
V4L2_IN_ST_MACROVISION0x01000000Macrovision is an analog copy prevention system mangling the video signal to confuse video recorders. When this flag is set Macrovision has been detected.
V4L2_IN_ST_NO_ACCESS0x02000000Conditional access denied.
V4L2_IN_ST_VTR0x04000000VTR time constant. [?]

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_input index is out of bounds.

ioctl VIDIOC_ENUMOUTPUT

Name

VIDIOC_ENUMOUTPUT -- Enumerate video outputs

Synopsis

int ioctl(int fd, int request, struct v4l2_output *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUMOUTPUT

argp

Description

To query the attributes of a video outputs applications initialize the index field of struct v4l2_output and call the VIDIOC_ENUMOUTPUT ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the index is out of bounds. To enumerate all outputs applications shall begin at index zero, incrementing by one until the driver returns EINVAL.

Table 1. struct v4l2_output

__u32indexIdentifies the output, set by the application.
__u8name[32]Name of the video output, a NUL-terminated ASCII string, for example: "Vout". This information is intended for the user, preferably the connector label on the device itself.
__u32typeType of the output, see Table 2>.
__u32audiosetVideo outputs combine with zero or more audio outputs. For example one composite video connectors may exist, but two audio connectors. On the other hand, video to a modulator will likely combine only with audio to the same modulator. Devices with N audio outputs number them 0 … N-1 with N ≤ 32. Each bit position of audioset represents one audio output. For details on audio outputs and how to switch see Section 1.5>.
__u32modulatorOutput devices can have zero or more RF modulators. When the type is V4L2_OUTPUT_TYPE_MODULATOR this is an RF connector and this field identifies the modulator. It corresponds to struct v4l2_modulator field index. For details on modulators see Section 1.6>.
v4l2_std_idstdEvery video output supports one or more different video standards. This field is a set of all supported standards. For details on video standards and how to switch see Section 1.7>.
__u32reserved[4]Reserved for future extensions. Drivers must set the array to zero.

Table 2. Output Type

V4L2_OUTPUT_TYPE_MODULATOR1This output is an analog TV modulator.
V4L2_OUTPUT_TYPE_ANALOG2Analog baseband output, for example Composite / CVBS, S-Video, RGB.
V4L2_OUTPUT_TYPE_ANALOGVGAOVERLAY3[?]

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_output index is out of bounds.

ioctl VIDIOC_ENUMSTD

Name

VIDIOC_ENUMSTD -- Enumerate supported video standards

Synopsis

int ioctl(int fd, int request, struct v4l2_standard *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_ENUMSTD

argp

Description

To query the attributes of a video standard, especially a custom (driver defined) one, applications initialize the index field of struct v4l2_standard and call the VIDIOC_ENUMSTD ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the index is out of bounds. To enumerate all standards applications shall begin at index zero, incrementing by one until the driver returns EINVAL. Drivers may enumerate a different set of standards after switching the video input or output.[24]

Table 1. struct v4l2_standard

__u32indexNumber of the video standard, set by the application.
v4l2_std_ididThe bits in this field identify the standard as one of the common standards listed in Table 3>, or if bits 32 to 63 are set as custom standards. Multiple bits can be set if the hardware does not distinguish between these standards, however separate indices do not indicate the opposite. The id must be unique. No other enumerated v4l2_standard structure, for this input or output anyway, can contain the same set of bits.
__u8name[24]Name of the standard, a NUL-terminated ASCII string, for example: "PAL-B/G", "NTSC Japan". This information is intended for the user.
struct v4l2_fractframeperiodThe frame period (not field period) is numerator / denominator. For example M/NTSC has a frame period of 1001 / 30000 seconds.
__u32framelinesTotal lines per frame including blanking, e. g. 625 for B/PAL.
__u32reserved[4]Reserved for future extensions. Drivers must set the array to zero.

Table 2. struct v4l2_fract

__u32numerator 
__u32denominator 

Table 3. typedef v4l2_std_id

__u64v4l2_std_idThis type is a set, each bit representing another video standard as listed below and in Table 4>. The 32 most significant bits are reserved for custom (driver defined) video standards.

#define V4L2_STD_PAL_B ((v4l2_std_id)0x00000001)
#define V4L2_STD_PAL_B1 ((v4l2_std_id)0x00000002)
#define V4L2_STD_PAL_G ((v4l2_std_id)0x00000004)
#define V4L2_STD_PAL_H ((v4l2_std_id)0x00000008)
#define V4L2_STD_PAL_I ((v4l2_std_id)0x00000010)
#define V4L2_STD_PAL_D ((v4l2_std_id)0x00000020)
#define V4L2_STD_PAL_D1 ((v4l2_std_id)0x00000040)
#define V4L2_STD_PAL_K ((v4l2_std_id)0x00000080)
#define V4L2_STD_PAL_M ((v4l2_std_id)0x00000100)
#define V4L2_STD_PAL_N ((v4l2_std_id)0x00000200)
#define V4L2_STD_PAL_Nc ((v4l2_std_id)0x00000400)
#define V4L2_STD_PAL_60 ((v4l2_std_id)0x00000800)

V4L2_STD_PAL_60 is a hybrid standard with 525 lines, 60 Hz refresh rate, and PAL color modulation with a 4.43 MHz color subcarrier. Some PAL video recorders can play back NTSC tapes in this mode for display on a 50/60 Hz agnostic PAL TV.

#define V4L2_STD_NTSC_M ((v4l2_std_id)0x00001000)
#define V4L2_STD_NTSC_M_JP ((v4l2_std_id)0x00002000)
#define V4L2_STD_NTSC_443 ((v4l2_std_id)0x00004000)

V4L2_STD_NTSC_443 is a hybrid standard with 525 lines, 60 Hz refresh rate, and NTSC color modulation with a 4.43 MHz color subcarrier.

#define V4L2_STD_SECAM_B ((v4l2_std_id)0x00010000)
#define V4L2_STD_SECAM_D ((v4l2_std_id)0x00020000)
#define V4L2_STD_SECAM_G ((v4l2_std_id)0x00040000)
#define V4L2_STD_SECAM_H ((v4l2_std_id)0x00080000)
#define V4L2_STD_SECAM_K ((v4l2_std_id)0x00100000)
#define V4L2_STD_SECAM_K1 ((v4l2_std_id)0x00200000)
#define V4L2_STD_SECAM_L ((v4l2_std_id)0x00400000)
#define V4L2_STD_SECAM_LC ((v4l2_std_id)0x00800000)
/* ATSC/HDTV */
#define V4L2_STD_ATSC_8_VSB ((v4l2_std_id)0x01000000)
#define V4L2_STD_ATSC_16_VSB ((v4l2_std_id)0x02000000)

V4L2_STD_ATSC_8_VSB and V4L2_STD_ATSC_16_VSB are U.S. terrestrial digital TV standards. Presently the V4L2 API does not support digital TV. See also the Linux DVB API at http://linuxtv.org.

#define V4L2_STD_PAL_BG (V4L2_STD_PAL_B |\
V4L2_STD_PAL_B1 |\
V4L2_STD_PAL_G)
#define V4L2_STD_PAL_DK (V4L2_STD_PAL_D |\
V4L2_STD_PAL_D1 |\
V4L2_STD_PAL_K)
#define V4L2_STD_PAL (V4L2_STD_PAL_BG |\
V4L2_STD_PAL_DK |\
V4L2_STD_PAL_H |\
V4L2_STD_PAL_I)
#define V4L2_STD_NTSC (V4L2_STD_NTSC_M |\
V4L2_STD_NTSC_M_JP)
#define V4L2_STD_SECAM_DK (V4L2_STD_SECAM_D |\
V4L2_STD_SECAM_K |\
V4L2_STD_SECAM_K1)
#define V4L2_STD_SECAM (V4L2_STD_SECAM_B |\
V4L2_STD_SECAM_G |\
V4L2_STD_SECAM_H |\
V4L2_STD_SECAM_DK |\
V4L2_STD_SECAM_L)
#define V4L2_STD_525_60 (V4L2_STD_PAL_M |\
V4L2_STD_PAL_60 |\
V4L2_STD_NTSC |\
V4L2_STD_NTSC_443)
#define V4L2_STD_625_50 (V4L2_STD_PAL |\
V4L2_STD_PAL_N |\
V4L2_STD_PAL_Nc |\
V4L2_STD_SECAM)
#define V4L2_STD_UNKNOWN 0
#define V4L2_STD_ALL (V4L2_STD_525_60 |\
V4L2_STD_625_50)

Table 4. Video Standards (based on [ITU470>])

Characteristics

M/NTSCa

M/PAL

N/PALb

B, B1, G/PALD, D1, K/PALH/PALI/PALB, G/SECAMD, K/SECAMK1/SECAML/SECAM
Frame lines525625
Frame period (s)1001/300001/25
Chrominance sub-carrier frequency (Hz)3579545 ± 103579611.49 ± 104433618.75 ± 5 (3582056.25 ± 5)4433618.75 ± 54433618.75 ± 1fOR = 4406250 ± 2000, fOB = 4250000 ± 2000
Nominal radio-frequency channel bandwidth (MHz)666B: 7; B1, G: 88888888
Sound carrier relative to vision carrier (MHz)+ 4.5+ 4.5+ 4.5

+ 5.5 ± 0.001 c d e f

+ 6.5 ± 0.001+ 5.5+ 5.9996 ± 0.0005+ 5.5 ± 0.001+ 6.5 ± 0.001+ 6.5

+ 6.5 g

Notes:
a. Japan uses a standard similar to M/NTSC (V4L2_STD_NTSC_M_JP).
b. The values in brackets apply to the combination N/PAL a.k.a. NC used in Argentina (V4L2_STD_PAL_Nc).
c. In the Federal Republic of Germany, Austria, Italy, the Netherlands, Slovakia and Switzerland a system of two sound carriers is used, the frequency of the second carrier being 242.1875 kHz above the frequency of the first sound carrier. For stereophonic sound transmissions a similar system is used in Australia.
d. New Zealand uses a sound carrier displaced 5.4996 ± 0.0005 MHz from the vision carrier.
e. In Denmark, Finland, New Zealand, Sweden and Spain a system of two sound carriers is used. In Iceland, Norway and Poland the same system is being introduced. The second carrier is 5.85 MHz above the vision carrier and is DQPSK modulated with 728 kbit/s sound and data multiplex. (NICAM system)
f. In the United Kingdom, a system of two sound carriers is used. The second sound carrier is 6.552 MHz above the vision carrier and is DQPSK modulated with a 728 kbit/s sound and data multiplex able to carry two sound channels. (NICAM system)
g. In France, a digital carrier 5.85 MHz away from the vision carrier may be used in addition to the main sound carrier. It is modulated in differentially encoded QPSK with a 728 kbit/s sound and data multiplexer capable of carrying two sound channels. (NICAM system)

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_standard index is out of bounds.

ioctl VIDIOC_G_AUDIO, VIDIOC_S_AUDIO

Name

VIDIOC_G_AUDIO, VIDIOC_S_AUDIO -- Query or select the current audio input and its attributes

Synopsis

int ioctl(int fd, int request, struct v4l2_audio *argp);

int ioctl(int fd, int request, const struct v4l2_audio *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_AUDIO, VIDIOC_S_AUDIO

argp

Description

To query the current audio input applications zero out the reserved array of a struct v4l2_audio and call the VIDIOC_G_AUDIO ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the device has no audio inputs, or none which combine with the current video input.

Audio inputs have one writable property, the audio mode. To select the current audio input and change the audio mode, applications initialize the index and mode fields, and the reserved array of a v4l2_audio structure and call the VIDIOC_S_AUDIO ioctl. Drivers may switch to a different audio mode if the request cannot be satisfied. However, this is a write-only ioctl, it does not return the actual new audio mode.

Table 1. struct v4l2_audio

__u32indexIdentifies the audio input, set by the driver or application.
__u8name[32]Name of the audio input, a NUL-terminated ASCII string, for example: "Line In". This information is intended for the user, preferably the connector label on the device itself.
__u32capabilityAudio capability flags, see Table 2>.
__u32modeAudio mode set by drivers and applications (on VIDIOC_S_AUDIO ioctl), see Table 3>.
__u32reserved[2]Reserved for future extensions. Drivers and applications must set the array to zero.

Table 2. Audio Capability Flags

V4L2_AUDCAP_STEREO0x00001This is a stereo input. The flag is intended to automatically disable stereo recording etc. when the signal is always monaural. The API provides no means to detect if stereo is received, unless the audio input belongs to a tuner.
V4L2_AUDCAP_AVL0x00002Automatic Volume Level mode is supported.

Table 3. Audio Modes

V4L2_AUDMODE_AVL0x00001AVL mode is on.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

No audio inputs combine with the current video input, or the number of the selected audio input is out of bounds or it does not combine, or there are no audio inputs at all and the ioctl is not supported.

EBUSY

I/O is in progress, the input cannot be switched.

ioctl VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT

Name

VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT -- Query or select the current audio output

Synopsis

int ioctl(int fd, int request, struct v4l2_audioout *argp);

int ioctl(int fd, int request, const struct v4l2_audioout *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_AUDOUT, VIDIOC_S_AUDOUT

argp

Description

To query the current audio output applications zero out the reserved array of a struct v4l2_audioout and call the VIDIOC_G_AUDOUT ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the device has no audio inputs, or none which combine with the current video output.

Audio outputs have no writable properties. Nevertheless, to select the current audio output applications can initialize the index field and reserved array (which in the future may contain writable properties) of a v4l2_audioout structure and call the VIDIOC_S_AUDOUT ioctl. Drivers switch to the requested output or return the EINVAL error code when the index is out of bounds. This is a write-only ioctl, it does not return the current audio output attributes as VIDIOC_G_AUDOUT does.

Note connectors on a TV card to loop back the received audio signal to a sound card are not audio outputs in this sense.

Table 1. struct v4l2_audioout

__u32indexIdentifies the audio output, set by the driver or application.
__u8name[32]Name of the audio output, a NUL-terminated ASCII string, for example: "Line Out". This information is intended for the user, preferably the connector label on the device itself.
__u32capabilityAudio capability flags, none defined yet. Drivers must set this field to zero.
__u32modeAudio mode, none defined yet. Drivers and applications (on VIDIOC_S_AUDOUT) must set this field to zero.
__u32reserved[2]Reserved for future extensions. Drivers and applications must set the array to zero.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

No audio outputs combine with the current video output, or the number of the selected audio output is out of bounds or it does not combine, or there are no audio outputs at all and the ioctl is not supported.

EBUSY

I/O is in progress, the output cannot be switched.

ioctl VIDIOC_G_MPEGCOMP, VIDIOC_S_MPEGCOMP

Name

VIDIOC_G_MPEGCOMP, VIDIOC_S_MPEGCOMP -- Get or set compression parameters

Synopsis

int ioctl(int fd, int request, v4l2_mpeg_compression *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_MPEGCOMP, VIDIOC_S_MPEGCOMP

argp

Description

[to do]

Table 1. struct v4l2_mpeg_compression

[to do]

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

This ioctl is not supported

ioctl VIDIOC_G_CROP, VIDIOC_S_CROP

Name

VIDIOC_G_CROP, VIDIOC_S_CROP -- Get or set the current cropping rectangle

Synopsis

int ioctl(int fd, int request, struct v4l2_crop *argp);

int ioctl(int fd, int request, const struct v4l2_crop *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_CROP, VIDIOC_S_CROP

argp

Description

To query the cropping rectangle size and position applications set the type field of a v4l2_crop structure to the respective buffer (stream) type and call the VIDIOC_G_CROP ioctl with a pointer to this structure. The driver fills the rest of the structure or returns the EINVAL error code if cropping is not supported.

To change the cropping rectangle applications initialize the type and struct v4l2_rect substructure named c of a v4l2_crop structure and call the VIDIOC_S_CROP ioctl with a pointer to this structure.

The driver first adjusts the requested dimensions against hardware limits, i. e. the bounds given by the capture/output window, and it rounds to the closest possible values of horizontal and vertical offset, width and height. In particular the driver must round the vertical offset of the cropping rectangle to frame lines modulo two, such that the field order cannot be confused.

Second the driver adjusts the image size (the opposite rectangle of the scaling process, source or target depending on the data direction) to the closest size possible while maintaining the current horizontal and vertical scaling factor.

Finally the driver programs the hardware with the actual cropping and image parameters. VIDIOC_S_CROP is a write-only ioctl, it does not return the actual parameters. To query them applications must call VIDIOC_G_CROP and VIDIOC_G_FMT. When the parameters are unsuitable the application may modify the cropping or image parameters and repeat the cycle until satisfactory parameters have been negotiated.

When cropping is not supported then no parameters are changed and VIDIOC_S_CROP returns the EINVAL error code.

Table 1. struct v4l2_crop

enum v4l2_buf_typetypeType of the data stream, set by the application. Only these types are valid here: V4L2_BUF_TYPE_VIDEO_CAPTURE, V4L2_BUF_TYPE_VIDEO_OUTPUT, V4L2_BUF_TYPE_VIDEO_OVERLAY, and custom (driver defined) types with code V4L2_BUF_TYPE_PRIVATE and higher.
struct v4l2_rectcCropping rectangle. The same co-ordinate system as for struct v4l2_cropcap bounds is used.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

Cropping is not supported.

ioctl VIDIOC_G_CTRL, VIDIOC_S_CTRL

Name

VIDIOC_G_CTRL, VIDIOC_S_CTRL -- Get or set the value of a control

Synopsis

int ioctl(int fd, int request, struct v4l2_control *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_CTRL, VIDIOC_S_CTRL

argp

Description

To get the current value of a control applications initialize the id field of a struct v4l2_control and call the VIDIOC_G_CTRL ioctl with a pointer to this structure. To change the value of a control applications initialize the id and value fields of a struct v4l2_control and call the VIDIOC_S_CTRL ioctl.

When the id is invalid drivers return an EINVAL error code. When the value is out of bounds drivers can choose to take the closest valid value or return an ERANGE error code, whatever seems more appropriate. However, VIDIOC_S_CTRL is a write-only ioctl, it does not return the actual new value.

Table 1. struct v4l2_control

__u32idIdentifies the control, set by the application.
__s32valueNew value or current value.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_control id is invalid.

ERANGE

The struct v4l2_control value is out of bounds.

EBUSY

The control is temporarily not changeable, possibly because another applications took over control of the device function this control belongs to.

ioctl VIDIOC_G_FBUF, VIDIOC_S_FBUF

Name

VIDIOC_G_FBUF, VIDIOC_S_FBUF -- Get or set frame buffer overlay parameters.

Synopsis

int ioctl(int fd, int request, struct v4l2_framebuffer *argp);

int ioctl(int fd, int request, const struct v4l2_framebuffer *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_FBUF, VIDIOC_S_FBUF

argp

Description

The VIDIOC_G_FBUF and VIDIOC_S_FBUF ioctl are used to get and set the frame buffer parameters for video overlay.

To get the current parameters applications call the VIDIOC_G_FBUF ioctl with a pointer to a v4l2_framebuffer structure, the driver fills all fields of the structure or returns the EINVAL error code when overlay is not supported. To set the parameters applications initialize the flags field, base unless the overlay is of V4L2_FBUF_CAP_EXTERNOVERLAY type, and the struct v4l2_pix_format fmt substructure. The driver accordingly prepares for overlay or returns an error code.

When the driver does not support V4L2_FBUF_CAP_EXTERNOVERLAY, i. e. it will write into video memory, the VIDIOC_S_FBUF ioctl is a privileged function and only the superuser can change the frame buffer parameters.

Table 1. struct v4l2_framebuffer

__u32capability Overlay capability flags set by the driver, see Table 2>.
__u32flags Overlay control flags set by application and driver, see Table 3>
void *base 

Physical base address of the frame buffer, the address of the pixel at coordinates (0; 0) in the frame buffer. This field is not used when VIDIOC_G_FBUF sets the V4L2_FBUF_CAP_EXTERNOVERLAY flag in the capability field.a

struct v4l2_pix_formatfmt Physical layout of the frame buffer. The v4l2_pix_format structure is defined in Chapter 2>, for clarification the fields and expected values are listed below.
 __u32widthWidth of the frame buffer in pixels.
 __u32heightHeight of the frame buffer in pixels. When the driver clears V4L2_FBUF_CAP_EXTERNOVERLAY, the visible portion of the frame buffer can be smaller than width and height.
 __u32pixelformatThe pixel format of the graphics surface, set by the application. Usually this is an RGB format (for example RGB 5:6:5) but YUV formats are also permitted. The behavior of the driver when requesting a compressed format is undefined. See Chapter 2> for information on pixel formats. This field is not used when the driver sets V4L2_FBUF_CAP_EXTERNOVERLAY.
 enum v4l2_fieldfieldIgnored. The field order is selected with the VIDIOC_S_FMT ioctl using struct v4l2_window.
 __u32bytesperlineDistance in bytes between the leftmost pixels in two adjacent lines.

Both applications and drivers can set this field to request padding bytes at the end of each line. Drivers however may ignore the value requested by the application, returning width times bytes per pixel or a larger value required by the hardware. That implies applications can just set this field to zero to get a reasonable default.

Video hardware may access padding bytes, therefore they must reside in accessible memory. Consider cases where padding bytes after the last line of an image cross a system page boundary. Input devices may write padding bytes, the value is undefined. Output devices ignore the contents of padding bytes.

When the image format is planar the bytesperline value applies to the largest plane and is divided by the same factor as the width field for any smaller planes. For example the Cb and Cr planes of a YUV 4:2:0 image have half as many padding bytes following each line as the Y plane. To avoid ambiguities drivers must return a bytesperline value rounded up to a multiple of the scale factor.

This field is not used when the driver sets V4L2_FBUF_CAP_EXTERNOVERLAY.

 __u32sizeimage

Applications must initialize this field. Together with base it defines the frame buffer memory accessible by the driver.

The field is not used when the driver sets V4L2_FBUF_CAP_EXTERNOVERLAY.

 enum v4l2_colorspacecolorspaceThis information supplements the pixelformat and must be set by the driver, see Section 2.2>.
 __u32privReserved for additional information about custom (driver defined) formats. When not used drivers and applications must set this field to zero.
Notes:
a. A physical base address may not suit all platforms. GK notes in theory we should pass something like PCI device + memory region + offset instead. If you encounter problems please discuss on the Video4Linux mailing list: https://listman.redhat.com/mailman/listinfo/video4linux-list.

Table 2. Frame Buffer Capability Flags

V4L2_FBUF_CAP_EXTERNOVERLAY0x0001The video is overlaid externally onto the video signal of the graphics card.
V4L2_FBUF_CAP_CHROMAKEY0x0002The device supports clipping by chroma-keying the image into the display.
V4L2_FBUF_CAP_LIST_CLIPPING0x0004The device supports clipping using a list of clip rectangles.
V4L2_FBUF_CAP_BITMAP_CLIPPING0x0008The device supports clipping using a bit mask.

Table 3. Frame Buffer Flags

V4L2_FBUF_FLAG_PRIMARY0x0001The frame buffer is the primary graphics surface. In other words, the overlay is destructive, the video hardware will write the image into visible graphics memory as opposed to merely displaying the image in place of the original display contents.
V4L2_FBUF_FLAG_OVERLAY0x0002The frame buffer is an overlay surface the same size as the capture. [?]
V4L2_FBUF_FLAG_CHROMAKEY0x0004Use chromakey (when V4L2_FBUF_CAP_CHROMAKEY indicates this capability). The other clipping methods are negotiated with the VIDIOC_S_FMT ioctl, see also Section 4.2>.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EACCES

VIDIOC_S_FBUF can only be called by a privileged user.

EBUSY

The frame buffer parameters cannot be changed at this time because overlay is already enabled, or capturing is enabled and the hardware cannot capture and overlay simultaneously.

EINVAL

The ioctl is not supported or the VIDIOC_S_FBUF parameters are unsuitable.

ioctl VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT

Name

VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT -- Get or set the data format, try a format.

Synopsis

int ioctl(int fd, int request, struct v4l2_format *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_FMT, VIDIOC_S_FMT, VIDIOC_TRY_FMT

argp

Description

These ioctls are used to negotiate the format of data (typically image format) exchanged between driver and application.

To query the current parameters applications set the type field of a struct v4l2_format to the respective buffer (stream) type. For example video capture devices use V4L2_BUF_TYPE_VIDEO_CAPTURE. When the application calls the VIDIOC_G_FMT ioctl with a pointer to this structure the driver fills the respective member of the fmt union. In case of video capture devices that is the struct v4l2_pix_format pix member. When the requested buffer type is not supported drivers return an EINVAL error code.

To change the current format parameters applications initialize the type field and all fields of the respective fmt union member. For details see the documentation of the various devices types in Chapter 4>. Good practice is to query the current parameters first, and to modify only those parameters not suitable for the application. When the application calls the VIDIOC_S_FMT ioctl with a pointer to a v4l2_format structure the driver checks and adjusts the parameters against hardware abilities. Drivers should not return an error code unless the input is ambiguous, this is a mechanism to fathom device capabilities and to approach parameters acceptable for both the application and driver. On success the driver may program the hardware, allocate resources and generally prepare for data exchange. Finally the VIDIOC_S_FMT ioctl returns the current format parameters as VIDIOC_G_FMT does. Very simple, inflexible devices may even ignore all input and always return the default parameters. However all V4L2 devices exchanging data with the application must implement the VIDIOC_G_FMT and VIDIOC_S_FMT ioctl. When the requested buffer type is not supported drivers return an EINVAL error code on a VIDIOC_S_FMT attempt. When I/O is already in progress or the resource is not available for other reasons drivers return the EBUSY error code.

The VIDIOC_TRY_FMT ioctl is equivalent to VIDIOC_S_FMT with one exception: it does not change driver state. It can also be called at any time, never returning EBUSY. This function is provided to negotiate parameters, to learn about hardware limitations, without disabling I/O or possibly time consuming hardware preparations. Although strongly recommended drivers are not required to implement this ioctl.

Table 1. struct v4l2_format

enum v4l2_buf_typetype Type of the data stream, see Table 3-2>.
unionfmt  
 struct v4l2_pix_formatpixDefinition of an image format, see Chapter 2>, used by video capture and output devices.
 struct v4l2_windowwinDefinition of an overlaid image, see Section 4.2>, used by video overlay devices.
 struct v4l2_vbi_formatvbiRaw VBI capture or output parameters. This is discussed in more detail in Section 4.6>. Used by raw VBI capture and output devices.
 struct v4l2_sliced_vbi_formatslicedSliced VBI capture or output parameters. See Section 4.7> for details. Used by sliced VBI capture and output devices.
 __u8raw_data[200]Place holder for future extensions and custom (driver defined) formats with type V4L2_BUF_TYPE_PRIVATE and higher.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EBUSY

The data format cannot be changed at this time, for example because I/O is already in progress.

EINVAL

The struct v4l2_format type field is invalid, the requested buffer type not supported, or VIDIOC_TRY_FMT was called and is not supported with this buffer type.

ioctl VIDIOC_G_FREQUENCY, VIDIOC_S_FREQUENCY

Name

VIDIOC_G_FREQUENCY, VIDIOC_S_FREQUENCY -- Get or set tuner or modulator radio frequency

Synopsis

int ioctl(int fd, int request, struct v4l2_frequency *argp);

int ioctl(int fd, int request, const struct v4l2_frequency *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_FREQUENCY, VIDIOC_S_FREQUENCY

argp

Description

To get the current tuner or modulator radio frequency applications set the tuner field of a struct v4l2_frequency to the respective tuner or modulator number (only input devices have tuners, only output devices have modulators), zero out the reserved array and call the VIDIOC_G_FREQUENCY ioctl with a pointer to this structure. The driver stores the current frequency in the frequency field.

To change the current tuner or modulator radio frequency applications initialize the tuner and frequency fields, and the reserved array of a struct v4l2_frequency and call the VIDIOC_S_FREQUENCY ioctl with a pointer to this structure. When the requested frequency is not possible the driver assumes the closest possible value. However, VIDIOC_S_FREQUENCY is a write-only ioctl, it does not return the actual new frequency.

Table 1. struct v4l2_frequency

__u32tunerThe tuner or modulator index number. This is the same value as in the struct v4l2_input tuner field and the struct v4l2_tuner index field, or the struct v4l2_output modulator field and the struct v4l2_modulator index field.
enum v4l2_tuner_typetypeThe tuner type. This is the same value as in the struct v4l2_tuner type field. The field is not applicable to modulators, i. e. ignored by drivers.
__u32frequencyTuning frequency in units of 62.5 kHz, or if the struct v4l2_tuner or struct v4l2_modulator capabilities flag V4L2_TUNER_CAP_LOW is set, in units of 62.5 Hz.
__u32reserved[8];Reserved for future extensions. Drivers and applications must set the array to zero.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The tuner field is out of bounds.

ioctl VIDIOC_G_INPUT, VIDIOC_S_INPUT

Name

VIDIOC_G_INPUT, VIDIOC_S_INPUT -- Query or select the current video input

Synopsis

int ioctl(int fd, int request, int *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_INPUT, VIDIOC_S_INPUT

argp

Description

To query the current video input applications call the VIDIOC_G_INPUT ioctl with a pointer to an integer where the driver stores the number of the input, as in the struct v4l2_input index field. This ioctl will fail only when there are no video inputs, returning EINVAL.

To select a video input applications store the number of the desired input in an integer and call the VIDIOC_S_INPUT ioctl with a pointer to this integer. Side effects are possible. For example inputs may support different video standards, so the driver may implicitly switch the current standard. It is good practice to select an input before querying or negotiating any other parameters.

Information about video inputs is available using the VIDIOC_ENUMINPUT ioctl.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The number of the video input is out of bounds, or there are no video inputs at all and this ioctl is not supported.

EBUSY

I/O is in progress, the input cannot be switched.

ioctl VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP

Name

VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP -- 

Synopsis

int ioctl(int fd, int request, v4l2_jpegcompression *argp);

int ioctl(int fd, int request, const v4l2_jpegcompression *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_JPEGCOMP, VIDIOC_S_JPEGCOMP

argp

Description

[to do]

Ronald Bultje elaborates:

APP is some application-specific information. The application can set it itself, and it'll be stored in the JPEG-encoded fields (e.g. interlacing information for in an AVI or so). COM is the same, but it's comments, like 'encoded by me' or so.

jpeg_markers describes whether the huffman tables, quantization tables and the restart interval information (all JPEG-specific stuff) should be stored in the JPEG-encoded fields. These define how the JPEG field is encoded. If you omit them, applications assume you've used standard encoding. You usually do want to add them.

Table 1. struct v4l2_jpegcompression

intquality 
intAPPn 
intAPP_len 
charAPP_data[60] 
intCOM_len 
charCOM_data[60] 
__u32jpeg_markersSee Table 2>.

Table 2. JPEG Markers Flags

V4L2_JPEG_MARKER_DHT(1<<3)Define Huffman Tables
V4L2_JPEG_MARKER_DQT(1<<4)Define Quantization Tables
V4L2_JPEG_MARKER_DRI(1<<5)Define Restart Interval
V4L2_JPEG_MARKER_COM(1<<6)Comment segment
V4L2_JPEG_MARKER_APP(1<<7)App segment, driver will always use APP0

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

This ioctl is not supported.

ioctl VIDIOC_G_MODULATOR, VIDIOC_S_MODULATOR

Name

VIDIOC_G_MODULATOR, VIDIOC_S_MODULATOR -- Get or set modulator attributes

Synopsis

int ioctl(int fd, int request, struct v4l2_modulator *argp);

int ioctl(int fd, int request, const struct v4l2_modulator *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_MODULATOR, VIDIOC_S_MODULATOR

argp

Description

To query the attributes of a modulator applications initialize the index field and zero out the reserved array of a struct v4l2_modulator and call the VIDIOC_G_MODULATOR ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the index is out of bounds. To enumerate all modulators applications shall begin at index zero, incrementing by one until the driver returns EINVAL.

Modulators have two writable properties, an audio modulation set and the radio frequency. To change the modulated audio subprograms, applications initialize the index and txsubchans fields and the reserved array and call the VIDIOC_S_MODULATOR ioctl. Drivers may choose a different audio modulation if the request cannot be satisfied. However this is a write-only ioctl, it does not return the actual audio modulation selected.

To change the radio frequency the VIDIOC_S_FREQUENCY ioctl is available.

Table 1. struct v4l2_modulator

__u32indexIdentifies the modulator, set by the application.
__u8name[32]Name of the modulator, a NUL-terminated ASCII string. This information is intended for the user.
__u32capabilityModulator capability flags. No flags are defined for this field, the tuner flags in struct v4l2_tuner are used accordingly. The audio flags indicate the ability to encode audio subprograms. They will not change for example with the current video standard.
__u32rangelowThe lowest tunable frequency in units of 62.5 KHz, or if the capability flag V4L2_TUNER_CAP_LOW is set, in units of 62.5 Hz.
__u32rangehighThe highest tunable frequency in units of 62.5 KHz, or if the capability flag V4L2_TUNER_CAP_LOW is set, in units of 62.5 Hz.
__u32txsubchansWith this field applications can determine how audio sub-carriers shall be modulated. It contains a set of flags as defined in Table 2>. Note the tuner rxsubchans flags are reused, but the semantics are different. Video output devices are assumed to have an analog or PCM audio input with 1-3 channels. The txsubchans flags select one or more channels for modulation, together with some audio subprogram indicator, for example a stereo pilot tone.
__u32reserved[4]Reserved for future extensions. Drivers and applications must set the array to zero.

Table 2. Modulator Audio Transmission Flags

V4L2_TUNER_SUB_MONO0x0001Modulate channel 1 as mono audio, when the input has more channels, a down-mix of channel 1 and 2. This flag does not combine with V4L2_TUNER_SUB_STEREO or V4L2_TUNER_SUB_LANG1.
V4L2_TUNER_SUB_STEREO0x0002Modulate channel 1 and 2 as left and right channel of a stereo audio signal. When the input has only one channel or two channels and V4L2_TUNER_SUB_SAP is also set, channel 1 is encoded as left and right channel. This flag does not combine with V4L2_TUNER_SUB_MONO or V4L2_TUNER_SUB_LANG1. When the driver does not support stereo audio it shall fall back to mono.
V4L2_TUNER_SUB_LANG10x0008Modulate channel 1 and 2 as primary and secondary language of a bilingual audio signal. When the input has only one channel it is used for both languages. It is not possible to encode the primary or secondary language only. This flag does not combine with V4L2_TUNER_SUB_MONO or V4L2_TUNER_SUB_STEREO. If the hardware does not support the respective audio matrix, or the current video standard does not permit bilingual audio the VIDIOC_S_MODULATOR ioctl shall return an EINVAL error code and the driver shall fall back to mono or stereo mode.
V4L2_TUNER_SUB_LANG20x0004Same effect as V4L2_TUNER_SUB_LANG1.
V4L2_TUNER_SUB_SAP0x0004When combined with V4L2_TUNER_SUB_MONO the first channel is encoded as mono audio, the last channel as Second Audio Program. When the input has only one channel it is used for both audio tracks. When the input has three channels the mono track is a down-mix of channel 1 and 2. When combined with V4L2_TUNER_SUB_STEREO channel 1 and 2 are encoded as left and right stereo audio, channel 3 as Second Audio Program. When the input has only two channels, the first is encoded as left and right channel and the second as SAP. When the input has only one channel it is used for all audio tracks. It is not possible to encode a Second Audio Program only. This flag must combine with V4L2_TUNER_SUB_MONO or V4L2_TUNER_SUB_STEREO. If the hardware does not support the respective audio matrix, or the current video standard does not permit SAP the VIDIOC_S_MODULATOR ioctl shall return an EINVAL error code and driver shall fall back to mono or stereo mode.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_modulator index is out of bounds.

ioctl VIDIOC_G_OUTPUT, VIDIOC_S_OUTPUT

Name

VIDIOC_G_OUTPUT, VIDIOC_S_OUTPUT -- Query or select the current video output

Synopsis

int ioctl(int fd, int request, int *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_OUTPUT, VIDIOC_S_OUTPUT

argp

Description

To query the current video output applications call the VIDIOC_G_OUTPUT ioctl with a pointer to an integer where the driver stores the number of the output, as in the struct v4l2_output index field. This ioctl will fail only when there are no video outputs, returning the EINVAL error code.

To select a video output applications store the number of the desired output in an integer and call the VIDIOC_S_OUTPUT ioctl with a pointer to this integer. Side effects are possible. For example outputs may support different video standards, so the driver may implicitly switch the current standard. It is good practice to select an output before querying or negotiating any other parameters.

Information about video outputs is available using the VIDIOC_ENUMOUTPUT ioctl.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The number of the video output is out of bounds, or there are no video outputs at all and this ioctl is not supported.

EBUSY

I/O is in progress, the output cannot be switched.

ioctl VIDIOC_G_PARM, VIDIOC_S_PARM

Name

VIDIOC_G_PARM, VIDIOC_S_PARM -- Get or set streaming parameters

Synopsis

int ioctl(int fd, int request, v4l2_streamparm *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_PARM, VIDIOC_S_PARM

argp

Description

The current video standard determines a nominal number of frames per second. If less than this number of frames is to be captured or output, applications can request frame skipping or duplicating on the driver side. This is especially useful when using the read() or write(), which are not augmented by timestamps or sequence counters, and to avoid unneccessary data copying.

Further these ioctls can be used to determine the number of buffers used internally by a driver in read/write mode. For implications see the section discussing the read() function.

To get and set the streaming parameters applications call the VIDIOC_G_PARM and VIDIOC_S_PARM ioctl, respectively. They take a pointer to a struct v4l2_streamparm which contains a union holding separate parameters for input and output devices.

Table 1. struct v4l2_streamparm

enum v4l2_buf_typetype The buffer (stream) type, same as struct v4l2_format type, set by the application.
unionparm  
 struct v4l2_captureparmcaptureParameters for capture devices, used when type is V4L2_BUF_TYPE_VIDEO_CAPTURE.
 struct v4l2_outputparmoutputParameters for output devices, used when type is V4L2_BUF_TYPE_VIDEO_OUTPUT.
 __u8raw_data[200]A place holder for future extensions and custom (driver defined) buffer types V4L2_BUF_TYPE_PRIVATE and higher.

Table 2. struct v4l2_captureparm

__u32capabilitySee Table 4>.
__u32capturemodeSet by drivers and applications, see Table 5>.
struct v4l2_fracttimeperframe

This is is the desired period between successive frames captured by the driver, in seconds. The field is intended to skip frames on the driver side, saving I/O bandwidth.

Applications store here the desired frame period, drivers return the actual frame period, which must be greater or equal to the nominal frame period determined by the current video standard (struct v4l2_standard frameperiod field). Changing the video standard (also implicitly by switching the video input) may reset this parameter to the nominal frame period. To reset manually applications can just set this field to zero.

Drivers support this function only when they set the V4L2_CAP_TIMEPERFRAME flag in the capability field.

__u32extendedmodeCustom (driver specific) streaming parameters. When unused, applications and drivers must set this field to zero. Applications using this field should check the driver name and version, see Section 1.2>.
__u32readbuffersApplications set this field to the desired number of buffers used internally by the driver in read() mode. Drivers return the actual number of buffers. When an application requests zero buffers, drivers should just return the current setting rather than the minimum or an error code. For details see Section 3.1>.
__u32reserved[4]Reserved for future extensions. Drivers and applications must set the array to zero.

Table 3. struct v4l2_outputparm

__u32capabilitySee Table 4>.
__u32outputmodeSet by drivers and applications, see Table 5>.
struct v4l2_fracttimeperframeThis is is the desired period between successive frames output by the driver, in seconds.

The field is intended to repeat frames on the driver side in write() mode (in streaming mode timestamps can be used to throttle the output), saving I/O bandwidth.

Applications store here the desired frame period, drivers return the actual frame period, which must be greater or equal to the nominal frame period determined by the current video standard (struct v4l2_standard frameperiod field). Changing the video standard (also implicitly by switching the video output) may reset this parameter to the nominal frame period. To reset manually applications can just set this field to zero.

Drivers support this function only when they set the V4L2_CAP_TIMEPERFRAME flag in the capability field.

__u32extendedmodeCustom (driver specific) streaming parameters. When unused, applications and drivers must set this field to zero. Applications using this field should check the driver name and version, see Section 1.2>.
__u32writebuffersApplications set this field to the desired number of buffers used internally by the driver in write() mode. Drivers return the actual number of buffers. When an application requests zero buffers, drivers should just return the current setting rather than the minimum or an error code. For details see Section 3.1>.
__u32reserved[4]Reserved for future extensions. Drivers and applications must set the array to zero.

Table 4. Streaming Parameters Capabilites

V4L2_CAP_TIMEPERFRAME0x1000The frame skipping/repeating controlled by the timeperframe field is supported.

Table 5. Capture Parameters Flags

V4L2_MODE_HIGHQUALITY0x0001

High quality imaging mode. High quality mode is intended for still imaging applications. The idea is to get the best possible image quality that the hardware can deliver. It is not defined how the driver writer may achieve that; it will depend on the hardware and the ingenuity of the driver writer. High quality mode is a different mode from the the regular motion video capture modes. In high quality mode:

  • The driver may be able to capture higher resolutions than for motion capture.

  • The driver may support fewer pixel formats than motion capture (e.g. true color).

  • The driver may capture and arithmetically combine multiple successive fields or frames to remove color edge artifacts and reduce the noise in the video data.

  • The driver may capture images in slices like a scanner in order to handle larger format images than would otherwise be possible.

  • An image capture operation may be significantly slower than motion capture.

  • Moving objects in the image might have excessive motion blur.

  • Capture might only work through the read() call.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

This ioctl is not supported.

ioctl VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY

Name

VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY -- Query or request the access priority associated with a file descriptor

Synopsis

int ioctl(int fd, int request, enum v4l2_priority *argp);

int ioctl(int fd, int request, const enum v4l2_priority *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_PRIORITY, VIDIOC_S_PRIORITY

argp

Pointer to an enum v4l2_priority type.

Description

To query the current access priority applications call the VIDIOC_G_PRIORITY ioctl with a pointer to an enum v4l2_priority variable where the driver stores the current priority.

To request an access priority applications store the desired priority in an enum v4l2_priority variable and call VIDIOC_S_PRIORITY ioctl with a pointer to this variable.

Table 1. enum v4l2_priority

V4L2_PRIORITY_UNSET0 
V4L2_PRIORITY_BACKGROUND1Lowest priority, usually applications running in background, for example monitoring VBI transmissions. A proxy application running in user space will be necessary if multiple applications want to read from a device at this priority.
V4L2_PRIORITY_INTERACTIVE2 
V4L2_PRIORITY_DEFAULT2Medium priority, usually applications started and interactively controlled by the user. For example TV viewers, Teletext browsers, or just "panel" applications to change the channel or video controls. This is the default priority unless an application requests another.
V4L2_PRIORITY_RECORD3Highest priority. Only one file descriptor can have this priority, it blocks any other fd from changing device properties. Usually applications which must not be interrupted, like video recording.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The requested priority value is invalid, or the driver does not support access priorities.

EBUSY

Another application already requested higher priority.

ioctl VIDIOC_G_SLICED_VBI_CAP

Name

VIDIOC_G_SLICED_VBI_CAP -- Query sliced VBI capabilities

Synopsis

int ioctl(int fd, int request, struct v4l2_sliced_vbi_cap *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_SLICED_VBI_CAP

argp

Description

To find out which data services are supported by a sliced VBI capture or output device, applications can call the VIDIOC_G_SLICED_VBI_CAP ioctl. It takes a pointer to a struct v4l2_sliced_vbi_cap which is filled by the driver.

Table 1. struct v4l2_sliced_vbi_cap

__u16service_setA set of all data services supported by the driver. Equal to the union of all elements of the service_lines array.
__u16service_lines[2][24]Each element of this array contains a set of data services the hardware can look for or insert into a particular scan line. Data services are defined in Table 2>. Array indices map to ITU-R line numbers (see also Figure 4-2> and Figure 4-3>) as follows:
  Element525 line systems625 line systems
  service_lines[0][1]11
  service_lines[0][23]2323
  service_lines[1][1]264314
  service_lines[1][23]286336
     
  The number of VBI lines the hardware can capture or output per frame, or the number of services it can identify on a given line may be limited. For example on PAL line 16 the hardware may be able to look for a VPS or Teletext signal, but not both at the same time. Applications can learn about these limits using the VIDIOC_S_FMT ioctl as described in Section 4.7>.
     
  Drivers must set service_lines[0][0] and service_lines[1][0] to zero.
__u32reserved[4]This array is reserved for future extensions. Drivers must set it to zero.

Table 2. Sliced VBI services

SymbolValueReferenceLines, usuallyPayload
V4L2_SLICED_TELETEXT_B (Teletext System B)0x0001TELETEXT>PAL/SECAM line 7-22, 320-335 (second field 7-22)Last 42 of the 45 byte Teletext packet, that is without clock run-in and framing code, lsb first transmitted.
V4L2_SLICED_VPS0x0400VPS>PAL line 16Byte number 3 to 15 according to Figure 9 of ETS 300 231, lsb first transmitted.
V4L2_SLICED_CAPTION_5250x1000EIA608>NTSC line 21, 284 (second field 21)Two bytes in transmission order, including parity bit, lsb first transmitted.
V4L2_SLICED_WSS_6250x4000WSS>PAL/SECAM line 23
Byte 0 1
msb lsb msb lsb
Bit 7 6 5 4 3 2 1 0 x x 13 12 11 10 9
V4L2_SLICED_VBI_5250x1000Set of services applicable to 525 line systems.
V4L2_SLICED_VBI_6250x4401Set of services applicable to 625 line systems.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The device does not support sliced VBI capturing or output.

ioctl VIDIOC_G_STD, VIDIOC_S_STD

Name

VIDIOC_G_STD, VIDIOC_S_STD -- Query or select the video standard of the current input

Synopsis

int ioctl(int fd, int request, v4l2_std_id *argp);

int ioctl(int fd, int request, const v4l2_std_id *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_STD, VIDIOC_S_STD

argp

Description

To query and select the current video standard applications use the VIDIOC_G_STD and VIDIOC_S_STD ioctls which take a pointer to a v4l2_std_id type as argument. VIDIOC_G_STD can return a single flag or a set of flags as in struct v4l2_standard field id. The flags must be unambiguous such that they appear in only one enumerated v4l2_standard structure.

VIDIOC_S_STD accepts one or more flags, being a write-only ioctl it does not return the actual new standard as VIDIOC_G_STD does. When no flags are given or the current input does not support the requested standard the driver returns an EINVAL error code. When the standard set is ambiguous drivers may return EINVAL or choose any of the requested standards.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

This ioctl is not supported, or the VIDIOC_S_STD parameter was unsuitable.

ioctl VIDIOC_G_TUNER, VIDIOC_S_TUNER

Name

VIDIOC_G_TUNER, VIDIOC_S_TUNER -- Get or set tuner attributes

Synopsis

int ioctl(int fd, int request, struct v4l2_tuner *argp);

int ioctl(int fd, int request, const struct v4l2_tuner *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_G_TUNER, VIDIOC_S_TUNER

argp

Description

To query the attributes of a tuner applications initialize the index field and zero out the reserved array of a struct v4l2_tuner and call the VIDIOC_G_TUNER ioctl with a pointer to this structure. Drivers fill the rest of the structure or return an EINVAL error code when the index is out of bounds. To enumerate all tuners applications shall begin at index zero, incrementing by one until the driver returns EINVAL.

Tuners have two writable properties, the audio mode and the radio frequency. To change the audio mode, applications initialize the index, audmode and reserved fields and call the VIDIOC_S_TUNER ioctl. This will not change the current tuner, which is determined by the current video input. Drivers may choose a different audio mode if the requested mode is invalid or unsupported. Since this is a write-only ioctl, it does not return the actually selected audio mode.

To change the radio frequency the VIDIOC_S_FREQUENCY ioctl is available.

Table 1. struct v4l2_tuner

__u32indexIdentifies the tuner, set by the application.
__u8name[32]

Name of the tuner, a NUL-terminated ASCII string. This information is intended for the user.

enum v4l2_tuner_typetypeType of the tuner, see Table 2>.
__u32capability

Tuner capability flags, see Table 3>. Audio flags indicate the ability to decode audio subprograms. They will not change, for example with the current video standard.

When the structure refers to a radio tuner only the V4L2_TUNER_CAP_LOW and V4L2_TUNER_CAP_STEREO flags can be set.

__u32rangelowThe lowest tunable frequency in units of 62.5 kHz, or if the capability flag V4L2_TUNER_CAP_LOW is set, in units of 62.5 Hz.
__u32rangehighThe highest tunable frequency in units of 62.5 kHz, or if the capability flag V4L2_TUNER_CAP_LOW is set, in units of 62.5 Hz.
__u32rxsubchans

Some tuners or audio decoders can determine the received audio subprograms by analyzing audio carriers, pilot tones or other indicators. To pass this information drivers set flags defined in Table 4> in this field. For example:

  V4L2_TUNER_SUB_MONOreceiving mono audio
  STEREO | SAPreceiving stereo audio and a secondary audio program
  MONO | STEREOreceiving mono or stereo audio, the hardware cannot distinguish
  LANG1 | LANG2receiving bilingual audio
  MONO | STEREO | LANG1 | LANG2receiving mono, stereo or bilingual audio
  

When the V4L2_TUNER_CAP_STEREO, _LANG1, _LANG2 or _SAP flag is cleared in the capability field, the corresponding V4L2_TUNER_SUB_ flag must not be set here.

This field is valid only if this is the tuner of the current video input, or when the structure refers to a radio tuner.

__u32audmode

The selected audio mode, see Table 5> for valid values. The audio mode does not affect audio subprogram detection, and like a control it does not automatically change unless the requested mode is invalid or unsupported. See Table 6> for possible results when the selected and received audio programs do not match.

Currently this is the only field of struct v4l2_tuner applications can change.

__u32signalThe signal strength if known, ranging from 0 to 65535. Higher values indicate a better signal.
__s32afcAutomatic frequency control: When the afc value is negative, the frequency is too low, when positive too high.
__u32reserved[4]Reserved for future extensions. Drivers and applications must set the array to zero.

Table 2. enum v4l2_tuner_type

V4L2_TUNER_RADIO1 
V4L2_TUNER_ANALOG_TV2 

Table 3. Tuner and Modulator Capability Flags

V4L2_TUNER_CAP_LOW0x0001When set, tuning frequencies are expressed in units of 62.5 Hz, otherwise in units of 62.5 kHz.
V4L2_TUNER_CAP_NORM0x0002This is a multi-standard tuner; the video standard can or must be switched. (B/G PAL tuners for example are typically not considered multi-standard because the video standard is automatically determined from the frequency band.) The set of supported video standards is available from the struct v4l2_input pointing to this tuner, see the description of ioctl VIDIOC_ENUMINPUT for details. Only V4L2_TUNER_ANALOG_TV tuners can have this capability.
V4L2_TUNER_CAP_STEREO0x0010Stereo audio reception is supported.
V4L2_TUNER_CAP_LANG10x0040Reception of the primary language of a bilingual audio program is supported. Bilingual audio is a feature of two-channel systems, transmitting the primary language monaural on the main audio carrier and a secondary language monaural on a second carrier. Only V4L2_TUNER_ANALOG_TV tuners can have this capability.
V4L2_TUNER_CAP_LANG20x0020Reception of the secondary language of a bilingual audio program is supported. Only V4L2_TUNER_ANALOG_TV tuners can have this capability.
V4L2_TUNER_CAP_SAP0x0020

Reception of a secondary audio program is supported. This is a feature of the BTSC system which accompanies the NTSC video standard. Two audio carriers are available for mono or stereo transmissions of a primary language, and an independent third carrier for a monaural secondary language. Only V4L2_TUNER_ANALOG_TV tuners can have this capability.

Note the V4L2_TUNER_CAP_LANG2 and V4L2_TUNER_CAP_SAP flags are synonyms. V4L2_TUNER_CAP_SAP applies when the tuner supports the V4L2_STD_NTSC_M video standard.

Table 4. Tuner Audio Reception Flags

V4L2_TUNER_SUB_MONO0x0001The tuner receives a mono audio signal.
V4L2_TUNER_SUB_STEREO0x0002The tuner receives a stereo audio signal.
V4L2_TUNER_SUB_LANG10x0008The tuner receives the primary language of a bilingual audio signal. Drivers must clear this flag when the current video standard is V4L2_STD_NTSC_M.
V4L2_TUNER_SUB_LANG20x0004The tuner receives the secondary language of a bilingual audio signal (or a second audio program).
V4L2_TUNER_SUB_SAP0x0004The tuner receives a Second Audio Program. Note the V4L2_TUNER_SUB_LANG2 and V4L2_TUNER_SUB_SAP flags are synonyms. The V4L2_TUNER_SUB_SAP flag applies when the current video standard is V4L2_STD_NTSC_M.

Table 5. Tuner Audio Modes

V4L2_TUNER_MODE_MONO0Play mono audio. When the tuner receives a stereo signal this a down-mix of the left and right channel. When the tuner receives a bilingual or SAP signal this mode selects the primary language.
V4L2_TUNER_MODE_STEREO1Play stereo audio. When the tuner receives bilingual audio it may play different languages on the left and right channel or the primary language on both channels. When it receives no stereo signal or does not support stereo reception the driver shall behave as in mono mode.
V4L2_TUNER_MODE_LANG13Play the primary language, mono or stereo. Only V4L2_TUNER_ANALOG_TV tuners support this mode.
V4L2_TUNER_MODE_LANG22Play the secondary language, mono. When the tuner receives no bilingual audio or SAP, or their reception is not supported the driver shall fall back to mono or stereo mode. Only V4L2_TUNER_ANALOG_TV tuners support this mode.
V4L2_TUNER_MODE_SAP2Play the Second Audio Program. When the tuner receives no bilingual audio or SAP, or their reception is not supported the driver shall fall back to mono or stereo mode. Only V4L2_TUNER_ANALOG_TV tuners support this mode. Note the V4L2_TUNER_MODE_LANG2 and V4L2_TUNER_MODE_SAP are synonyms.

Table 6. Tuner Audio Matrix

 Selected V4L2_TUNER_MODE_
Received V4L2_TUNER_SUB_MONOSTEREOLANG1LANG2 / SAP
MONOMonoMono/MonoMonoMono
MONO | SAPMonoMono/MonoMonoSAP
STEREOL+RL/RStereo L/R (preferred) or Mono L+RStereo L/R (preferred) or Mono L+R
STEREO | SAPL+RL/RStereo L/R (preferred) or Mono L+RSAP
LANG1 | LANG2Language 1Lang1/Lang2 (preferred) or Lang1/Lang1Language 1Language 2

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_tuner index is out of bounds.

ioctl VIDIOC_LOG_STATUS

Name

VIDIOC_LOG_STATUS -- Log driver status information

Synopsis

int ioctl(int fd, int request);

Description

As the video/audio devices become more complicated it becomes harder to debug problems. When this ioctl is called the driver will output the current device status to the kernel log. This is particular useful when dealing with problems like no sound, no video and incorrectly tuned channels. Also many modern devices autodetect video and audio standards and this ioctl will report what the device thinks what the standard is. Mismatches may give an indication where the problem is.

This ioctl is optional and not all drivers support it. It was introduced in Linux 2.6.15.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The driver does not support this ioctl.

ioctl VIDIOC_OVERLAY

Name

VIDIOC_OVERLAY -- Start or stop video overlay

Synopsis

int ioctl(int fd, int request, const int *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_OVERLAY

argp

Description

This ioctl is part of the video overlay I/O method. Applications call VIDIOC_OVERLAY to start or stop the overlay. It takes a pointer to an integer which must be set to zero by the application to stop overlay, to one to start.

Drivers do not support VIDIOC_STREAMON or VIDIOC_STREAMOFF with V4L2_BUF_TYPE_VIDEO_OVERLAY.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

Video overlay is not supported, or the parameters have not been set up. See Section 4.2> for the necessary steps.

ioctl VIDIOC_QBUF, VIDIOC_DQBUF

Name

VIDIOC_QBUF, VIDIOC_DQBUF -- Exchange a buffer with the driver

Synopsis

int ioctl(int fd, int request, struct v4l2_buffer *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_QBUF, VIDIOC_DQBUF

argp

Description

Applications call the VIDIOC_QBUF ioctl to enqueue an empty (capturing) or filled (output) buffer in the driver's incoming queue. The semantics depend on the selected I/O method.

To enqueue a memory mapped buffer applications set the type field of a struct v4l2_buffer to the same buffer type as previously struct v4l2_format type and struct v4l2_requestbuffers type, the memory field to V4L2_MEMORY_MMAP and the index field. Valid index numbers range from zero to the number of buffers allocated with VIDIOC_REQBUFS (struct v4l2_requestbuffers count) minus one. The contents of the struct v4l2_buffer returned by a VIDIOC_QUERYBUF ioctl will do as well. When the buffer is intended for output (type is V4L2_BUF_TYPE_VIDEO_OUTPUT or V4L2_BUF_TYPE_VBI_OUTPUT) applications must also initialize the bytesused, field and timestamp fields. See Section 3.5> for details. When VIDIOC_QBUF is called with a pointer to this structure the driver sets the V4L2_BUF_FLAG_MAPPED and V4L2_BUF_FLAG_QUEUED flags and clears the V4L2_BUF_FLAG_DONE flag in the flags field, or it returns an EINVAL error code.

To enqueue a user pointer buffer applications set the type field of a struct v4l2_buffer to the same buffer type as previously struct v4l2_format type and struct v4l2_requestbuffers type, the memory field to V4L2_MEMORY_USERPTR and the m.userptr field to the address of the buffer and length to its size. When the buffer is intended for output additional fields must be set as above. When VIDIOC_QBUF is called with a pointer to this structure the driver sets the V4L2_BUF_FLAG_QUEUED flag and clears the V4L2_BUF_FLAG_MAPPED and V4L2_BUF_FLAG_DONE flags in the flags field, or it returns an error code. This ioctl locks the memory pages of the buffer in physical memory, they cannot be swapped out to disk. Buffers remain locked until dequeued, until the VIDIOC_STREAMOFF or VIDIOC_REQBUFS ioctl are called, or until the device is closed.

Applications call the VIDIOC_DQBUF ioctl to dequeue a filled (capturing) or displayed (output) buffer from the driver's outgoing queue. They just set the type and memory fields of a struct v4l2_buffer as above, when VIDIOC_DQBUF is called with a pointer to this structure the driver fills the remaining fields or returns an error code.

By default VIDIOC_DQBUF blocks when no buffer is in the outgoing queue. When the O_NONBLOCK flag was given to the open() function, VIDIOC_DQBUF returns immediately with an EAGAIN error code when no buffer is available.

The v4l2_buffer structure is specified in Section 3.5>.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EAGAIN

Non-blocking I/O has been selected using O_NONBLOCK and no buffer was in the outgoing queue.

EINVAL

The buffer type is not supported, or the index is out of bounds, or no buffers have been allocated yet, or the userptr or length are invalid.

ENOMEM

Insufficient memory to enqueue a user pointer buffer.

EIO

VIDIOC_DQBUF failed due to an internal error. Can also indicate temporary problems like signal loss. Note the driver might dequeue an (empty) buffer despite returning an error, or even stop capturing.

ioctl VIDIOC_QUERYBUF

Name

VIDIOC_QUERYBUF -- Query the status of a buffer

Synopsis

int ioctl(int fd, int request, struct v4l2_buffer *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_QUERYBUF

argp

Description

This ioctl is part of the memory mapping I/O method. It can be used to query the status of a buffer at any time after buffers have been allocated with the VIDIOC_REQBUFS ioctl.

Applications set the type field of a struct v4l2_buffer to the same buffer type as previously struct v4l2_format type and struct v4l2_requestbuffers type, and the index field. Valid index numbers range from zero to the number of buffers allocated with VIDIOC_REQBUFS (struct v4l2_requestbuffers count) minus one. After calling VIDIOC_QUERYBUF with a pointer to this structure drivers return an error code or fill the rest of the structure.

In the flags field the V4L2_BUF_FLAG_MAPPED, V4L2_BUF_FLAG_QUEUED and V4L2_BUF_FLAG_DONE flags will be valid. The memory field will be set to V4L2_MEMORY_MMAP, the m.offset contains the offset of the buffer from the start of the device memory, the length field its size. The driver may or may not set the remaining fields and flags, they are meaningless in this context.

The v4l2_buffer structure is specified in Section 3.5>.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The buffer type is not supported, or the index is out of bounds.

ioctl VIDIOC_QUERYCAP

Name

VIDIOC_QUERYCAP -- Query device capabilities

Synopsis

int ioctl(int fd, int request, struct v4l2_capability *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_QUERYCAP

argp

Description

All V4L2 devices support the VIDIOC_QUERYCAP ioctl. It is used to identify kernel devices compatible with this specification and to obtain information about individual hardware capabilities. The ioctl takes a pointer to a struct v4l2_capability which is filled by the driver. When the driver is not compatible with this specification the ioctl returns the EINVAL error code.

Table 1. struct v4l2_capability

__u8driver[16]

Name of the driver, a unique NUL-terminated ASCII string. For example: "bttv". Driver specific applications shall use this information to verify the driver identity. It is also useful to work around known bugs, or to print the driver name and version in an error report to aid debugging. The driver version is stored in the version field.

Storing strings in fixed sized arrays is bad practice but unavoidable here. Drivers and applications should take precautions to never read or write beyond the end of the array and to properly terminate the strings.

__u8card[32]Name of the device, a NUL-terminated ASCII string. For example: "Yoyodyne TV/FM". Remember that one driver may support different brands or models of video hardware. This information can be used to build a menu of available devices for a device-select user interface. Since drivers may support multiple installed devices this name should be combined with the bus_info string to avoid ambiguities.
__u8bus_info[32]Location of the device in the system, a NUL-terminated ASCII string. For example: "PCI Slot 4". This information is intended for the user, to distinguish multiple identical devices. If no such information is available the field may simply count the devices controlled by the driver, or contain the empty string (bus_info[0] = 0). [pci_dev->slot_name example].
__u32version

Version number of the driver. Together with the driver field this identifies a particular driver. The version number is formatted using the KERNEL_VERSION() macro:

#define KERNEL_VERSION(a,b,c) (((a) << 16) + ((b) << 8) + (c))
__u32 version = KERNEL_VERSION(0, 8, 1);
printf ("Version: %u.%u.%u\n",
(version >> 16) & 0xFF,
(version >> 8) & 0xFF,
version & 0xFF);

__u32capabilitiesDevice capabilities, see Table 2>.
__u32reserved[4]Reserved for future extensions. Drivers must set this array to zero.

Table 2. Device Capabilities Flags

V4L2_CAP_VIDEO_CAPTURE0x00000001The device supports the video capture interface.
V4L2_CAP_VIDEO_OUTPUT0x00000002The device supports the video output interface.
V4L2_CAP_VIDEO_OVERLAY0x00000004The device supports the video overlay interface. Overlay typically stores captured images directly in the video memory of a graphics card, with support for clipping.
V4L2_CAP_VBI_CAPTURE0x00000010The device supports the VBI capture interface, see Section 4.6>, Section 4.7>.
V4L2_CAP_VBI_OUTPUT0x00000020The device supports the VBI output interface, see Section 4.6>, Section 4.7>.
V4L2_CAP_RDS_CAPTURE0x00000100[to be defined]
V4L2_CAP_TUNER0x00010000The device has some sort of tuner or modulator to receive or emit RF-modulated video signals. For more information see Section 1.6>.
V4L2_CAP_AUDIO0x00020000The device has audio inputs or outputs. For more information see Section 1.5>. It may or may not support PCM sampling or output, this function must be implemented as ALSA or OSS PCM interface.
V4L2_CAP_READWRITE0x01000000The device supports the read() and/or write() I/O methods.
V4L2_CAP_ASYNCIO0x02000000The device supports the asynchronous I/O methods.
V4L2_CAP_STREAMING0x04000000The device supports the streaming I/O method.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The kernel device is not compatible with this specification.

ioctl VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU

Name

VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU -- Enumerate controls and menu control items

Synopsis

int ioctl(int fd, int request, struct v4l2_queryctrl *argp);

int ioctl(int fd, int request, struct v4l2_querymenu *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_QUERYCTRL, VIDIOC_QUERYMENU

argp

Description

To query the attributes of a control applications set the id field of a struct v4l2_queryctrl and call the VIDIOC_QUERYCTRL ioctl with a pointer to this structure. The driver fills the rest of the structure or returns an EINVAL error code when the id is invalid.

It is possible to enumerate controls by calling VIDIOC_QUERYCTRL with successive id values starting from V4L2_CID_BASE up to and exclusive V4L2_CID_BASE_LASTP1, or starting from V4L2_CID_PRIVATE_BASE until the driver returns EINVAL. When the V4L2_CTRL_FLAG_DISABLED flag is set in the flags field, this control is permanently disabled and should be ignored by the application.[25]

Additional information is required for menu controls, the name of menu items. To query them applications set the id and index fields of struct v4l2_querymenu and call the VIDIOC_QUERYMENU ioctl with a pointer to this structure. The driver fills the rest of the structure or returns an EINVAL error code when the id or index is invalid. Menu items are enumerated by calling VIDIOC_QUERYMENU with successive index values from struct v4l2_queryctrl minimum (0) to maximum, inclusive.

Table 1. struct v4l2_queryctrl

__u32idIdentifies the control, set by the application. See Table 1-1> for predefined IDs.
enum v4l2_ctrl_typetypeType of control, see Table 3>.
__u8name[32]Name of the control, a NUL-terminated ASCII string. This information is intended for the user.
__s32minimumMinimum value, inclusive. This field is mostly useful to define a lower bound for integer controls. Note the value is signed.
__s32maximumMaximum value, inclusive. Note the value is signed.
__s32step

Generally drivers should not scale hardware control values. It may be necessary for example when the name or id imply a particular unit and the hardware actually accepts only multiples of said unit. If so, drivers must take care values are properly rounded when scaling, such that errors will not accumulate on repeated read-write cycles.

This field reports the smallest change of an integer control actually affecting hardware. Often the information is needed when the user can change controls by keyboard or GUI buttons, rather than a slider. When for example a hardware register accepts values 0-511 and the driver reports 0-65535, step should be 128.

Note although signed, the step value is supposed to be always positive.

__s32default_valueThe default value of the control. Drivers reset controls only when the driver is loaded, not later, in particular not when the open() is called.
__u32flagsControl flags, see Table 4>.
__u32reserved[2]Reserved for future extensions. Drivers must set the array to zero.

Table 2. struct v4l2_querymenu

__u32idIdentifies the control, set by the application from the respective struct v4l2_queryctrl id.
__u32indexIndex of the menu item, starting at zero, set by the application.
__u8name[32]Name of the menu item, a NUL-terminated ASCII string. This information is intended for the user.
__u32reservedReserved for future extensions. Drivers must set the array to zero.

Table 3. enum v4l2_ctrl_type

TypeminimumstepmaximumDescription
V4L2_CTRL_TYPE_INTEGERlow valueincrement (positive) [__u32?]high valueAn integer-valued control ranging from minimum to maximum inclusive. The step value indicates the increment between values which are actually different on the hardware.
V4L2_CTRL_TYPE_BOOLEAN011A boolean-valued control. Zero corresponds to "disabled", and one means "enabled".
V4L2_CTRL_TYPE_MENU01N-1The control has a menu of N choices. The names of the menu items can be enumerated with the VIDIOC_QUERYMENU ioctl.
V4L2_CTRL_TYPE_BUTTON000A control which performs an action when set. Drivers must ignore the value passed with VIDIOC_S_CTRL and return an EINVAL error code on a VIDIOC_G_CTRL attempt.

Table 4. Control Flags

V4L2_CTRL_FLAG_DISABLED0x0001This control is permanently disabled and should be ignored by the application. An attempt to change this control results in an EINVAL error code.
V4L2_CTRL_FLAG_GRABBED0x0002This control is temporarily unchangeable, for example because another application took over control of the respective resource. Such controls may be displayed specially in a user interface. An attempt to change a "grabbed" control results in an EBUSY error code.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

The struct v4l2_queryctrl id is invalid. The struct v4l2_querymenu id or index is invalid.

ioctl VIDIOC_QUERYSTD

Name

VIDIOC_QUERYSTD -- Sense the video standard received by the current input

Synopsis

int ioctl(int fd, int request, v4l2_std_id *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_QUERYSTD

argp

Description

The hardware may be able to detect the current video standard automatically. To do so, applications call VIDIOC_QUERYSTD with a pointer to a v4l2_std_id type. The driver stores here a set of candidates, this can be a single flag or a set of supported standards if for example the hardware can only distinguish between 50 and 60 Hz systems. When detection is not possible or fails, the set must contain all standards supported by the current video input or output.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

This ioctl is not supported.

ioctl VIDIOC_REQBUFS

Name

VIDIOC_REQBUFS -- Initiate Memory Mapping or User Pointer I/O

Synopsis

int ioctl(int fd, int request, struct v4l2_requestbuffers *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_REQBUFS

argp

Description

This ioctl is used to initiate memory mapped or user pointer I/O. Memory mapped buffers are located in device memory and must be allocated with this ioctl before they can be mapped into the application's address space. User buffers are allocated by applications themselves, and this ioctl is merely used to switch the driver into user pointer I/O mode.

To allocate device buffers applications initialize three fields of a v4l2_requestbuffers structure. They set the type field to the respective stream or buffer type, the count field to the desired number of buffers, and memory must be set to V4L2_MEMORY_MMAP. When the ioctl is called with a pointer to this structure the driver attempts to allocate the requested number of buffers and stores the actual number allocated in the count field. It can be smaller than the number requested, even zero, when the driver runs out of free memory. A larger number is possible when the driver requires more buffers to function correctly.[26] When memory mapping I/O is not supported the ioctl returns an EINVAL error code.

Applications can call VIDIOC_REQBUFS again to change the number of buffers, however this cannot succeed when any buffers are still mapped. A count value of zero frees all buffers, after aborting or finishing any DMA in progress, an implicit VIDIOC_STREAMOFF.

To negotiate user pointer I/O, applications initialize only the type field and set memory to V4L2_MEMORY_USERPTR. When the ioctl is called with a pointer to this structure the driver prepares for user pointer I/O, when this I/O method is not supported the ioctl returns an EINVAL error code.

Table 1. struct v4l2_requestbuffers

__u32countThe number of buffers requested or granted. This field is only used when memory is set to V4L2_MEMORY_MMAP.
enum v4l2_buf_typetypeType of the stream or buffers, this is the same as the struct v4l2_format type field. See Table 3-2> for valid values.
enum v4l2_memorymemoryApplications set this field to V4L2_MEMORY_MMAP or V4L2_MEMORY_USERPTR.
__u32reserved[32]A place holder for future extensions and custom (driver defined) buffer types V4L2_BUF_TYPE_PRIVATE and higher.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EBUSY

The driver supports multiple opens and I/O is already in progress, or reallocation of buffers was attempted although one or more are still mapped.

EINVAL

The buffer type (type field) or the requested I/O method (memory) is not supported.

ioctl VIDIOC_STREAMON, VIDIOC_STREAMOFF

Name

VIDIOC_STREAMON, VIDIOC_STREAMOFF -- Start or stop streaming I/O

Synopsis

int ioctl(int fd, int request, const int *argp);

Arguments

fd

File descriptor returned by open().

request

VIDIOC_STREAMON, VIDIOC_STREAMOFF

argp

Description

The VIDIOC_STREAMON and VIDIOC_STREAMOFF ioctl start and stop the capture or output process during streaming (memory mapping or user pointer) I/O.

Specifically the capture hardware is disabled and no input buffers are filled (if there are any empty buffers in the incoming queue) until VIDIOC_STREAMON has been called. Accordingly the output hardware is disabled, no video signal is produced until VIDIOC_STREAMON has been called. The ioctl will succeed only when at least one output buffer is in the incoming queue.

The VIDIOC_STREAMOFF ioctl, apart of aborting or finishing any DMA in progress, unlocks any user pointer buffers locked in physical memory, and it removes all buffers from the incoming and outgoing queues. That means all images captured but not dequeued yet will be lost, likewise all images enqueued for output but not transmitted yet. I/O returns to the same state as after calling VIDIOC_REQBUFS and can be restarted accordingly.

Both ioctls take a pointer to an integer, the desired buffer or stream type. This is the same as struct v4l2_requestbuffers type.

Note applications can be preempted for unknown periods right before or after the VIDIOC_STREAMON or VIDIOC_STREAMOFF calls, there is no notion of starting or stopping "now". Buffer timestamps can be used to synchronize with other events.

Return Value

On success 0 is returned, on error -1 and the errno variable is set appropriately:

EINVAL

Streaming I/O is not supported, the buffer type is not supported, or no buffers have been allocated (memory mapping) or enqueued (output) yet.

V4L2 mmap()

Name

v4l2-mmap -- Map device memory into application address space

Synopsis

#include <unistd.h>
#include <sys/mman.h>

void *mmap(void *start, size_t length, int prot, int flags, int fd, off_t offset);

Arguments

start

Map the buffer to this address in the application's address space. When the MAP_FIXED flag is specified, start must be a multiple of the pagesize and mmap will fail when the specified address cannot be used. Use of this option is discouraged; applications should just specify a NULL pointer here.

length

Length of the memory area to map. This must be the same value as returned by the driver in the struct v4l2_buffer length field.

prot

The prot argument describes the desired memory protection. It must be set to PROT_READ | PROT_WRITE, indicating pages may be read and written. This is a technicality independent of the device type and direction of data exchange. Note device memory accesses may incur a performance penalty. It can happen when writing to capture buffers, when reading from output buffers, or always. When the application intends to modify buffers, other I/O methods may be more efficient.

flags

The flags parameter specifies the type of the mapped object, mapping options and whether modifications made to the mapped copy of the page are private to the process or are to be shared with other references.

MAP_FIXED requests that the driver selects no other address than the one specified. If the specified address cannot be used, mmap will fail. If MAP_FIXED is specified, start must be a multiple of the pagesize. Use of this option is discouraged.

One of the MAP_SHARED or MAP_PRIVATE flags must be set. MAP_SHARED allows to share this mapping with all other processes that map this object. MAP_PRIVATE requests copy-on-write semantics. We recommend to set MAP_SHARED. The MAP_PRIVATE, MAP_DENYWRITE, MAP_EXECUTABLE and MAP_ANON flags should not be set.

fd

File descriptor returned by open().

offset

Offset of the buffer in device memory. This must be the same value as returned by the driver in the struct v4l2_buffer m union offset field.

Description

The mmap() function asks to map length bytes starting at offset in the memory of the device specified by fd into the application address space, preferably at address start. This latter address is a hint only, and is usually specified as 0.

Suitable length and offset parameters are queried with the VIDIOC_QUERYBUF ioctl. Buffers must be allocated with the VIDIOC_REQBUFS ioctl before they can be queried.

To unmap buffers the munmap() function is used.

Return Value

On success mmap() returns a pointer to the mapped buffer. On error MAP_FAILED (-1) is returned, and the errno variable is set appropriately. Possible error codes are:

EBADF

fd is not a valid file descriptor.

EACCESS

fd is not open for reading and writing.

EINVAL

The start or length or offset are not suitable. (E.g., they are too large, or not aligned on a PAGESIZE boundary.) Or no buffers have been allocated with the VIDIOC_REQBUFS ioctl.

ENOMEM

No memory is available.

V4L2 munmap()

Name

v4l2-munmap -- Unmap device memory

Synopsis

#include <unistd.h>
#include <sys/mman.h>

int munmap(void *start, size_t length);

Arguments

start

Address of the mapped buffer as returned by the mmap() function.

length

Length of the mapped buffer. This must be the same value as given to mmap() and returned by the driver in the struct v4l2_buffer length field.

Description

Unmaps a previously with the mmap() function mapped buffer and frees it, if possible.

Return Value

On success munmap() returns 0, on failure -1 and the errno variable is set appropriately:

EINVAL

The start or length is incorrect, or no buffers have been mapped yet.

V4L2 open()

Name

v4l2-open -- Open a V4L2 device

Synopsis

#include <fcntl.h>

int open(const char *device_name, int flags);

Arguments

device_name

Device to be opened.

flags

Open flags. Access mode must be O_RDWR. This is just a technicality, input devices still support only reading and output devices only writing.

When the O_NONBLOCK flag is given, the read() function and the VIDIOC_DQBUF ioctl will return the EAGAIN error code when no data is available or no buffer is in the driver outgoing queue, otherwise these functions block until data becomes available. All V4L2 drivers exchanging data with applications must support the O_NONBLOCK flag.

Other flags have no effect.

Description

To open a V4L2 device applications call open() with the desired device name. This function has no side effects; all data format parameters, current input or output, control values or other properties remain unchanged. At the first open() call after loading the driver they will be reset to default values, drivers are never in an undefined state.

Return Value

On success open returns the new file descriptor. On error -1 is returned, and the errno variable is set appropriately. Possible error codes are:

EACCES

The caller has no permission to access the device.

EBUSY

The driver does not support multiple opens and the device is already in use.

ENXIO

No device corresponding to this device special file exists.

ENOMEM

Insufficient kernel memory was available.

EMFILE

The process already has the maximum number of files open.

ENFILE

The limit on the total number of files open on the system has been reached.

V4L2 poll()

Name

v4l2-poll -- Wait for some event on a file descriptor

Synopsis

#include <sys/poll.h>

int poll(struct pollfd *ufds, unsigned int nfds, int timeout);

Description

All drivers implementing the read() or write() function or streaming I/O must also support the poll() function. See the poll() manual page for details.

V4L2 read()

Name

v4l2-read -- Read from a V4L2 device

Synopsis

#include <unistd.h>

ssize_t read(int fd, void *buf, size_t count);

Arguments

fd

File descriptor returned by open().

buf

count

Description

read() attempts to read up to count bytes from file descriptor fd into the buffer starting at buf. The layout of the data in the buffer is discussed in the respective device interface section, see ##. If count is zero, read() returns zero and has no other results. If count is greater than SSIZE_MAX, the result is unspecified. Regardless of the count value each read() call will provide at most one frame (two fields) worth of data.

By default read() blocks until data becomes available. When the O_NONBLOCK flag was given to the open() function it returns immediately with an EAGAIN error code when no data is available. The select() or poll() functions can always be used to suspend execution until data becomes available. All drivers supporting the read() function must also support select() and poll().

Drivers can implement read functionality in different ways, using a single or multiple buffers and discarding the oldest or newest frames once the internal buffers are filled.

read() never returns a "snapshot" of a buffer being filled. Using a single buffer the driver will stop capturing when the application starts reading the buffer until the read is finished. Thus only the period of the vertical blanking interval is available for reading, or the capture rate must fall below the nominal frame rate of the video standard.

The behavior of read() when called during the active picture period or the vertical blanking separating the top and bottom field depends on the discarding policy. A driver discarding the oldest frames keeps capturing into an internal buffer, continuously overwriting the previously, not read frame, and returns the frame being received at the time of the read() call as soon as it is complete.

A driver discarding the newest frames stops capturing until the next read() call. The frame being received at read() time is discarded, returning the following frame instead. Again this implies a reduction of the capture rate to one half or less of the nominal frame rate. An example of this model is the video read mode of the "bttv" driver, initiating a DMA to user memory when read() is called and returning when the DMA finished.

In the multiple buffer model drivers maintain a ring of internal buffers, automatically advancing to the next free buffer. This allows continuous capturing when the application can empty the buffers fast enough. Again, the behavior when the driver runs out of free buffers depends on the discarding policy.

Applications can get and set the number of buffers used internally by the driver with the streaming parameter ioctls, see ##streaming-par. They are optional, however. The discarding policy is not reported and cannot be changed. For minimum requirements see the respective device interface section in ##.

Return Value

On success, the number of bytes read is returned. It is not an error if this number is smaller than the number of bytes requested, or the amount of data required for one frame. This may happen for example because read() was interrupted by a signal. On error, -1 is returned, and the errno variable is set appropriately. In this case the next read will start at the beginning of a new frame. Possible error codes are:

EAGAIN

Non-blocking I/O has been selected using O_NONBLOCK and no data was immediately available for reading.

EBADF

fd is not a valid file descriptor or is not open for reading, or the process already has the maximum number of files open.

EBUSY

The driver does not support multiple read streams and the device is already in use.

EFAULT

buf is outside your accessible address space.

EINTR

The call was interrupted by a signal before any data was read.

EIO

I/O error. This indicates some hardware problem or a failure to communicate with a remote device (USB camera etc.).

EINVAL

The read() function is not supported by this driver, not on this device, or generally not on this type of device.

V4L2 select()

Name

v4l2-select -- Synchronous I/O multiplexing

Synopsis

#include <sys/time.h>
#include <sys/types.h>
#include <unistd.h>

int select(int n, fd_set *readfds, fd_set *writefds, fd_set *exceptfds, struct timeval *timeout);

Description

All drivers implementing the read() or write() function or streaming I/O must also support the select() function. See the select() manual page for details.

V4L2 write()

Name

v4l2-write -- Write to a V4L2 device

Synopsis

#include <unistd.h>

ssize_t write(int fd, void *buf, size_t count);

Arguments

fd

File descriptor returned by open().

buf

count

Description

write() writes up to count bytes to the device referenced by the file descriptor fd from the buffer starting at buf. If count is zero, 0 will be returned without causing any other effect. [implementation tbd]

When the application does not provide more data in time, the previous frame is displayed again.

Return Value

On success, the number of bytes written are returned. Zero indicates nothing was written. [tbd] On error, -1 is returned, and the errno variable is set appropriately. In this case the next write will start at the beginning of a new frame. Possible error codes are:

EAGAIN

Non-blocking I/O has been selected using O_NONBLOCK and no buffer space was available to write the data immediately. [tbd]

EBADF

fd is not a valid file descriptor or is not open for writing.

EBUSY

The driver does not support multiple write streams and the device is already in use.

EFAULT

buf is outside your accessible address space.

EINTR

The call was interrupted by a signal before any data was written.

EIO

I/O error. This indicates some hardware problem.

EINVAL

The write() function is not supported by this driver, not on this device, or generally not on this type of device.


Chapter 5. V4L2 Driver Programming

to do


Chapter 6. History

The following chapters document the evolution of the V4L2 API, errata or extensions. They shall also aid application and driver writers porting their software to later versions of V4L2.


6.1. Differences between V4L and V4L2

The Video For Linux API was first introduced in Linux 2.1 to unify and replace various TV and radio device related interfaces, developped independently by driver writers in prior years. Starting with Linux 2.5 the much improved V4L2 API replaces the V4L API, although existing drivers will continue to support V4L in the future, either directly or through the V4L2 compatibility layer. For a transition period not all drivers will support the V4L2 API.


6.1.1. Opening and Closing Devices

For compatibility reasons the character device file names recommended for V4L2 video capture, overlay, radio, teletext and raw vbi capture devices did not change from those used by V4L. They are listed in Chapter 4> and below in Table 6-1>.

The V4L "videodev" module automatically assigns minor numbers to drivers in load order, depending on the registered device type. We recommend V4L2 drivers by default register devices with the same numbers, but in principle the system administrator can assign arbitrary minor numbers using driver module options. The major device number remains 81.

Table 6-1. V4L Device Types, Names and Numbers

Device TypeFile NameMinor Numbers
Video capture and overlay

/dev/video and /dev/bttv0a, /dev/video0 to /dev/video63

0-63
Radio receiver

/dev/radiob, /dev/radio0 to /dev/radio63

64-127
Teletext decoder

/dev/vtx, /dev/vtx0 to /dev/vtx31

192-223
Raw VBI capture

/dev/vbi, /dev/vbi0 to /dev/vbi15

224-239c

Notes:
a. According to Documentation/devices.txt these should be symbolic links to /dev/video0. Note the original bttv interface is not compatible with V4L or V4L2.
b. According to Documentation/devices.txt a symbolic link to /dev/radio0.
c. The range used to be 224-255. More device types may be added in the future, so you should expect more range splitting in the future.

V4L prohibits (or used to) multiple opens. V4L2 drivers may support multiple opens, see Section 1.1> for details and consequences.

V4L drivers respond to V4L2 ioctls with the EINVAL error code. The V4L2 "videodev" module backward compatibility layer can translate V4L ioctl requests to their V4L2 counterpart, however a V4L2 driver usually needs more preparation to become fully V4L compatible. This is covered in more detail in Chapter 5>.


6.1.2. Querying Capabilities

The V4L VIDIOCGCAP ioctl is equivalent to V4L2's VIDIOC_QUERYCAP.

The name field in struct video_capability became card in struct v4l2_capability, type was replaced by capabilities. Note V4L2 does not distinguish between device types like this, better think of basic video input, video output and radio devices supporting a set of related functions like video capturing, video overlay and VBI capturing. See Section 1.1> for an introduction.

struct video_capability typestruct v4l2_capability capabilities flagsPurpose
VID_TYPE_CAPTUREV4L2_CAP_VIDEO_CAPTUREThe video capture interface is supported.
VID_TYPE_TUNERV4L2_CAP_TUNERThe device has a tuner or modulator.
VID_TYPE_TELETEXTV4L2_CAP_VBI_CAPTUREThe raw VBI capture interface is supported.
VID_TYPE_OVERLAYV4L2_CAP_VIDEO_OVERLAYThe video overlay interface is supported.
VID_TYPE_CHROMAKEYV4L2_FBUF_CAP_CHROMAKEY in field capability of struct v4l2_framebufferWhether chromakey overlay is supported. For more information on overlay see Section 4.2>.
VID_TYPE_CLIPPINGV4L2_FBUF_CAP_LIST_CLIPPING and V4L2_FBUF_CAP_BITMAP_CLIPPING in field capability of struct v4l2_framebufferWhether clipping the overlaid image is supported, see Section 4.2>.
VID_TYPE_FRAMERAMV4L2_FBUF_CAP_EXTERNOVERLAY not set in field capability of struct v4l2_framebufferWhether overlay overwrites frame buffer memory, see Section 4.2>.
VID_TYPE_SCALES-This flag indicates if the hardware can scale images. The V4L2 API implies the scale factor by setting the cropping dimensions and image size with the VIDIOC_S_CROP and VIDIOC_S_FMT ioctl, respectively. The driver returns the closest sizes possible. For more information on cropping and scaling see Section 1.10>.
VID_TYPE_MONOCHROME-Applications can enumerate the supported image formats with the VIDIOC_ENUM_FMT ioctl to determine if the device supports grey scale capturing only. For more information on image formats see Chapter 2>.
VID_TYPE_SUBCAPTURE-Applications can call the VIDIOC_G_CROP ioctl to determine if the device supports capturing a subsection of the full picture ("cropping" in V4L2). If not, the ioctl returns the EINVAL error code. For more information on cropping and scaling see Section 1.10>.

The audios field was replaced by capabilities flag V4L2_CAP_AUDIO, indicating if the device has any audio inputs or outputs. To determine their number applications can enumerate audio inputs with the VIDIOC_G_AUDIO ioctl. The audio ioctls are described in Section 1.5>.

The maxwidth, maxheight, minwidth and minheight fields were removed. Calling the VIDIOC_S_FMT or VIDIOC_TRY_FMT ioctl with the desired dimensions returns the closest size possible, taking into account the current video standard, cropping and scaling.


6.1.3. Video Sources

V4L provides the VIDIOCGCHAN and VIDIOCSCHAN ioctl using struct video_channel to enumerate the video inputs of a V4L device. The equivalent V4L2 ioctls are VIDIOC_ENUMINPUT, VIDIOC_G_INPUT and VIDIOC_S_INPUT using struct v4l2_input as discussed in Section 1.4>.

The channel field counting inputs was renamed to index, the video input types were renamed:

struct video_channel typestruct v4l2_input type
VIDEO_TYPE_TVV4L2_INPUT_TYPE_TUNER
VIDEO_TYPE_CAMERAV4L2_INPUT_TYPE_CAMERA

Unlike the tuners field expressing the number of tuners of this input, V4L2 assumes each video input is associated with at most one tuner. On the contrary a tuner can have more than one input, i.e. RF connectors, and a device can have multiple tuners. The index of the tuner associated with the input, if any, is stored in field tuner of struct v4l2_input. Enumeration of tuners is discussed in Section 1.6>.

The redundant VIDEO_VC_TUNER flag was dropped. Video inputs associated with a tuner are of type V4L2_INPUT_TYPE_TUNER. The VIDEO_VC_AUDIO flag was replaced by the audioset field. V4L2 considers devices with up to 32 audio inputs. Each set bit in the audioset field represents one audio input this video input combines with. For information about audio inputs and how to switch see Section 1.5>.

The norm field describing the supported video standards was replaced by std. The V4L specification mentions a flag VIDEO_VC_NORM indicating whether the standard can be changed. This flag was a later addition together with the norm field and has been removed in the meantime. V4L2 has a similar, albeit more comprehensive approach to video standards, see Section 1.7> for more information.


6.1.4. Tuning

The V4L VIDIOCGTUNER and VIDIOCSTUNER ioctl and struct video_tuner can be used to enumerate the tuners of a V4L TV or radio device. The equivalent V4L2 ioctls are VIDIOC_G_TUNER and VIDIOC_S_TUNER using struct v4l2_tuner. Tuners are covered in Section 1.6>.

The tuner field counting tuners was renamed to index. The fields name, rangelow and rangehigh remained unchanged.

The VIDEO_TUNER_PAL, VIDEO_TUNER_NTSC and VIDEO_TUNER_SECAM flags indicating the supported video standards were dropped. This information is now contained in the associated struct v4l2_input. No replacement exists for the VIDEO_TUNER_NORM flag indicating whether the video standard can be switched. The mode field to select a different video standard was replaced by a whole new set of ioctls and structures described in Section 1.7>. Due to its ubiquity it should be mentioned the BTTV driver supports several standards in addition to the regular VIDEO_MODE_PAL (0), VIDEO_MODE_NTSC, VIDEO_MODE_SECAM and VIDEO_MODE_AUTO (3). Namely N/PAL Argentina, M/PAL, N/PAL, and NTSC Japan with numbers 3-6 (sic).

The VIDEO_TUNER_STEREO_ON flag indicating stereo reception became V4L2_TUNER_SUB_STEREO in field rxsubchans. This field also permits the detection of monaural and bilingual audio, see the definition of struct v4l2_tuner for details. Presently no replacement exists for the VIDEO_TUNER_RDS_ON and VIDEO_TUNER_MBS_ON flags.

The VIDEO_TUNER_LOW flag was renamed to V4L2_TUNER_CAP_LOW in the struct v4l2_tuner capability field.

The VIDIOCGFREQ and VIDIOCSFREQ ioctl to change the tuner frequency where renamed to VIDIOC_G_FREQUENCY and VIDIOC_S_FREQUENCY. They take a pointer to a struct v4l2_frequency instead of an unsigned long integer.


6.1.5. Image Properties

V4L2 has no equivalent of the VIDIOCGPICT and VIDIOCSPICT ioctl and struct video_picture. The following fields where replaced by V4L2 controls accessible with the VIDIOC_QUERYCTRL, VIDIOC_G_CTRL and VIDIOC_S_CTRL ioctls:

struct video_pictureV4L2 Control ID
brightnessV4L2_CID_BRIGHTNESS
hueV4L2_CID_HUE
colourV4L2_CID_SATURATION
contrastV4L2_CID_CONTRAST
whitenessV4L2_CID_WHITENESS

The V4L picture controls are assumed to range from 0 to 65535 with no particular reset value. The V4L2 API permits arbitrary limits and defaults which can be queried with the VIDIOC_QUERYCTRL ioctl. For general information about controls see Section 1.8>.

The depth (average number of bits per pixel) of a video image is implied by the selected image format. V4L2 does not explicitely provide such information assuming applications recognizing the format are aware of the image depth and others need not know. The palette field moved into the struct v4l2_pix_format:

struct video_picture palettestruct v4l2_pix_format pixfmt
VIDEO_PALETTE_GREY

V4L2_PIX_FMT_GREY

VIDEO_PALETTE_HI240

V4L2_PIX_FMT_HI240a

VIDEO_PALETTE_RGB565

V4L2_PIX_FMT_RGB565

VIDEO_PALETTE_RGB555

V4L2_PIX_FMT_RGB555

VIDEO_PALETTE_RGB24

V4L2_PIX_FMT_BGR24

VIDEO_PALETTE_RGB32

V4L2_PIX_FMT_BGR32b

VIDEO_PALETTE_YUV422

V4L2_PIX_FMT_YUYV

VIDEO_PALETTE_YUYVc

V4L2_PIX_FMT_YUYV

VIDEO_PALETTE_UYVY

V4L2_PIX_FMT_UYVY

VIDEO_PALETTE_YUV420None
VIDEO_PALETTE_YUV411

V4L2_PIX_FMT_Y41Pd

VIDEO_PALETTE_RAW

Nonee

VIDEO_PALETTE_YUV422P

V4L2_PIX_FMT_YUV422P

VIDEO_PALETTE_YUV411P

V4L2_PIX_FMT_YUV411Pf

VIDEO_PALETTE_YUV420P

V4L2_PIX_FMT_YVU420

VIDEO_PALETTE_YUV410P

V4L2_PIX_FMT_YVU410

Notes:
a. This is a custom format used by the BTTV driver, not one of the V4L2 standard formats.
b. Presumably all V4L RGB formats are little-endian, although some drivers might interpret them according to machine endianess. V4L2 defines little-endian, big-endian and red/blue swapped variants. For details see Section 2.3>.
c. VIDEO_PALETTE_YUV422 and VIDEO_PALETTE_YUYV are the same formats. Some V4L drivers respond to one, some to the other.
d. Not to be confused with V4L2_PIX_FMT_YUV411P, which is a planar format.
e. V4L explains this as: "RAW capture (BT848)"
f. Not to be confused with V4L2_PIX_FMT_Y41P, which is a packed format.

V4L2 image formats are defined in Chapter 2>. The image format can be selected with the VIDIOC_S_FMT ioctl.


6.1.6. Audio

The VIDIOCGAUDIO and VIDIOCSAUDIO ioctl and struct video_audio are used to enumerate the audio inputs of a V4L device. The equivalent V4L2 ioctls are VIDIOC_G_AUDIO and VIDIOC_S_AUDIO using struct v4l2_audio as discussed in Section 1.5>.

The audio "channel number" field counting audio inputs was renamed to index.

On VIDIOCSAUDIO the mode field selects one of the VIDEO_SOUND_MONO, VIDEO_SOUND_STEREO, VIDEO_SOUND_LANG1 or VIDEO_SOUND_LANG2 audio demodulation modes. When the current audio standard is BTSC VIDEO_SOUND_LANG2 refers to SAP and VIDEO_SOUND_LANG1 is meaningless. Also undocumented in the V4L specification, there is no way to query the selected mode. On VIDIOCGAUDIO the driver returns the actually received audio programmes in this field. In the V4L2 API this information is stored in the struct v4l2_tuner rxsubchans and audmode fields, respectively. See Section 1.6> for more information on tuners. Related to audio modes struct v4l2_audio also reports if this is a mono or stereo input, regardless if the source is a tuner.

The following fields where replaced by V4L2 controls accessible with the VIDIOC_QUERYCTRL, VIDIOC_G_CTRL and VIDIOC_S_CTRL ioctls:

struct video_audioV4L2 Control ID
volumeV4L2_CID_AUDIO_VOLUME
bassV4L2_CID_AUDIO_BASS
trebleV4L2_CID_AUDIO_TREBLE
balanceV4L2_CID_AUDIO_BALANCE

To determine which of these controls are supported by a driver V4L provides the flags VIDEO_AUDIO_VOLUME, VIDEO_AUDIO_BASS, VIDEO_AUDIO_TREBLE and VIDEO_AUDIO_BALANCE. In the V4L2 API the VIDIOC_QUERYCTRL ioctl reports if the respective control is supported. Accordingly the VIDEO_AUDIO_MUTABLE and VIDEO_AUDIO_MUTE flags where replaced by the boolean V4L2_CID_AUDIO_MUTE control.

All V4L2 controls have a step attribute replacing the struct video_audio step field. The V4L audio controls are assumed to range from 0 to 65535 with no particular reset value. The V4L2 API permits arbitrary limits and defaults which can be queried with the VIDIOC_QUERYCTRL ioctl. For general information about controls see Section 1.8>.


6.1.7. Frame Buffer Overlay

The V4L2 ioctls equivalent to VIDIOCGFBUF and VIDIOCSFBUF are VIDIOC_G_FBUF and VIDIOC_S_FBUF. The base field of struct video_buffer remained unchanged, except V4L2 using a flag to indicate non-destructive overlay instead of a NULL pointer. All other fields moved into the struct v4l2_pix_format substructure fmt of struct v4l2_framebuffer. The depth field was replaced by pixelformat. A conversion table is available in the Section 2.3>.

Instead of the special ioctls VIDIOCGWIN and VIDIOCSWIN V4L2 uses the general-purpose data format negotiation ioctls VIDIOC_G_FMT and VIDIOC_S_FMT. They take a pointer to a struct v4l2_format as argument, here the struct v4l2_window named win of its fmt union is used.

The x, y, width and height fields of struct video_window moved into struct v4l2_rect substructure w of struct v4l2_window. The chromakey, clips, and clipcount fields remained unchanged. Struct video_clip was renamed to struct v4l2_clip, also containing a struct v4l2_rect, but the semantics are still the same.

The VIDEO_WINDOW_INTERLACE flag was dropped, instead applications must set the field field to V4L2_FIELD_ANY or V4L2_FIELD_INTERLACED. The VIDEO_WINDOW_CHROMAKEY flag moved into struct v4l2_framebuffer, renamed to V4L2_FBUF_FLAG_CHROMAKEY.

In V4L, storing a bitmap pointer in clips and setting clipcount to VIDEO_CLIP_BITMAP (-1) requests bitmap clipping, using a fixed size bitmap of 1024 × 625 bits. Struct v4l2_window has a separate bitmap pointer field for this purpose and the bitmap size is determined by w.width and w.height.

The VIDIOCCAPTURE ioctl to enable or disable overlay was renamed to VIDIOC_OVERLAY.


6.1.8. Cropping

To capture only a subsection of the full picture V4L provides the VIDIOCGCAPTURE and VIDIOCSCAPTURE ioctl using struct video_capture. The equivalent V4L2 ioctls are VIDIOC_G_CROP and VIDIOC_S_CROP using struct v4l2_crop, and the related VIDIOC_CROPCAP ioctl. This is a rather complex matter, see Section 1.10> for details.

The x, y, width and height fields moved into struct v4l2_rect substructure c of struct v4l2_crop. The decimation field was dropped. The scaling factor is implied by the size of the cropping rectangle and the size of the captured or overlaid image.

The VIDEO_CAPTURE_ODD and VIDEO_CAPTURE_EVEN flags to capture only the odd or even field, respectively, were replaced by V4L2_FIELD_TOP and V4L2_FIELD_BOTTOM in the field named field of struct v4l2_pix_format and struct v4l2_window. These structures are used to determine the capture or overlay format with the VIDIOC_S_FMT ioctl.


6.1.9. Reading Images, Memory Mapping

6.1.9.1. Capturing using the read method

There is no essential difference between reading images from a V4L or V4L2 device using the read() function. Supporting this method is optional for V4L2 devices. Whether the function is available can be determined with the VIDIOC_QUERYCAP ioctl. All V4L2 devices exchanging data with applications must support the select() and poll() function.

To select an image format and size, V4L provides the VIDIOCSPICT and VIDIOCSWIN ioctls. V4L2 uses the general-purpose data format negotiation ioctls VIDIOC_G_FMT and VIDIOC_S_FMT. They take a pointer to a struct v4l2_format as argument, here the struct v4l2_pix_format named pix of its fmt union is used.

For more information about the V4L2 read interface see Section 3.1>.


6.1.9.2. Capturing using memory mapping

Applications can read from V4L devices by mapping buffers in device memory, or more often just buffers allocated in DMA-able system memory, into their address space. This avoids the data copy overhead of the read method. V4L2 supports memory mapping as well, with a few differences.

V4LV4L2
 The image format must be selected before buffers are allocated, with the VIDIOC_S_FMT ioctl. When no format is selected the driver may use the last, possibly by another application requested format.

Applications cannot change the number of buffers allocated. The number is built into the driver, unless it has a module option to change the number when the driver module is loaded.

The VIDIOC_REQBUFS ioctl allocates the desired number of buffers, this is a required step in the initialization sequence.

Drivers map all buffers as one contiguous range of memory. The VIDIOCGMBUF ioctl is available to query the number of buffers, the offset of each buffer from the start of the virtual file, and the overall amount of memory used, which can be used as arguments to the mmap() function.

Buffers are individually mapped. The offset and size of each buffer can be determined with the VIDIOC_QUERYBUF ioctl.

The VIDIOCMCAPTURE ioctl prepares a buffer for capturing. It also determines the image format for this buffer. The ioctl returns immediately, eventually with an EAGAIN error code if no video signal had been detected. When the driver supports more than one buffer applications can call the ioctl multiple times and thus have multiple outstanding capture requests.

The VIDIOCSYNC ioctl suspends execution until a particular buffer has been filled.

Drivers maintain an incoming and outgoing queue. VIDIOC_QBUF enqueues any empty buffer into the incoming queue. Filled buffers are dequeued from the outgoing queue with the VIDIOC_DQBUF ioctl. To wait until filled buffers become available this function, select() or poll() can be used. The VIDIOC_STREAMON ioctl must be called once after enqueuing one or more buffers to start capturing. Its counterpart VIDIOC_STREAMOFF stops capturing and dequeues all buffers from both queues. Applications can query the signal status, if known, with the VIDIOC_ENUMINPUT ioctl.

For a more in-depth discussion of memory mapping and examples, see Section 3.2>.


6.1.10. Reading Raw VBI Data

Originally the V4L API did not specify a raw VBI capture interface, merely the device file /dev/vbi was reserved for this purpose. The only driver supporting this interface was the BTTV driver, de-facto defining the V4L VBI interface. Reading from the device yields a raw VBI image with the following parameters:

struct v4l2_vbi_formatV4L, BTTV driver
sampling_rate28636363 Hz NTSC (precisely all 525-line standards); 35468950 Hz PAL and SECAM (625-line)
offset?
samples_per_line2048
sample_formatV4L2_PIX_FMT_GREY. The last four bytes (machine endianess integer) contain a frame counter.
start[]10, 273 NTSC; 22, 335 PAL and SECAM
count[]

16, 16a

flags0
Notes:
a. Old driver versions used different values, eventually the custom BTTV_VBISIZE ioctl was added to query the correct values.

Undocumented in the V4L specification, in Linux 2.3 the VIDIOCGVBIFMT and VIDIOCSVBIFMT ioctls using struct vbi_format were added to determine the VBI image parameters. These ioctls are only partially compatible with the V4L2 VBI interface specified in Section 4.6>.

An offset field does not exist, sample_format is supposed to be VIDEO_PALETTE_RAW, here equivalent to V4L2_PIX_FMT_GREY. The remaining fields are probably equivalent to struct v4l2_vbi_format.

Apparently only the Zoran (ZR 36120) driver implements these ioctls. The semantics differ from those specified for V4L2 in two ways. The parameters are reset on open() and VIDIOCSVBIFMT always returns the EINVAL error code if the parameters are invalid.


6.1.11. Miscellaneous

V4L2 has no equivalent of the VIDIOCGUNIT ioctl. Applications can find the VBI device associated with a video capture device (or vice versa) by reopening the device and requesting VBI data. For details see Section 1.1>.

Presently no replacement exists for VIDIOCKEY, the V4L functions regarding MPEG compression and decompression, and microcode programming. Drivers may implement the respective V4L ioctls for these purposes.


6.2. History of the V4L2 API

Soon after the V4L API was added to the kernel it was criticised as too inflexible. In August 1998 Bill Dirks proposed a number of improvements and began work on documentation, example drivers and applications. With the help of other volunteers this eventually became the V4L2 API, not just an extension but a replacement for the V4L API. However it took another four years and two stable kernel releases until the new API was finally accepted for inclusion into the kernel in its present form.


6.2.1. Early Versions

1998-08-20: First version.

1998-08-27: select() function was introduced.

1998-09-10: New video standard interface.

1998-09-18: The VIDIOC_NONCAP ioctl was replaced by the O_TRUNC open() flag (with synonym O_NONCAP/O_NOIO) to indicate a non-capturing open. The VIDEO_STD_XXX identifiers are now ordinals rather than bits, and video_std_construct helper function takes id and transmission as arguments.

1998-09-28: Revamped video standard. Made video controls individually enumerable.

1998-10-02: Removed id from video_standard, renamed color subcarrier fields. Renamed VIDIOC_QUERYSTD to VIDIOC_ENUMSTD and VIDIOC_G_INPUT to VIDIOC_ENUMINPUT. Added preliminary /proc/videodev file. First draft of CODEC driver API spec.

1998-11-08: Updating for many minor changes to the V4L2 spec. Most symbols have been renamed. Some material changes to v4l2_capability.

1998-11-12 bugfix: some of the VIDIOC_* symbols were not constructed with the right macros, which could lead to errors on what should have been valid ioctl() calls.

1998-11-14: V4L2_PIX_FMT_RGB24 changed to V4L2_PIX_FMT_BGR24. Same for RGB32. Audio UI controls moved to VIDIOC_S_CTRL system and assigned V4L2_CID_AUDIO_* symbols. Removed V4L2_MAJOR from videodev.h since it is only used at one place in videodev. Added YUV422 and YUV411 planar formats.

1998-11-28: Changed a few ioctl symbols. Added stuff for codec and video output devices.

1999-01-14: Added raw VBI interface.

1999-01-19: Removed VIDIOC_NEXTBUF ioctl.


6.2.2. V4L2 Version 0.16 1999-01-31

1999-01-27: There is now one QBUF ioctl, VIDIOC_QWBUF and VIDIOC_QRBUF are gone. VIDIOC_QBUF takes a v4l2_buffer as a parameter. Added digital zoom (cropping) controls.


6.2.3. V4L2 Version 0.18 1999-03-16

Added a v4l to V4L2 ioctl compatibility layer to videodev.c. Driver writers, this changes how you implement your ioctl handler. See the Driver Writer's Guide. Added some more control id codes.


6.2.4. V4L2 Version 0.19 1999-06-05

1999-03-18: Fill in the category and catname fields of v4l2_queryctrl objects before passing them to the driver. Required a minor change to the VIDIOC_QUERYCTRL handlers in the sample drivers.

1999-03-31: Better compatibility for v4l memory capture ioctls. Requires changes to drivers to fully support new compatibility features, see Driver Writer's Guide and v4l2cap.c. Added new control IDs: V4L2_CID_HFLIP, _VFLIP. Changed V4L2_PIX_FMT_YUV422P to _YUV422P, and _YUV411P to _YUV411P.

1999-04-04: Added a few more control IDs.

1999-04-07: Added the button control type.

1999-05-02: Fixed a typo in videodev.h, and added the V4L2_CTRL_FLAG_GRAYED (later V4L2_CTRL_FLAG_GRABBED) flag.

1999-05-20: Definition of VIDIOC_G_CTRL was wrong causing a malfunction of this ioctl.

1999-06-05: Changed the value of V4L2_CID_WHITENESS.


6.2.5. V4L2 Version 0.20 1999-09-10

Version 0.20 introduced a number of changes not backward compatible with 0.19 and earlier. The purpose was to simplify the API, while at the same time make it more extensible, and follow common Linux driver API conventions.

  1. Fixed typos in some V4L2_FMT_FLAG symbols. Changed struct v4l2_clip to be compatible with v4l. (1999-08-30)

  2. Added V4L2_TUNER_SUB_LANG1. (1999-09-05)

  3. All ioctl() commands that took an integer argument before, will now take a pointer to an integer. Where it makes sense, the driver will return the actual value used in the integer pointed to by the argument. This is a common convention, and also makes certain things easier in libv4l2 and other system code when the parameter to ioctl() is always a pointer. The ioctl commands affected are: VIDIOC_PREVIEW, VIDIOC_STREAMON, VIDIOC_STREAMOFF, VIDIOC_S_FREQ, VIDIOC_S_INPUT, VIDIOC_S_OUTPUT, VIDIOC_S_EFFECT. For example, where before you might have had code like:

    err = ioctl (fd, VIDIOC_XXX, V4L2_XXX);
    that becomes
    int a = V4L2_XXX; err = ioctl(fd, VIDIOC_XXX, &a);

  4. All the different set-format ioctl() commands are swept into a single set-format command whose parameter consists of an integer value indicating the type of format, followed by the format data. The same for the get-format commands, of course. This will simplify the API by eliminating several ioctl codes and also make it possible to add additional kinds of data streams, or driver-private kinds of streams without having to add more set-format ioctls. The parameter to VIDIOC_S_FMT is as follows. The first field is a V4L2_BUF_TYPE_XXX value that indicates which stream the set-format is for, and implicitly, what type of format data. After that is a union of the different format structures. More can be added later without breaking backward compatibility. Nonstandard driver-private formats can be used by casting raw_data.

    struct v4l2_format {
    __u32 type;
    union {
    struct v4l2_pix_format pix;
    struct v4l2_vbi_format vbi;
    ... and so on ...
    __u8 raw_data[200];
    } fmt;
    };
    

    For a get-format, the application fills in the type field, and the driver fills in the rest. What was before the image format structure, struct v4l2_format, becomes struct v4l2_pix_format. These ioctls become obsolete: VIDIOC_S_INFMT, VIDIOC_G_INFMT, VIDIOC_S_OUTFMT, VIDIOC_G_OUTFMT, VIDIOC_S_VBIFMT VIDIOC_G_VBIFMT.

  5. Similar to item 2, VIDIOC_G/S_PARM and VIDIOC_G/S_OUTPARM are merged, along with the corresponding 'get' functions. A type field will indicate which stream the parameters are for, set to a V4L2_BUF_TYPE_* value.

    struct v4l2_streamparm {
    __u32 type;
    union {
    struct v4l2_captureparm capture;
    struct v4l2_outputparm output;
    __u8 raw_data[200];
    } parm;
    };
    

    These ioctls become obsolete: VIDIOC_G_OUTPARM, VIDIOC_S_OUTPARM.

  6. The way controls are enumerated is simplified. Simultaneously, two new control flags are introduced and the existing flag is dropped. Also, the catname field is dropped in favor of a group name. To enumerate controls call VIDIOC_QUERYCTRL with successive id's starting from V4L2_CID_BASE or V4L2_CID_PRIVATE_BASE and stop when the driver returns the EINVAL error code. Controls that are not supported on the hardware are marked with the V4L2_CTRL_FLAG_DISABLED flag.

    Additionally, controls that are temporarily unavailable, or that can only be controlled from another file descriptor are marked with the V4L2_CTRL_FLAG_GRABBED flag. Usually, a control that is GRABBED, but not DISABLED can be read, but changed. The group name indicates a possibly narrower classification than the category. In other words, there may be multiple groups within a category. Controls within a group would typically be drawn within a group box. Controls in different categories might have a greater separation, or even be in separate windows.

  7. The v4l2_buffer timestamp field is changed to a 64-bit integer, and holds the time of the frame based on the system time, in 1 nanosecond units. Additionally, timestamps will be in absolute system time, not starting from zero at the beginning of a stream as it is now. The data type name for timestamps is stamp_t, defined as a signed 64-bit integer. Output devices should not send a buffer out until the time in the timestamp field has arrived. I would like to follow SGI's lead, and adopt a multimedia timestamping system like their UST (Unadjusted System Time). See http://reality.sgi.com/cpirazzi_engr/lg/time/intro.html. [This link is no longer valid.] UST uses timestamps that are 64-bit signed integers (not struct timeval's) and given in nanosecond units. The UST clock starts at zero when the system is booted and runs continuously and uniformly. It takes a little over 292 years for UST to overflow. There is no way to set the UST clock. The regular Linux time-of-day clock can be changed periodically, which would cause errors if it were being used for timestamping a multimedia stream. A real UST style clock will require some support in the kernel that is not there yet. But in anticipation, I will change the timestamp field to a 64-bit integer, and I will change the v4l2_masterclock_gettime() function (used only by drivers) to return a 64-bit integer.

  8. The sequence field is added to the struct v4l2_buffer. The sequence field indicates which frame this is in the sequence-- 0, 1, 2, 3, 4, etc. Set by capturing devices. Ignored by output devices. If a capture driver drops a frame, the sequence number of that frame is skipped. A break in the sequence will indicate to the application which frame was dropped.


6.2.6. V4L2 Version 0.20 incremental changes

1999-12-23: In struct v4l2_vbi_format field reserved1 became offset. Previously reserved1 was required to always read zero.

2000-01-13: Added V4L2_FMT_FLAG_NOT_INTERLACED.

2000-07-31: Included linux/poll.h in videodev.h for compatibility with the original videodev.h.

2000-11-20: Added V4L2_TYPE_VBI_OUTPUT. Added V4L2_PIX_FMT_Y41P.

2000-11-25: Added V4L2_TYPE_VBI_INPUT.

2000-12-04: Fixed a couple typos in symbol names.

2001-01-18: Fixed a namespace conflict (the fourcc macro changed to v4l2_fourcc).

2001-01-25: Fixed a possible driver-level compatibility problem between the original 2.4.0 videodev.h and the videodev.h that comes with videodevX. If you were using an earlier version of videodevX on 2.4.0, then you should recompile your v4l and V4L2 drivers to be safe.

2001-01-26: videodevX: Fixed a possible kernel-level incompatibility between the videodevX videodev.h and the 2.2.x videodev.h that had the devfs patches applied. videodev: Changed fourcc to v4l2_fourcc to avoid namespace pollution. Some other cleanup.

2001-03-02: Certain v4l ioctls that really pass data both ways, but whose types are read-only, did not work correctly through the backward compatibility layer. [Solution?]

2001-04-13: Added big endian 16-bit RGB formats.

2001-09-17: Added new YUV formats. Added VIDIOC_G_FREQUENCY and VIDIOC_S_FREQUENCY. (The VIDIOC_G/S_FREQ ioctls did not take multiple tuners into account.)

2000-09-18: Added V4L2_BUF_TYPE_VBI. Raw VBI VIDIOC_G_FMT and VIDIOC_S_FMT may fail if field type is not V4L2_BUF_TYPE_VBI. Changed the ambiguous phrase "rising edge" to "leading edge" in the definition of struct v4l2_vbi_format field offset.


6.2.7. V4L2 Version 0.20 2000-11-23

A number of changes were made to the raw VBI interface.

  1. Added figures clarifying the line numbering scheme. The description of start[0] and start[1] as base 0 offset has been dropped. Rationale: a) The previous definition was unclear. b) The start[] values are not an offset into anything, as a means of identifying scanning lines it can only be counterproductive to deviate from common numbering conventions. Compatibility: Add one to the start values. Applications depending on the previous semantics of start values may not function correctly.

  2. The restriction "count[0] > 0 and count[1] > 0" has been relaxed to "(count[0] + count[1]) > 0". Rationale: Drivers allocating resources at scanning line granularity and first field only data services. The comment that both 'count' values will usually be equal is misleading and pointless and has been removed. Compatibility: Drivers may return EINVAL, applications depending on the previous restriction may not function correctly.

  3. Restored description of the driver option to return negative start values. Existed in the initial revision of this document, not traceable why it disappeared in later versions. Compatibility: Applications depending on the returned start values being positive may not function correctly. Clarification on the use of EBUSY and EINVAL in VIDIOC_S_FMT ioctl. Added EBUSY paragraph to section. Added description of reserved2, previously mentioned only in videodev.h.

  4. Added V4L2_TYPE_VBI_INPUT and V4L2_TYPE_VBI_OUTPUT here and in videodev.h. The first is an alias for the older V4L2_TYPE_VBI, the latter was missing in videodev.h.


6.2.8. V4L2 Version 0.20 2002-07-25

Added sliced VBI interface proposal.


6.2.9. V4L2 in Linux 2.5.46, 2002-10

Around October-November 2002, prior to an announced feature freeze of Linux 2.5, the API was revised, drawing from experience with V4L2 0.20. This unnamed version was finally merged into Linux 2.5.46.

  1. As specified in Section 1.1.2> drivers must make related device functions available under all minor device numbers.

  2. The open() function requires access mode O_RDWR regardless of device type. All V4L2 drivers exchanging data with applications must support the O_NONBLOCK flag. The O_NOIO flag (alias of meaningless O_TRUNC) to indicate accesses without data exchange (panel applications) was dropped. Drivers must assume panel mode until the application attempts to initiate data exchange, see Section 1.1>.

  3. The struct v4l2_capability changed dramatically. Note that also the size of the structure changed, which is encoded in the ioctl request code, thus older V4L2 devices will respond with an EINVAL error code to the new VIDIOC_QUERYCAP ioctl.

    There are new fields to identify the driver, a new (as of yet unspecified) device function V4L2_CAP_RDS_CAPTURE, the V4L2_CAP_AUDIO flag indicates if the device has any audio connectors, another I/O capability V4L2_CAP_ASYNCIO can be flagged. Field type became a set in response to the change above and was merged with flags. V4L2_FLAG_TUNER was renamed to V4L2_CAP_TUNER, V4L2_CAP_VIDEO_OVERLAY replaced V4L2_FLAG_PREVIEW and V4L2_CAP_VBI_CAPTURE and V4L2_CAP_VBI_OUTPUT replaced V4L2_FLAG_DATA_SERVICE. V4L2_FLAG_READ and V4L2_FLAG_WRITE merged to V4L2_CAP_READWRITE.

    Redundant fields inputs, outputs, audios were removed, these can be determined as described in Section 1.4> and Section 1.5>.

    The somewhat volatile and therefore barely useful fields maxwidth, maxheight, minwidth, minheight, maxframerate were removed, this information is available as described in Section 1.9> and Section 1.7>.

    V4L2_FLAG_SELECT was removed, this function is considered important enough that all V4L2 drivers exchanging data with applications must support select(). The redundant flag V4L2_FLAG_MONOCHROME was removed, this information is available as described in Section 1.9>.

  4. In struct v4l2_input the assoc_audio field and the capability field and its only flag V4L2_INPUT_CAP_AUDIO was replaced by the new audioset field. Instead of linking one video input to one audio input this field reports all audio inputs this video input combines with.

    New fields are tuner (reversing the former link from tuners to video inputs), std and status.

    Accordingly struct v4l2_output lost its capability and assoc_audio fields, audioset, modulator and std where added.

  5. The struct v4l2_audio field audio was renamed to index, consistent with other structures. Capability flag V4L2_AUDCAP_STEREO was added to indicated if this is a stereo input. V4L2_AUDCAP_EFFECTS and the corresponding V4L2_AUDMODE flags where removed, this can be easily implemented using controls. (However the same applies to AVL which is still there.)

    The struct v4l2_audioout field audio was renamed to index.

  6. The struct v4l2_tuner input field was replaced by an index field, permitting devices with multiple tuners. The link between video inputs and tuners is now reversed, inputs point to the tuner they are on. The std substructure became a simple set (more about this below) and moved into struct v4l2_input. A type field was added.

    Accordingly in struct v4l2_modulator the output was replaced by an index field.

    In struct v4l2_frequency the port field was replaced by a tuner field containing the respective tuner or modulator index number. A tuner type field was added and the reserved field became larger for future extensions (satellite tuners in particular).

  7. The idea of completely transparent video standards was dropped. Experience showed that applications must be able to work with video standards beyond presenting the user a menu. To this end V4L2 returned to defined standards as v4l2_std_id, replacing references to standards throughout the API. For details see Section 1.7>. VIDIOC_G_STD and VIDIOC_S_STD now take a pointer to this type as argument. VIDIOC_QUERYSTD was added to autodetect the received standard. In struct v4l2_standard an index field was added for VIDIOC_ENUMSTD. A v4l2_std_id field named id was added as machine readable identifier, also replacing the transmission field. framerate, which is misleading, was renamed to frameperiod. The now obsolete colorstandard information, originally needed to distguish between variations of standards, were removed.

    Struct v4l2_enumstd ceased to be. VIDIOC_ENUMSTD now takes a pointer to a struct v4l2_standard directly. The information which standards are supported by a particular video input or output moved into struct v4l2_input and struct v4l2_output fields named std, respectively.

  8. The struct v4l2_queryctrl fields category and group did not catch on and/or were not implemented as expected and therefore removed.

  9. The VIDIOC_TRY_FMT ioctl was added to negotiate data formats as with VIDIOC_S_FMT, but without the overhead of programming the hardware and regardless of I/O in progress.

    In struct v4l2_format the fmt union was extended to contain struct v4l2_window. As a result all data format negotiation is now possible with VIDIOC_G_FMT, VIDIOC_S_FMT and VIDIOC_TRY_FMT; the VIDIOC_G_WIN, VIDIOC_S_WIN and ioctl to prepare for overlay were removed. The type field changed to type enum v4l2_buf_type and the buffer type names changed as follows.

    Old definesenum v4l2_buf_type
    V4L2_BUF_TYPE_CAPTUREV4L2_BUF_TYPE_VIDEO_CAPTURE
    V4L2_BUF_TYPE_CODECINPreliminary omitted
    V4L2_BUF_TYPE_CODECOUTPreliminary omitted
    V4L2_BUF_TYPE_EFFECTSINPreliminary omitted
    V4L2_BUF_TYPE_EFFECTSIN2Preliminary omitted
    V4L2_BUF_TYPE_EFFECTSOUTPreliminary omitted
    V4L2_BUF_TYPE_VIDEOOUTV4L2_BUF_TYPE_VIDEO_OUTPUT
    -V4L2_BUF_TYPE_VIDEO_OVERLAY
    -V4L2_BUF_TYPE_VBI_CAPTURE
    -V4L2_BUF_TYPE_VBI_OUTPUT
    V4L2_BUF_TYPE_PRIVATE_BASEV4L2_BUF_TYPE_PRIVATE

  10. In struct v4l2_fmtdesc a enum v4l2_buf_type field named type was added as in struct v4l2_format. As a result the VIDIOC_ENUM_FBUFFMT ioctl is no longer needed and was removed. These calls can be replaced by VIDIOC_ENUM_FMT with type V4L2_BUF_TYPE_VIDEO_OVERLAY.

  11. In struct v4l2_pix_format the depth was removed, assuming applications recognizing the format are aware of the image depth and others need not know. The same rationale lead to the removal of the V4L2_FMT_FLAG_COMPRESSED flag. The V4L2_FMT_FLAG_SWCONVECOMPRESSED flag was removed because drivers are not supposed to convert image formats in kernel space. The V4L2_FMT_FLAG_BYTESPERLINE flag was redundant, applications can set the bytesperline field to zero to get a reasonable default. Since also the remaining flags were replaced, the flags field itself was removed.

    The interlace flags were replaced by a enum v4l2_field value in a newly added field field.

    Old flagenum v4l2_field
    V4L2_FMT_FLAG_NOT_INTERLACED?
    V4L2_FMT_FLAG_INTERLACED = V4L2_FMT_FLAG_COMBINEDV4L2_FIELD_INTERLACED
    V4L2_FMT_FLAG_TOPFIELD = V4L2_FMT_FLAG_ODDFIELDV4L2_FIELD_TOP
    V4L2_FMT_FLAG_BOTFIELD = V4L2_FMT_FLAG_EVENFIELDV4L2_FIELD_BOTTOM
    -V4L2_FIELD_SEQ_TB
    -V4L2_FIELD_SEQ_BT
    -V4L2_FIELD_ALTERNATE

    The color space flags were replaced by a enum v4l2_colorspace value in a newly added colorspace field, where one of V4L2_COLORSPACE_SMPTE170M, V4L2_COLORSPACE_BT878, V4L2_COLORSPACE_470_SYSTEM_M or V4L2_COLORSPACE_470_SYSTEM_BG replaces V4L2_FMT_CS_601YUV.

  12. In struct v4l2_requestbuffers the type field was properly typed as enum v4l2_buf_type. Buffer types changed as mentioned above. A new memory field of type enum v4l2_memory was added to distinguish between mapping methods using buffers allocated by the driver or the application. See Chapter 3> for details.

  13. In struct v4l2_buffer the type field was properly typed as enum v4l2_buf_type. Buffer types changed as mentioned above. A field field of type enum v4l2_field was added to indicate if a buffer contains a top or bottom field, the field flags were removed. Realizing the efforts to introduce an unadjusted system time clock failed, the timestamp field changed back from type stamp_t, an unsigned 64 bit integer expressing time in nanoseconds, to struct timeval. With the addition of a second memory mapping method the offset field moved into union m, and a new memory field of type enum v4l2_memory was added to distinguish between mapping methods. See Chapter 3> for details.

    The V4L2_BUF_REQ_CONTIG flag was used by the V4L compatibility layer, after changes to this code it was no longer needed. The V4L2_BUF_ATTR_DEVICEMEM flag would indicate if the buffer was indeed allocated in device memory rather than DMA-able system memory. It was barely useful and so has been removed.

  14. In struct v4l2_framebuffer the base[3] array anticipating double- and triple-buffering in off-screen video memory, however without defining a synchronization mechanism, was replaced by a single pointer. The V4L2_FBUF_CAP_SCALEUP and V4L2_FBUF_CAP_SCALEDOWN flags were removed. Applications can determine this capability more accurately using the new cropping and scaling interface. The V4L2_FBUF_CAP_CLIPPING flag was replaced by V4L2_FBUF_CAP_LIST_CLIPPING and V4L2_FBUF_CAP_BITMAP_CLIPPING.

  15. In struct v4l2_clip the x, y, width and height field moved into a c substructure of type struct v4l2_rect. The x and y field were renamed to left and top, i. e. offsets to a context dependent origin.

  16. In struct v4l2_window the x, y, width and height field moved into a w substructure as above. A field field of type %v4l2-field; was added to distinguish between field and frame (interlaced) overlay.

  17. The digital zoom interface, including struct v4l2_zoomcap, struct v4l2_zoom, V4L2_ZOOM_NONCAP and V4L2_ZOOM_WHILESTREAMING was replaced by a new cropping and scaling interface. The previously unused struct v4l2_cropcap and v4l2_crop where redefined for this purpose. See Section 1.10> for details.

  18. In struct v4l2_vbi_format the SAMPLE_FORMAT field now contains a four-character-code as used to identify video image formats. V4L2_PIX_FMT_GREY replaces the V4L2_VBI_SF_UBYTE define. The reserved field was extended.

  19. In struct v4l2_captureparm the type of the timeperframe field changed from unsigned long to struct v4l2_fract. A new field readbuffers was added to control the driver behaviour in read I/O mode.

    According changes were made to struct v4l2_outputparm.

  20. The struct v4l2_performance and VIDIOC_G_PERF ioctl were dropped. Except when using the read/write I/O method, which is limited anyway, this information is already available to the application.

  21. The example transformation from RGB to YCbCr color space in the old V4L2 documentation was inaccurate, this has been corrected in Chapter 2>.


6.2.10. V4L2 2003-06-19

  1. A new capability flag V4L2_CAP_RADIO was added for radio devices. Prior to this change radio devices would identify soley by having exactly one tuner whose type field reads V4L2_TUNER_RADIO.

  2. An optional priority mechanism was added, see Section 1.3> for details.

  3. The audio input and output interface was found to be incomplete.

    Previously the VIDIOC_G_AUDIO ioctl would enumerate the available audio inputs. An ioctl to determine the current audio input, if more than one combines with the current video input, did not exist. So VIDIOC_G_AUDIO was renamed to VIDIOC_G_AUDIO_OLD, this ioctl will be removed in the future. The VIDIOC_ENUMAUDIO ioctl was added to enumerate audio inputs, while VIDIOC_G_AUDIO now reports the current audio input.

    The same changes were made to VIDIOC_G_AUDOUT and VIDIOC_ENUMAUDOUT.

    Until further the "videodev" module will automatically translate to the new versions, drivers and applications must be updated when they are recompiled.

  4. The VIDIOC_OVERLAY ioctl was incorrectly defined with read-write parameter. It was changed to write-only, while the read-write version was renamed to VIDIOC_OVERLAY_OLD. This function will be removed in the future. Until further the "videodev" module will automatically translate to the new version, so drivers must be recompiled, but not applications.

  5. Section 4.2> incorrectly stated that clipping rectangles define regions where the video can be seen. Correct is that clipping rectangles define regions where no video shall be displayed and so the graphics surface can be seen.

  6. The VIDIOC_S_PARM and VIDIOC_S_CTRL were defined with write-only parameter, inconsistent with other ioctls modifying their argument. They were changed to read-write, while a _OLD suffix was added to the write-only version. These functions will be removed in the future. Drivers, and applications assuming a constant parameter, need an update.


6.2.11. V4L2 2003-11-05

  1. In Section 2.3> the following pixel formats were incorrectly transferred from Bill Dirks' V4L2 specification. Descriptions refer to bytes in memory, in ascending address order.

    SymbolIn this document prior to revision 0.5Correct
    V4L2_PIX_FMT_RGB24B, G, RR, G, B
    V4L2_PIX_FMT_BGR24R, G, BB, G, R
    V4L2_PIX_FMT_RGB32B, G, R, XR, G, B, X
    V4L2_PIX_FMT_BGR32R, G, B, XB, G, R, X

    The V4L2_PIX_FMT_BGR24 example was always correct.

    In Section 6.1.5> the mapping of VIDEO_PALETTE_RGB24 and VIDEO_PALETTE_RGB32 to V4L2 pixel formats was accordingly corrected.

  2. Unrelated to the fixes above, drivers may still interpret some V4L2 RGB pixel formats differently. These issues have yet to be addressed, for details see Section 2.3>.


6.2.12. V4L2 in Linux 2.6.6, 2004-05-09

  1. The VIDIOC_CROPCAP ioctl was incorrectly defined with read-only parameter. It was changed to read-write, while the read-only version was renamed to VIDIOC_CROPCAP_OLD. This function will be removed in the future.


6.2.13. V4L2 in Linux 2.6.8

  1. A new field input (former reserved[0]) was added to the struct v4l2_buffer structure. It must be enabled with the new V4L2_BUF_FLAG_INPUT flag. The flags field is no longer read-only.


6.2.14. V4L2 spec erratum 2004-08-01

  1. The return value of the V4L2 open()(2)> function was incorrect.

  2. Audio output ioctls end in -AUDOUT, not -AUDIOOUT.

  3. In the current audio input example the VIDIOC_G_AUDIO ioctl took the wrong argument.

  4. The VIDIOC_QBUF and VIDIOC_DQBUF ioctl did not mention the struct v4l2_buffer memory field, it was also missing from examples. Added description of the VIDIOC_DQBUF EIO error.


6.2.15. V4L2 in Linux 2.6.14

  1. A new sliced VBI interface (see Section 4.7>) was added. It replaces the interface proposed in V4L2 specification 0.8.


6.2.16. V4L2 in Linux 2.6.15

  1. The VIDIOC_LOG_STATUS ioctl was added.

  2. New video standards V4L2_STD_NTSC_443, V4L2_STD_SECAM_LC, V4L2_STD_SECAM_DK (a set of SECAM D, K and K1), and V4L2_STD_ATSC (a set of V4L2_STD_ATSC_8_VSB and V4L2_STD_ATSC_16_VSB) were defined. Note the V4L2_STD_525_60 set now includes V4L2_STD_NTSC_443. See also Table 3>.

  3. The VIDIOC_G_COMP and VIDIOC_S_COMP ioctl were renamed to VIDIOC_G_MPEGCOMP and VIDIOC_S_MPEGCOMP respectively. Their argument was replaced by a struct v4l2_mpeg_compression pointer.


6.2.17. V4L2 spec erratum 2005-11-27

The capture example in Appendix B> called VIDIOC_S_CROP without checking if cropping (VIDIOC_CROPCAP) is supported. In the video standard selection example in Section 1.7> the VIDIOC_S_STD call used the wrong argument type.


6.2.18. V4L2 spec erratum 2006-01-10

  1. The V4L2_IN_ST_COLOR_KILL flag in struct v4l2_input does not only indicate if the color killer is enabled, but also if it is active (disabling color decoding because it detects no color modulation).

  2. VIDIOC_S_PARM is a read/write ioctl, not write-only as stated on the respective function reference page. The ioctl changed in 2003 as noted above.


6.2.19. V4L2 spec erratum 2006-02-03

  1. In struct v4l2_captureparm and struct v4l2_outputparm the timeperframe field gives the time in seconds, not microseconds.


6.3. Relation of V4L2 to other Linux multimedia APIs

6.3.1. X Video Extension

The X Video Extension (abbreviated XVideo or just Xv) is an extension of the X Window system, implemented for example by the XFree86 project. Its scope is similar to V4L2, an API to video capture and output devices for X clients. Xv allows applications to display live video in a window, send window contents to a TV output, and capture or output still images in XPixmaps[27]. With their implementation XFree86 makes the extension available across many operating systems and architectures.

Because the driver is embedded into the X server Xv has a number of advantages over the V4L2 video overlay interface. The driver can easily determine the overlay target, i. e. visible graphics memory or off-screen buffers for non-destructive overlay. It can program the RAMDAC for overlay, scaling or color-keying, or the clipping functions of the video capture hardware, always in sync with drawing operations or windows moving or changing their stacking order.

To combine the advantages of Xv and V4L a special Xv driver exists in XFree86, just programming any overlay capable Video4Linux device it finds. To enable it /etc/X11/XF86Config must contain these lines:

Section "Module"
Load "v4l"
EndSection

As of XFree86 4.2 this driver still supports only V4L ioctls, however it should work just fine with all V4L2 devices through the V4L2 backward-compatibility layer. Since V4L2 permits multiple opens it is possible (if supported by the V4L2 driver) to capture video while an X client requested video overlay. Restrictions of simultaneous capturing and overlay mentioned in Section 4.2> apply.

Only marginally related to V4L2, XFree86 extended Xv to support hardware YUV to RGB conversion and scaling for faster video playback, and added an interface to MPEG-2 decoding hardware. This can be used to improve displaying captured images.


6.3.2. Digital Video

V4L2 does not, at this time and possibly never, support digital terrestrial, cable or satellite broadcast. A separate project aiming at digital receivers exists. You can find its homepage at http://linuxtv.org. This group found the requirements sufficiently different from analog television to choose independent development of their interfaces.


6.3.3. Audio Interfaces

[to do - OSS/ALSA]


Appendix A. Video For Linux Two Header File

#ifndef __LINUX_VIDEODEV2_H
#define __LINUX_VIDEODEV2_H
/*
* Video for Linux Two
*
* Header file for v4l or V4L2 drivers and applications, for
* Linux kernels 2.2.x or 2.4.x.
*
* See http://bytesex.org/v4l/ for API specs and other
* v4l2 documentation.
*
* Author: Bill Dirks <bdirks@pacbell.net>
* Justin Schoeman
* et al.
*/
#ifdef __KERNEL__
#include <linux/time.h> /* need struct timeval */
#include <linux/poll.h>
#if LINUX_VERSION_CODE < KERNEL_VERSION(2,5,69)
#include <linux/devfs_fs_kernel.h>
#endif
#if LINUX_VERSION_CODE > KERNEL_VERSION(2,5,0)
#include <linux/device.h>
#endif
#endif
#include <linux/compiler.h> /* need __user */
#define OBSOLETE_OWNER 1 /* It will be removed for 2.6.15 */
#define HAVE_V4L2 1
/*
* Common stuff for both V4L1 and V4L2
* Moved from videodev.h
*/
#define VIDEO_MAX_FRAME 32
#define VID_TYPE_CAPTURE 1 /* Can capture */
#define VID_TYPE_TUNER 2 /* Can tune */
#define VID_TYPE_TELETEXT 4 /* Does teletext */
#define VID_TYPE_OVERLAY 8 /* Overlay onto frame buffer */
#define VID_TYPE_CHROMAKEY 16 /* Overlay by chromakey */
#define VID_TYPE_CLIPPING 32 /* Can clip */
#define VID_TYPE_FRAMERAM 64 /* Uses the frame buffer memory */
#define VID_TYPE_SCALES 128 /* Scalable */
#define VID_TYPE_MONOCHROME 256 /* Monochrome only */
#define VID_TYPE_SUBCAPTURE 512 /* Can capture subareas of the image */
#define VID_TYPE_MPEG_DECODER 1024 /* Can decode MPEG streams */
#define VID_TYPE_MPEG_ENCODER 2048 /* Can encode MPEG streams */
#define VID_TYPE_MJPEG_DECODER 4096 /* Can decode MJPEG streams */
#define VID_TYPE_MJPEG_ENCODER 8192 /* Can encode MJPEG streams */
#ifdef __KERNEL__
#define VFL_TYPE_GRABBER 0
#define VFL_TYPE_VBI 1
#define VFL_TYPE_RADIO 2
#define VFL_TYPE_VTX 3
struct video_device
{
/* device info */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,0)
struct device *dev;
#endif
char name[32];
int type; /* v4l1 */
int type2; /* v4l2 */
int hardware;
int minor;
/* device ops + callbacks */
struct file_operations *fops;
void (*release)(struct video_device *vfd);
#if LINUX_VERSION_CODE < KERNEL_VERSION(2,5,0)
/* old, obsolete interface -- dropped in 2.5.x, don't use it */
int (*open)(struct video_device *, int mode);
void (*close)(struct video_device *);
long (*read)(struct video_device *, char *, unsigned long, int noblock);
long (*write)(struct video_device *, const char *, unsigned long, int noblock);
unsigned int (*poll)(struct video_device *, struct file *, poll_table *);
int (*ioctl)(struct video_device *, unsigned int , void *);
int (*mmap)(struct video_device *, const char *, unsigned long);
int (*initialize)(struct video_device *);
#endif
#if OBSOLETE_OWNER /* to be removed in 2.6.15 */
/* obsolete -- fops->owner is used instead */
struct module *owner;
/* dev->driver_data will be used instead some day.
* Use the video_{get|set}_drvdata() helper functions,
* so the switch over will be transparent for you.
* Or use {pci|usb}_{get|set}_drvdata() directly. */
void *priv;
#endif
/* for videodev.c intenal usage -- please don't touch */
int users; /* video_exclusive_{open|close} ... */
struct semaphore lock; /* ... helper function uses these */
#if LINUX_VERSION_CODE < KERNEL_VERSION(2,5,69)
devfs_handle_t devfs_handle; /* devfs */
#else
char devfs_name[64]; /* devfs */
#endif
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,6,0)
struct class_device class_dev; /* sysfs */
#endif
};
#define VIDEO_MAJOR 81
extern int video_register_device(struct video_device *, int type, int nr);
extern void video_unregister_device(struct video_device *);
extern int video_usercopy(struct inode *inode, struct file *file,
unsigned int cmd, unsigned long arg,
int (*func)(struct inode *inode, struct file *file,
unsigned int cmd, void *arg));
/* helper functions to alloc / release struct video_device, the
later can be used for video_device->release() */
struct video_device *video_device_alloc(void);
void video_device_release(struct video_device *vfd);
#endif
/*
* M I S C E L L A N E O U S
*/
/* Four-character-code (FOURCC) */
#define v4l2_fourcc(a,b,c,d)\
(((__u32)(a)<<0)|((__u32)(b)<<8)|((__u32)(c)<<16)|((__u32)(d)<<24))
/*
* E N U M S
*/
enum v4l2_field {
V4L2_FIELD_ANY = 0, /* driver can choose from none,
top, bottom, interlaced
depending on whatever it thinks
is approximate ... */
V4L2_FIELD_NONE = 1, /* this device has no fields ... */
V4L2_FIELD_TOP = 2, /* top field only */
V4L2_FIELD_BOTTOM = 3, /* bottom field only */
V4L2_FIELD_INTERLACED = 4, /* both fields interlaced */
V4L2_FIELD_SEQ_TB = 5, /* both fields sequential into one
buffer, top-bottom order */
V4L2_FIELD_SEQ_BT = 6, /* same as above + bottom-top order */
V4L2_FIELD_ALTERNATE = 7, /* both fields alternating into
separate buffers */
};
#define V4L2_FIELD_HAS_TOP(field) \
((field) == V4L2_FIELD_TOP ||\
(field) == V4L2_FIELD_INTERLACED ||\
(field) == V4L2_FIELD_SEQ_TB ||\
(field) == V4L2_FIELD_SEQ_BT)
#define V4L2_FIELD_HAS_BOTTOM(field) \
((field) == V4L2_FIELD_BOTTOM ||\
(field) == V4L2_FIELD_INTERLACED ||\
(field) == V4L2_FIELD_SEQ_TB ||\
(field) == V4L2_FIELD_SEQ_BT)
#define V4L2_FIELD_HAS_BOTH(field) \
((field) == V4L2_FIELD_INTERLACED ||\
(field) == V4L2_FIELD_SEQ_TB ||\
(field) == V4L2_FIELD_SEQ_BT)
enum v4l2_buf_type {
V4L2_BUF_TYPE_VIDEO_CAPTURE = 1,
V4L2_BUF_TYPE_VIDEO_OUTPUT = 2,
V4L2_BUF_TYPE_VIDEO_OVERLAY = 3,
V4L2_BUF_TYPE_VBI_CAPTURE = 4,
V4L2_BUF_TYPE_VBI_OUTPUT = 5,
#if 1 /*KEEP*/
/* Experimental Sliced VBI */
V4L2_BUF_TYPE_SLICED_VBI_CAPTURE = 6,
V4L2_BUF_TYPE_SLICED_VBI_OUTPUT = 7,
#endif
V4L2_BUF_TYPE_PRIVATE = 0x80,
};
enum v4l2_ctrl_type {
V4L2_CTRL_TYPE_INTEGER = 1,
V4L2_CTRL_TYPE_BOOLEAN = 2,
V4L2_CTRL_TYPE_MENU = 3,
V4L2_CTRL_TYPE_BUTTON = 4,
};
enum v4l2_tuner_type {
V4L2_TUNER_RADIO = 1,
V4L2_TUNER_ANALOG_TV = 2,
V4L2_TUNER_DIGITAL_TV = 3,
};
enum v4l2_memory {
V4L2_MEMORY_MMAP = 1,
V4L2_MEMORY_USERPTR = 2,
V4L2_MEMORY_OVERLAY = 3,
};
/* see also http://vektor.theorem.ca/graphics/ycbcr/ */
enum v4l2_colorspace {
/* ITU-R 601 -- broadcast NTSC/PAL */
V4L2_COLORSPACE_SMPTE170M = 1,
/* 1125-Line (US) HDTV */
V4L2_COLORSPACE_SMPTE240M = 2,
/* HD and modern captures. */
V4L2_COLORSPACE_REC709 = 3,
/* broken BT878 extents (601, luma range 16-253 instead of 16-235) */
V4L2_COLORSPACE_BT878 = 4,
/* These should be useful. Assume 601 extents. */
V4L2_COLORSPACE_470_SYSTEM_M = 5,
V4L2_COLORSPACE_470_SYSTEM_BG = 6,
/* I know there will be cameras that send this. So, this is
* unspecified chromaticities and full 0-255 on each of the
* Y'CbCr components
*/
V4L2_COLORSPACE_JPEG = 7,
/* For RGB colourspaces, this is probably a good start. */
V4L2_COLORSPACE_SRGB = 8,
};
enum v4l2_priority {
V4L2_PRIORITY_UNSET = 0, /* not initialized */
V4L2_PRIORITY_BACKGROUND = 1,
V4L2_PRIORITY_INTERACTIVE = 2,
V4L2_PRIORITY_RECORD = 3,
V4L2_PRIORITY_DEFAULT = V4L2_PRIORITY_INTERACTIVE,
};
struct v4l2_rect {
__s32 left;
__s32 top;
__s32 width;
__s32 height;
};
struct v4l2_fract {
__u32 numerator;
__u32 denominator;
};
/*
* D R I V E R C A P A B I L I T I E S
*/
struct v4l2_capability
{
__u8 driver[16]; /* i.e. "bttv" */
__u8 card[32]; /* i.e. "Hauppauge WinTV" */
__u8 bus_info[32]; /* "PCI:" + pci_name(pci_dev) */
__u32 version; /* should use KERNEL_VERSION() */
__u32 capabilities; /* Device capabilities */
__u32 reserved[4];
};
/* Values for 'capabilities' field */
#define V4L2_CAP_VIDEO_CAPTURE 0x00000001 /* Is a video capture device */
#define V4L2_CAP_VIDEO_OUTPUT 0x00000002 /* Is a video output device */
#define V4L2_CAP_VIDEO_OVERLAY 0x00000004 /* Can do video overlay */
#define V4L2_CAP_VBI_CAPTURE 0x00000010 /* Is a raw VBI capture device */
#define V4L2_CAP_VBI_OUTPUT 0x00000020 /* Is a raw VBI output device */
#if 1 /*KEEP*/
#define V4L2_CAP_SLICED_VBI_CAPTURE 0x00000040 /* Is a sliced VBI capture device */
#define V4L2_CAP_SLICED_VBI_OUTPUT 0x00000080 /* Is a sliced VBI output device */
#endif
#define V4L2_CAP_RDS_CAPTURE 0x00000100 /* RDS data capture */
#define V4L2_CAP_TUNER 0x00010000 /* has a tuner */
#define V4L2_CAP_AUDIO 0x00020000 /* has audio support */
#define V4L2_CAP_RADIO 0x00040000 /* is a radio device */
#define V4L2_CAP_READWRITE 0x01000000 /* read/write systemcalls */
#define V4L2_CAP_ASYNCIO 0x02000000 /* async I/O */
#define V4L2_CAP_STREAMING 0x04000000 /* streaming I/O ioctls */
/*
* V I D E O I M A G E F O R M A T
*/
struct v4l2_pix_format
{
__u32 width;
__u32 height;
__u32 pixelformat;
enum v4l2_field field;
__u32 bytesperline; /* for padding, zero if unused */
__u32 sizeimage;
enum v4l2_colorspace colorspace;
__u32 priv; /* private data, depends on pixelformat */
};
/* Pixel format FOURCC depth Description */
#define V4L2_PIX_FMT_RGB332 v4l2_fourcc('R','G','B','1') /* 8 RGB-3-3-2 */
#define V4L2_PIX_FMT_RGB555 v4l2_fourcc('R','G','B','O') /* 16 RGB-5-5-5 */
#define V4L2_PIX_FMT_RGB565 v4l2_fourcc('R','G','B','P') /* 16 RGB-5-6-5 */
#define V4L2_PIX_FMT_RGB555X v4l2_fourcc('R','G','B','Q') /* 16 RGB-5-5-5 BE */
#define V4L2_PIX_FMT_RGB565X v4l2_fourcc('R','G','B','R') /* 16 RGB-5-6-5 BE */
#define V4L2_PIX_FMT_BGR24 v4l2_fourcc('B','G','R','3') /* 24 BGR-8-8-8 */
#define V4L2_PIX_FMT_RGB24 v4l2_fourcc('R','G','B','3') /* 24 RGB-8-8-8 */
#define V4L2_PIX_FMT_BGR32 v4l2_fourcc('B','G','R','4') /* 32 BGR-8-8-8-8 */
#define V4L2_PIX_FMT_RGB32 v4l2_fourcc('R','G','B','4') /* 32 RGB-8-8-8-8 */
#define V4L2_PIX_FMT_GREY v4l2_fourcc('G','R','E','Y') /* 8 Greyscale */
#define V4L2_PIX_FMT_YVU410 v4l2_fourcc('Y','V','U','9') /* 9 YVU 4:1:0 */
#define V4L2_PIX_FMT_YVU420 v4l2_fourcc('Y','V','1','2') /* 12 YVU 4:2:0 */
#define V4L2_PIX_FMT_YUYV v4l2_fourcc('Y','U','Y','V') /* 16 YUV 4:2:2 */
#define V4L2_PIX_FMT_UYVY v4l2_fourcc('U','Y','V','Y') /* 16 YUV 4:2:2 */
#define V4L2_PIX_FMT_YUV422P v4l2_fourcc('4','2','2','P') /* 16 YVU422 planar */
#define V4L2_PIX_FMT_YUV411P v4l2_fourcc('4','1','1','P') /* 16 YVU411 planar */
#define V4L2_PIX_FMT_Y41P v4l2_fourcc('Y','4','1','P') /* 12 YUV 4:1:1 */
/* two planes -- one Y, one Cr + Cb interleaved */
#define V4L2_PIX_FMT_NV12 v4l2_fourcc('N','V','1','2') /* 12 Y/CbCr 4:2:0 */
#define V4L2_PIX_FMT_NV21 v4l2_fourcc('N','V','2','1') /* 12 Y/CrCb 4:2:0 */
/* The following formats are not defined in the V4L2 specification */
#define V4L2_PIX_FMT_YUV410 v4l2_fourcc('Y','U','V','9') /* 9 YUV 4:1:0 */
#define V4L2_PIX_FMT_YUV420 v4l2_fourcc('Y','U','1','2') /* 12 YUV 4:2:0 */
#define V4L2_PIX_FMT_YYUV  v4l2_fourcc('Y','Y','U','V') /* 16 YUV 4:2:2 */
#define V4L2_PIX_FMT_HI240  v4l2_fourcc('H','I','2','4') /* 8 8-bit color */
/* see http://www.siliconimaging.com/RGB%20Bayer.htm */
#define V4L2_PIX_FMT_SBGGR8  v4l2_fourcc('B','A','8','1') /* 8 BGBG.. GRGR.. */
/* compressed formats */
#define V4L2_PIX_FMT_MJPEG  v4l2_fourcc('M','J','P','G') /* Motion-JPEG */
#define V4L2_PIX_FMT_JPEG  v4l2_fourcc('J','P','E','G') /* JFIF JPEG */
#define V4L2_PIX_FMT_DV  v4l2_fourcc('d','v','s','d') /* 1394 */
#define V4L2_PIX_FMT_MPEG  v4l2_fourcc('M','P','E','G') /* MPEG */
/* Vendor-specific formats */
#define V4L2_PIX_FMT_WNVA  v4l2_fourcc('W','N','V','A') /* Winnov hw compress */
#define V4L2_PIX_FMT_SN9C10X  v4l2_fourcc('S','9','1','0') /* SN9C10x compression */
#define V4L2_PIX_FMT_PWC1  v4l2_fourcc('P','W','C','1') /* pwc older webcam */
#define V4L2_PIX_FMT_PWC2  v4l2_fourcc('P','W','C','2') /* pwc newer webcam */
/*
* F O R M A T E N U M E R A T I O N
*/
struct v4l2_fmtdesc
{
__u32 index; /* Format number */
enum v4l2_buf_type type; /* buffer type */
__u32 flags;
__u8 description[32]; /* Description string */
__u32 pixelformat; /* Format fourcc */
__u32 reserved[4];
};
#define V4L2_FMT_FLAG_COMPRESSED 0x0001
/*
* T I M E C O D E
*/
struct v4l2_timecode
{
__u32 type;
__u32 flags;
__u8 frames;
__u8 seconds;
__u8 minutes;
__u8 hours;
__u8 userbits[4];
};
/* Type */
#define V4L2_TC_TYPE_24FPS 1
#define V4L2_TC_TYPE_25FPS 2
#define V4L2_TC_TYPE_30FPS 3
#define V4L2_TC_TYPE_50FPS 4
#define V4L2_TC_TYPE_60FPS 5
/* Flags */
#define V4L2_TC_FLAG_DROPFRAME 0x0001 /* "drop-frame" mode */
#define V4L2_TC_FLAG_COLORFRAME 0x0002
#define V4L2_TC_USERBITS_field 0x000C
#define V4L2_TC_USERBITS_USERDEFINED 0x0000
#define V4L2_TC_USERBITS_8BITCHARS 0x0008
/* The above is based on SMPTE timecodes */
#if 1
/*
* M P E G C O M P R E S S I O N P A R A M E T E R S
*
* ### WARNING: this is still work-in-progress right now, most likely
* ### there will be some incompatible changes.
*
*/
enum  v4l2_bitrate_mode {
V4L2_BITRATE_NONE = 0, /* not specified */
V4L2_BITRATE_CBR, /* constant bitrate */
V4L2_BITRATE_VBR, /* variable bitrate */
};
struct v4l2_bitrate {
/* rates are specified in kbit/sec */
enum  v4l2_bitrate_mode mode;
__u32 min;
__u32 target; /* use this one for CBR */
__u32 max;
};
enum v4l2_mpeg_streamtype {
V4L2_MPEG_SS_1, /* MPEG-1 system stream */
V4L2_MPEG_PS_2, /* MPEG-2 program stream */
V4L2_MPEG_TS_2, /* MPEG-2 transport stream */
V4L2_MPEG_PS_DVD, /* MPEG-2 program stream with DVD header fixups */
};
enum v4l2_mpeg_audiotype {
V4L2_MPEG_AU_2_I, /* MPEG-2 layer 1 */
V4L2_MPEG_AU_2_II, /* MPEG-2 layer 2 */
V4L2_MPEG_AU_2_III, /* MPEG-2 layer 3 */
V4L2_MPEG_AC3, /* AC3 */
V4L2_MPEG_LPCM, /* LPCM */
};
enum v4l2_mpeg_videotype {
V4L2_MPEG_VI_1, /* MPEG-1 */
V4L2_MPEG_VI_2, /* MPEG-2 */
};
enum v4l2_mpeg_aspectratio {
V4L2_MPEG_ASPECT_SQUARE = 1, /* square pixel */
V4L2_MPEG_ASPECT_4_3 = 2, /* 4 : 3 */
V4L2_MPEG_ASPECT_16_9 = 3, /* 16 : 9 */
V4L2_MPEG_ASPECT_1_221 = 4, /* 1 : 2,21 */
};
struct v4l2_mpeg_compression {
/* general */
enum v4l2_mpeg_streamtype st_type;
struct v4l2_bitrate st_bitrate;
/* transport streams */
__u16 ts_pid_pmt;
__u16 ts_pid_audio;
__u16 ts_pid_video;
__u16 ts_pid_pcr;
/* program stream */
__u16 ps_size;
__u16 reserved_1; /* align */
/* audio */
enum v4l2_mpeg_audiotype au_type;
struct v4l2_bitrate au_bitrate;
__u32 au_sample_rate;
__u8 au_pesid;
__u8 reserved_2[3]; /* align */
/* video */
enum v4l2_mpeg_videotype vi_type;
enum v4l2_mpeg_aspectratio vi_aspect_ratio;
struct v4l2_bitrate vi_bitrate;
__u32 vi_frame_rate;
__u16 vi_frames_per_gop;
__u16 vi_bframes_count;
__u8 vi_pesid;
__u8 reserved_3[3]; /* align */
/* misc flags */
__u32 closed_gops:1;
__u32 pulldown:1;
__u32 reserved_4:30; /* align */
/* I don't expect the above being perfect yet ;) */
__u32 reserved_5[8];
};
#endif
struct v4l2_jpegcompression
{
int quality;
int APPn; /* Number of APP segment to be written,
* must be 0..15 */
int APP_len; /* Length of data in JPEG APPn segment */
char APP_data[60]; /* Data in the JPEG APPn segment. */
int COM_len; /* Length of data in JPEG COM segment */
char COM_data[60]; /* Data in JPEG COM segment */
__u32 jpeg_markers; /* Which markers should go into the JPEG
* output. Unless you exactly know what
* you do, leave them untouched.
* Inluding less markers will make the
* resulting code smaller, but there will
* be fewer aplications which can read it.
* The presence of the APP and COM marker
* is influenced by APP_len and COM_len
* ONLY, not by this property! */
#define V4L2_JPEG_MARKER_DHT (1<<3) /* Define Huffman Tables */
#define V4L2_JPEG_MARKER_DQT (1<<4) /* Define Quantization Tables */
#define V4L2_JPEG_MARKER_DRI (1<<5) /* Define Restart Interval */
#define V4L2_JPEG_MARKER_COM (1<<6) /* Comment segment */
#define V4L2_JPEG_MARKER_APP (1<<7) /* App segment, driver will
* allways use APP0 */
};
/*
* M E M O R Y - M A P P I N G B U F F E R S
*/
struct v4l2_requestbuffers
{
__u32 count;
enum v4l2_buf_type type;
enum v4l2_memory memory;
__u32 reserved[2];
};
struct v4l2_buffer
{
__u32 index;
enum v4l2_buf_type type;
__u32 bytesused;
__u32 flags;
enum v4l2_field field;
struct timeval timestamp;
struct v4l2_timecode timecode;
__u32 sequence;
/* memory location */
enum v4l2_memory memory;
union {
__u32 offset;
unsigned long userptr;
} m;
__u32 length;
__u32 input;
__u32 reserved;
};
/* Flags for 'flags' field */
#define V4L2_BUF_FLAG_MAPPED 0x0001 /* Buffer is mapped (flag) */
#define V4L2_BUF_FLAG_QUEUED 0x0002 /* Buffer is queued for processing */
#define V4L2_BUF_FLAG_DONE 0x0004 /* Buffer is ready */
#define V4L2_BUF_FLAG_KEYFRAME 0x0008 /* Image is a keyframe (I-frame) */
#define V4L2_BUF_FLAG_PFRAME 0x0010 /* Image is a P-frame */
#define V4L2_BUF_FLAG_BFRAME 0x0020 /* Image is a B-frame */
#define V4L2_BUF_FLAG_TIMECODE 0x0100 /* timecode field is valid */
#define V4L2_BUF_FLAG_INPUT 0x0200 /* input field is valid */
/*
* O V E R L A Y P R E V I E W
*/
struct v4l2_framebuffer
{
__u32 capability;
__u32 flags;
/* FIXME: in theory we should pass something like PCI device + memory
* region + offset instead of some physical address */
void* base;
struct v4l2_pix_format fmt;
};
/* Flags for the 'capability' field. Read only */
#define V4L2_FBUF_CAP_EXTERNOVERLAY 0x0001
#define V4L2_FBUF_CAP_CHROMAKEY 0x0002
#define V4L2_FBUF_CAP_LIST_CLIPPING 0x0004
#define V4L2_FBUF_CAP_BITMAP_CLIPPING 0x0008
/* Flags for the 'flags' field. */
#define V4L2_FBUF_FLAG_PRIMARY 0x0001
#define V4L2_FBUF_FLAG_OVERLAY 0x0002
#define V4L2_FBUF_FLAG_CHROMAKEY 0x0004
struct v4l2_clip
{
struct v4l2_rect c;
struct v4l2_clip *next;
};
struct v4l2_window
{
struct v4l2_rect w;
enum v4l2_field field;
__u32 chromakey;
struct v4l2_clip __user *clips;
__u32 clipcount;
void __user *bitmap;
};
/*
* C A P T U R E P A R A M E T E R S
*/
struct v4l2_captureparm
{
__u32 capability; /* Supported modes */
__u32 capturemode; /* Current mode */
struct v4l2_fract timeperframe; /* Time per frame in .1us units */
__u32 extendedmode; /* Driver-specific extensions */
__u32 readbuffers; /* # of buffers for read */
__u32 reserved[4];
};
/* Flags for 'capability' and 'capturemode' fields */
#define V4L2_MODE_HIGHQUALITY 0x0001 /* High quality imaging mode */
#define V4L2_CAP_TIMEPERFRAME 0x1000 /* timeperframe field is supported */
struct v4l2_outputparm
{
__u32 capability; /* Supported modes */
__u32 outputmode; /* Current mode */
struct v4l2_fract timeperframe; /* Time per frame in seconds */
__u32 extendedmode; /* Driver-specific extensions */
__u32 writebuffers; /* # of buffers for write */
__u32 reserved[4];
};
/*
* I N P U T I M A G E C R O P P I N G
*/
struct v4l2_cropcap {
enum v4l2_buf_type type;
struct v4l2_rect bounds;
struct v4l2_rect defrect;
struct v4l2_fract pixelaspect;
};
struct v4l2_crop {
enum v4l2_buf_type type;
struct v4l2_rect c;
};
/*
* A N A L O G V I D E O S T A N D A R D
*/
typedef __u64 v4l2_std_id;
/* one bit for each */
#define V4L2_STD_PAL_B ((v4l2_std_id)0x00000001)
#define V4L2_STD_PAL_B1 ((v4l2_std_id)0x00000002)
#define V4L2_STD_PAL_G ((v4l2_std_id)0x00000004)
#define V4L2_STD_PAL_H ((v4l2_std_id)0x00000008)
#define V4L2_STD_PAL_I ((v4l2_std_id)0x00000010)
#define V4L2_STD_PAL_D ((v4l2_std_id)0x00000020)
#define V4L2_STD_PAL_D1 ((v4l2_std_id)0x00000040)
#define V4L2_STD_PAL_K ((v4l2_std_id)0x00000080)
#define V4L2_STD_PAL_M ((v4l2_std_id)0x00000100)
#define V4L2_STD_PAL_N ((v4l2_std_id)0x00000200)
#define V4L2_STD_PAL_Nc ((v4l2_std_id)0x00000400)
#define V4L2_STD_PAL_60 ((v4l2_std_id)0x00000800)
#define V4L2_STD_NTSC_M ((v4l2_std_id)0x00001000)
#define V4L2_STD_NTSC_M_JP ((v4l2_std_id)0x00002000)
#define V4L2_STD_NTSC_443 ((v4l2_std_id)0x00004000)
#define V4L2_STD_SECAM_B ((v4l2_std_id)0x00010000)
#define V4L2_STD_SECAM_D ((v4l2_std_id)0x00020000)
#define V4L2_STD_SECAM_G ((v4l2_std_id)0x00040000)
#define V4L2_STD_SECAM_H ((v4l2_std_id)0x00080000)
#define V4L2_STD_SECAM_K ((v4l2_std_id)0x00100000)
#define V4L2_STD_SECAM_K1 ((v4l2_std_id)0x00200000)
#define V4L2_STD_SECAM_L ((v4l2_std_id)0x00400000)
#define V4L2_STD_SECAM_LC ((v4l2_std_id)0x00800000)
/* ATSC/HDTV */
#define V4L2_STD_ATSC_8_VSB ((v4l2_std_id)0x01000000)
#define V4L2_STD_ATSC_16_VSB ((v4l2_std_id)0x02000000)
/* some common needed stuff */
#define V4L2_STD_PAL_BG (V4L2_STD_PAL_B |\
V4L2_STD_PAL_B1 |\
V4L2_STD_PAL_G)
#define V4L2_STD_PAL_DK (V4L2_STD_PAL_D |\
V4L2_STD_PAL_D1 |\
V4L2_STD_PAL_K)
#define V4L2_STD_PAL (V4L2_STD_PAL_BG |\
V4L2_STD_PAL_DK |\
V4L2_STD_PAL_H |\
V4L2_STD_PAL_I)
#define V4L2_STD_NTSC (V4L2_STD_NTSC_M |\
V4L2_STD_NTSC_M_JP)
#define V4L2_STD_SECAM_DK (V4L2_STD_SECAM_D |\
V4L2_STD_SECAM_K |\
V4L2_STD_SECAM_K1)
#define V4L2_STD_SECAM (V4L2_STD_SECAM_B |\
V4L2_STD_SECAM_G |\
V4L2_STD_SECAM_H |\
V4L2_STD_SECAM_DK |\
V4L2_STD_SECAM_L)
#define V4L2_STD_525_60 (V4L2_STD_PAL_M |\
V4L2_STD_PAL_60 |\
V4L2_STD_NTSC |\
V4L2_STD_NTSC_443)
#define V4L2_STD_625_50 (V4L2_STD_PAL |\
V4L2_STD_PAL_N |\
V4L2_STD_PAL_Nc |\
V4L2_STD_SECAM)
#define V4L2_STD_ATSC (V4L2_STD_ATSC_8_VSB |\
V4L2_STD_ATSC_16_VSB)
#define V4L2_STD_UNKNOWN 0
#define V4L2_STD_ALL (V4L2_STD_525_60 |\
V4L2_STD_625_50)
struct v4l2_standard
{
__u32 index;
v4l2_std_id id;
__u8 name[24];
struct v4l2_fract frameperiod; /* Frames, not fields */
__u32 framelines;
__u32 reserved[4];
};
/*
* V I D E O I N P U T S
*/
struct v4l2_input
{
__u32 index; /* Which input */
__u8 name[32]; /* Label */
__u32 type; /* Type of input */
__u32 audioset; /* Associated audios (bitfield) */
__u32 tuner; /* Associated tuner */
v4l2_std_id std;
__u32 status;
__u32 reserved[4];
};
/* Values for the 'type' field */
#define V4L2_INPUT_TYPE_TUNER 1
#define V4L2_INPUT_TYPE_CAMERA 2
/* field 'status' - general */
#define V4L2_IN_ST_NO_POWER 0x00000001 /* Attached device is off */
#define V4L2_IN_ST_NO_SIGNAL 0x00000002
#define V4L2_IN_ST_NO_COLOR 0x00000004
/* field 'status' - analog */
#define V4L2_IN_ST_NO_H_LOCK 0x00000100 /* No horizontal sync lock */
#define V4L2_IN_ST_COLOR_KILL 0x00000200 /* Color killer is active */
/* field 'status' - digital */
#define V4L2_IN_ST_NO_SYNC 0x00010000 /* No synchronization lock */
#define V4L2_IN_ST_NO_EQU 0x00020000 /* No equalizer lock */
#define V4L2_IN_ST_NO_CARRIER 0x00040000 /* Carrier recovery failed */
/* field 'status' - VCR and set-top box */
#define V4L2_IN_ST_MACROVISION 0x01000000 /* Macrovision detected */
#define V4L2_IN_ST_NO_ACCESS 0x02000000 /* Conditional access denied */
#define V4L2_IN_ST_VTR 0x04000000 /* VTR time constant */
/*
* V I D E O O U T P U T S
*/
struct v4l2_output
{
__u32 index; /* Which output */
__u8 name[32]; /* Label */
__u32 type; /* Type of output */
__u32 audioset; /* Associated audios (bitfield) */
__u32 modulator; /* Associated modulator */
v4l2_std_id std;
__u32 reserved[4];
};
/* Values for the 'type' field */
#define V4L2_OUTPUT_TYPE_MODULATOR 1
#define V4L2_OUTPUT_TYPE_ANALOG 2
#define V4L2_OUTPUT_TYPE_ANALOGVGAOVERLAY 3
/*
* C O N T R O L S
*/
struct v4l2_control
{
__u32 id;
__s32 value;
};
/* Used in the VIDIOC_QUERYCTRL ioctl for querying controls */
struct v4l2_queryctrl
{
__u32 id;
enum v4l2_ctrl_type type;
__u8 name[32]; /* Whatever */
__s32 minimum; /* Note signedness */
__s32 maximum;
__s32 step;
__s32 default_value;
__u32 flags;
__u32 reserved[2];
};
/* Used in the VIDIOC_QUERYMENU ioctl for querying menu items */
struct v4l2_querymenu
{
__u32 id;
__u32 index;
__u8 name[32]; /* Whatever */
__u32 reserved;
};
/* Control flags */
#define V4L2_CTRL_FLAG_DISABLED 0x0001
#define V4L2_CTRL_FLAG_GRABBED 0x0002
/* Control IDs defined by V4L2 */
#define V4L2_CID_BASE 0x00980900
/* IDs reserved for driver specific controls */
#define V4L2_CID_PRIVATE_BASE 0x08000000
#define V4L2_CID_BRIGHTNESS (V4L2_CID_BASE+0)
#define V4L2_CID_CONTRAST (V4L2_CID_BASE+1)
#define V4L2_CID_SATURATION (V4L2_CID_BASE+2)
#define V4L2_CID_HUE (V4L2_CID_BASE+3)
#define V4L2_CID_AUDIO_VOLUME (V4L2_CID_BASE+5)
#define V4L2_CID_AUDIO_BALANCE (V4L2_CID_BASE+6)
#define V4L2_CID_AUDIO_BASS (V4L2_CID_BASE+7)
#define V4L2_CID_AUDIO_TREBLE (V4L2_CID_BASE+8)
#define V4L2_CID_AUDIO_MUTE (V4L2_CID_BASE+9)
#define V4L2_CID_AUDIO_LOUDNESS (V4L2_CID_BASE+10)
#define V4L2_CID_BLACK_LEVEL (V4L2_CID_BASE+11)
#define V4L2_CID_AUTO_WHITE_BALANCE (V4L2_CID_BASE+12)
#define V4L2_CID_DO_WHITE_BALANCE (V4L2_CID_BASE+13)
#define V4L2_CID_RED_BALANCE (V4L2_CID_BASE+14)
#define V4L2_CID_BLUE_BALANCE (V4L2_CID_BASE+15)
#define V4L2_CID_GAMMA (V4L2_CID_BASE+16)
#define V4L2_CID_WHITENESS (V4L2_CID_GAMMA) /* ? Not sure */
#define V4L2_CID_EXPOSURE (V4L2_CID_BASE+17)
#define V4L2_CID_AUTOGAIN (V4L2_CID_BASE+18)
#define V4L2_CID_GAIN (V4L2_CID_BASE+19)
#define V4L2_CID_HFLIP (V4L2_CID_BASE+20)
#define V4L2_CID_VFLIP (V4L2_CID_BASE+21)
#define V4L2_CID_HCENTER (V4L2_CID_BASE+22)
#define V4L2_CID_VCENTER (V4L2_CID_BASE+23)
#define V4L2_CID_LASTP1 (V4L2_CID_BASE+24) /* last CID + 1 */
/*
* T U N I N G
*/
struct v4l2_tuner
{
__u32 index;
__u8 name[32];
enum v4l2_tuner_type type;
__u32 capability;
__u32 rangelow;
__u32 rangehigh;
__u32 rxsubchans;
__u32 audmode;
__s32 signal;
__s32 afc;
__u32 reserved[4];
};
struct v4l2_modulator
{
__u32 index;
__u8 name[32];
__u32 capability;
__u32 rangelow;
__u32 rangehigh;
__u32 txsubchans;
__u32 reserved[4];
};
/* Flags for the 'capability' field */
#define V4L2_TUNER_CAP_LOW 0x0001
#define V4L2_TUNER_CAP_NORM 0x0002
#define V4L2_TUNER_CAP_STEREO 0x0010
#define V4L2_TUNER_CAP_LANG2 0x0020
#define V4L2_TUNER_CAP_SAP 0x0020
#define V4L2_TUNER_CAP_LANG1 0x0040
/* Flags for the 'rxsubchans' field */
#define V4L2_TUNER_SUB_MONO 0x0001
#define V4L2_TUNER_SUB_STEREO 0x0002
#define V4L2_TUNER_SUB_LANG2 0x0004
#define V4L2_TUNER_SUB_SAP 0x0004
#define V4L2_TUNER_SUB_LANG1 0x0008
/* Values for the 'audmode' field */
#define V4L2_TUNER_MODE_MONO 0x0000
#define V4L2_TUNER_MODE_STEREO 0x0001
#define V4L2_TUNER_MODE_LANG2 0x0002
#define V4L2_TUNER_MODE_SAP 0x0002
#define V4L2_TUNER_MODE_LANG1 0x0003
struct v4l2_frequency
{
__u32 tuner;
enum v4l2_tuner_type type;
__u32 frequency;
__u32 reserved[8];
};
/*
* A U D I O
*/
struct v4l2_audio
{
__u32 index;
__u8 name[32];
__u32 capability;
__u32 mode;
__u32 reserved[2];
};
/* Flags for the 'capability' field */
#define V4L2_AUDCAP_STEREO 0x00001
#define V4L2_AUDCAP_AVL 0x00002
/* Flags for the 'mode' field */
#define V4L2_AUDMODE_AVL 0x00001
#define V4L2_AUDMODE_32BITS 0x00002
struct v4l2_audioout
{
__u32 index;
__u8 name[32];
__u32 capability;
__u32 mode;
__u32 reserved[2];
};
/*
* D A T A S E R V I C E S ( V B I )
*
* Data services API by Michael Schimek
*/
/* Raw VBI */
struct v4l2_vbi_format
{
__u32 sampling_rate; /* in 1 Hz */
__u32 offset;
__u32 samples_per_line;
__u32 sample_format; /* V4L2_PIX_FMT_* */
__s32 start[2];
__u32 count[2];
__u32 flags; /* V4L2_VBI_* */
__u32 reserved[2]; /* must be zero */
};
/* VBI flags */
#define V4L2_VBI_UNSYNC (1<< 0)
#define V4L2_VBI_INTERLACED (1<< 1)
#if 1 /*KEEP*/
/* Sliced VBI
*
* This implements is a proposal V4L2 API to allow SLICED VBI
* required for some hardware encoders. It should change without
* notice in the definitive implementation.
*/
struct v4l2_sliced_vbi_format
{
__u16 service_set;
/* service_lines[0][...] specifies lines 0-23 (1-23 used) of the first field
service_lines[1][...] specifies lines 0-23 (1-23 used) of the second field
(equals frame lines 313-336 for 625 line video
standards, 263-286 for 525 line standards) */
__u16 service_lines[2][24];
__u32 io_size;
__u32 reserved[2]; /* must be zero */
};
#define V4L2_SLICED_TELETEXT_B (0x0001)
#define V4L2_SLICED_VPS (0x0400)
#define V4L2_SLICED_CAPTION_525 (0x1000)
#define V4L2_SLICED_WSS_625 (0x4000)
#define V4L2_SLICED_VBI_525 (V4L2_SLICED_CAPTION_525)
#define V4L2_SLICED_VBI_625 (V4L2_SLICED_TELETEXT_B | V4L2_SLICED_VPS | V4L2_SLICED_WSS_625)
struct v4l2_sliced_vbi_cap
{
__u16 service_set;
/* service_lines[0][...] specifies lines 0-23 (1-23 used) of the first field
service_lines[1][...] specifies lines 0-23 (1-23 used) of the second field
(equals frame lines 313-336 for 625 line video
standards, 263-286 for 525 line standards) */
__u16 service_lines[2][24];
__u32 reserved[4]; /* must be 0 */
};
struct v4l2_sliced_vbi_data
{
__u32 id;
__u32 field; /* 0: first field, 1: second field */
__u32 line; /* 1-23 */
__u32 reserved; /* must be 0 */
__u8 data[48];
};
#endif
/*
* A G G R E G A T E S T R U C T U R E S
*/
/* Stream data format
*/
struct v4l2_format
{
enum v4l2_buf_type type;
union
{
struct v4l2_pix_format pix; // V4L2_BUF_TYPE_VIDEO_CAPTURE
struct v4l2_window win; // V4L2_BUF_TYPE_VIDEO_OVERLAY
struct v4l2_vbi_format vbi; // V4L2_BUF_TYPE_VBI_CAPTURE
#if 1 /*KEEP*/
struct v4l2_sliced_vbi_format sliced; // V4L2_BUF_TYPE_SLICED_VBI_CAPTURE
#endif
__u8 raw_data[200]; // user-defined
} fmt;
};
/* Stream type-dependent parameters
*/
struct v4l2_streamparm
{
enum v4l2_buf_type type;
union
{
struct v4l2_captureparm capture;
struct v4l2_outputparm output;
__u8 raw_data[200]; /* user-defined */
} parm;
};
/*
* I O C T L C O D E S F O R V I D E O D E V I C E S
*
*/
#define VIDIOC_QUERYCAP _IOR ('V', 0, struct v4l2_capability)
#define VIDIOC_RESERVED _IO ('V', 1)
#define VIDIOC_ENUM_FMT _IOWR ('V', 2, struct v4l2_fmtdesc)
#define VIDIOC_G_FMT _IOWR ('V', 4, struct v4l2_format)
#define VIDIOC_S_FMT _IOWR ('V', 5, struct v4l2_format)
#if 1 /* experimental */
#define VIDIOC_G_MPEGCOMP _IOR ('V', 6, struct v4l2_mpeg_compression)
#define VIDIOC_S_MPEGCOMP _IOW ('V', 7, struct v4l2_mpeg_compression)
#endif
#define VIDIOC_REQBUFS _IOWR ('V', 8, struct v4l2_requestbuffers)
#define VIDIOC_QUERYBUF _IOWR ('V', 9, struct v4l2_buffer)
#define VIDIOC_G_FBUF _IOR ('V', 10, struct v4l2_framebuffer)
#define VIDIOC_S_FBUF _IOW ('V', 11, struct v4l2_framebuffer)
#define VIDIOC_OVERLAY _IOW ('V', 14, int)
#define VIDIOC_QBUF _IOWR ('V', 15, struct v4l2_buffer)
#define VIDIOC_DQBUF _IOWR ('V', 17, struct v4l2_buffer)
#define VIDIOC_STREAMON _IOW ('V', 18, int)
#define VIDIOC_STREAMOFF _IOW ('V', 19, int)
#define VIDIOC_G_PARM _IOWR ('V', 21, struct v4l2_streamparm)
#define VIDIOC_S_PARM _IOWR ('V', 22, struct v4l2_streamparm)
#define VIDIOC_G_STD _IOR ('V', 23, v4l2_std_id)
#define VIDIOC_S_STD _IOW ('V', 24, v4l2_std_id)
#define VIDIOC_ENUMSTD _IOWR ('V', 25, struct v4l2_standard)
#define VIDIOC_ENUMINPUT _IOWR ('V', 26, struct v4l2_input)
#define VIDIOC_G_CTRL _IOWR ('V', 27, struct v4l2_control)
#define VIDIOC_S_CTRL _IOWR ('V', 28, struct v4l2_control)
#define VIDIOC_G_TUNER _IOWR ('V', 29, struct v4l2_tuner)
#define VIDIOC_S_TUNER _IOW ('V', 30, struct v4l2_tuner)
#define VIDIOC_G_AUDIO _IOR ('V', 33, struct v4l2_audio)
#define VIDIOC_S_AUDIO _IOW ('V', 34, struct v4l2_audio)
#define VIDIOC_QUERYCTRL _IOWR ('V', 36, struct v4l2_queryctrl)
#define VIDIOC_QUERYMENU _IOWR ('V', 37, struct v4l2_querymenu)
#define VIDIOC_G_INPUT _IOR ('V', 38, int)
#define VIDIOC_S_INPUT _IOWR ('V', 39, int)
#define VIDIOC_G_OUTPUT _IOR ('V', 46, int)
#define VIDIOC_S_OUTPUT _IOWR ('V', 47, int)
#define VIDIOC_ENUMOUTPUT _IOWR ('V', 48, struct v4l2_output)
#define VIDIOC_G_AUDOUT _IOR ('V', 49, struct v4l2_audioout)
#define VIDIOC_S_AUDOUT _IOW ('V', 50, struct v4l2_audioout)
#define VIDIOC_G_MODULATOR _IOWR ('V', 54, struct v4l2_modulator)
#define VIDIOC_S_MODULATOR _IOW ('V', 55, struct v4l2_modulator)
#define VIDIOC_G_FREQUENCY _IOWR ('V', 56, struct v4l2_frequency)
#define VIDIOC_S_FREQUENCY _IOW ('V', 57, struct v4l2_frequency)
#define VIDIOC_CROPCAP _IOWR ('V', 58, struct v4l2_cropcap)
#define VIDIOC_G_CROP _IOWR ('V', 59, struct v4l2_crop)
#define VIDIOC_S_CROP _IOW ('V', 60, struct v4l2_crop)
#define VIDIOC_G_JPEGCOMP _IOR ('V', 61, struct v4l2_jpegcompression)
#define VIDIOC_S_JPEGCOMP _IOW ('V', 62, struct v4l2_jpegcompression)
#define VIDIOC_QUERYSTD _IOR ('V', 63, v4l2_std_id)
#define VIDIOC_TRY_FMT _IOWR ('V', 64, struct v4l2_format)
#define VIDIOC_ENUMAUDIO _IOWR ('V', 65, struct v4l2_audio)
#define VIDIOC_ENUMAUDOUT _IOWR ('V', 66, struct v4l2_audioout)
#define VIDIOC_G_PRIORITY _IOR ('V', 67, enum v4l2_priority)
#define VIDIOC_S_PRIORITY _IOW ('V', 68, enum v4l2_priority)
#if 1 /*KEEP*/
#define VIDIOC_G_SLICED_VBI_CAP _IOR ('V', 69, struct v4l2_sliced_vbi_cap)
#endif
#define VIDIOC_LOG_STATUS _IO ('V', 70)
/* for compatibility, will go away some day */
#define VIDIOC_OVERLAY_OLD _IOWR ('V', 14, int)
#define VIDIOC_S_PARM_OLD _IOW ('V', 22, struct v4l2_streamparm)
#define VIDIOC_S_CTRL_OLD _IOW ('V', 28, struct v4l2_control)
#define VIDIOC_G_AUDIO_OLD _IOWR ('V', 33, struct v4l2_audio)
#define VIDIOC_G_AUDOUT_OLD _IOWR ('V', 49, struct v4l2_audioout)
#define VIDIOC_CROPCAP_OLD _IOR ('V', 58, struct v4l2_cropcap)
#define BASE_VIDIOC_PRIVATE 192 /* 192-255 are private */
#ifdef __KERNEL__
/*
*
* V 4 L 2 D R I V E R H E L P E R A P I
*
* Some commonly needed functions for drivers (v4l2-common.o module)
*/
#include <linux/fs.h>
/* Video standard functions */
extern unsigned int v4l2_video_std_fps(struct v4l2_standard *vs);
extern int v4l2_video_std_construct(struct v4l2_standard *vs,
int id, char *name);
/* prority handling */
struct v4l2_prio_state {
atomic_t prios[4];
};
int v4l2_prio_init(struct v4l2_prio_state *global);
int v4l2_prio_change(struct v4l2_prio_state *global, enum v4l2_priority *local,
enum v4l2_priority new);
int v4l2_prio_open(struct v4l2_prio_state *global, enum v4l2_priority *local);
int v4l2_prio_close(struct v4l2_prio_state *global, enum v4l2_priority *local);
enum v4l2_priority v4l2_prio_max(struct v4l2_prio_state *global);
int v4l2_prio_check(struct v4l2_prio_state *global, enum v4l2_priority *local);
/* names for fancy debug output */
extern char *v4l2_field_names[];
extern char *v4l2_type_names[];
extern char *v4l2_ioctl_names[];
/* Compatibility layer interface -- v4l1-compat module */
typedef int (*v4l2_kioctl)(struct inode *inode, struct file *file,
unsigned int cmd, void *arg);
int v4l_compat_translate_ioctl(struct inode *inode, struct file *file,
int cmd, void *arg, v4l2_kioctl driver_ioctl);
#endif /* __KERNEL__ */
#endif /* __LINUX_VIDEODEV2_H */
/*
* Local variables:
* c-basic-offset: 8
* End:
*/

Appendix B. Video Capture Example

/*
* V4L2 video capture example
*
* This program can be used and distributed without restrictions.
*/
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include <getopt.h> /* getopt_long() */
#include <fcntl.h> /* low-level i/o */
#include <unistd.h>
#include <errno.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/mman.h>
#include <sys/ioctl.h>
#include <asm/types.h> /* for videodev2.h */
#include <linux/videodev2.h>
#define CLEAR(x) memset (&(x), 0, sizeof (x))
typedef enum {
IO_METHOD_READ,
IO_METHOD_MMAP,
IO_METHOD_USERPTR,
} io_method;
struct buffer {
void * start;
size_t length;
};
static char * dev_name = NULL;
static io_method io = IO_METHOD_MMAP;
static int fd = -1;
struct buffer * buffers = NULL;
static unsigned int n_buffers = 0;
static void
errno_exit (const char * s)
{
fprintf (stderr, "%s error %d, %s\n",
s, errno, strerror (errno));
exit (EXIT_FAILURE);
}
static int
xioctl (int fd,
int request,
void * arg)
{
int r;
do r = ioctl (fd, request, arg);
while (-1 == r && EINTR == errno);
return r;
}
static void
process_image (const void * p)
{
fputc ('.', stdout);
fflush (stdout);
}
static int
read_frame (void)
{
struct v4l2_buffer buf;
unsigned int i;
switch (io) {
case IO_METHOD_READ:
if (-1 == read (fd, buffers[0].start, buffers[0].length)) {
switch (errno) {
case EAGAIN:
return 0;
case EIO:
/* Could ignore EIO, see spec. */
/* fall through */
default:
errno_exit ("read");
}
}
process_image (buffers[0].start);
break;
case IO_METHOD_MMAP:
CLEAR (buf);
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP;
if (-1 == xioctl (fd, VIDIOC_DQBUF, &buf)) {
switch (errno) {
case EAGAIN:
return 0;
case EIO:
/* Could ignore EIO, see spec. */
/* fall through */
default:
errno_exit ("VIDIOC_DQBUF");
}
}
assert (buf.index < n_buffers);
process_image (buffers[buf.index].start);
if (-1 == xioctl (fd, VIDIOC_QBUF, &buf))
errno_exit ("VIDIOC_QBUF");
break;
case IO_METHOD_USERPTR:
CLEAR (buf);
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_USERPTR;
if (-1 == xioctl (fd, VIDIOC_DQBUF, &buf)) {
switch (errno) {
case EAGAIN:
return 0;
case EIO:
/* Could ignore EIO, see spec. */
/* fall through */
default:
errno_exit ("VIDIOC_DQBUF");
}
}
for (i = 0; i < n_buffers; ++i)
if (buf.m.userptr == (unsigned long) buffers[i].start
&& buf.length == buffers[i].length)
break;
assert (i < n_buffers);
process_image ((void *) buf.m.userptr);
if (-1 == xioctl (fd, VIDIOC_QBUF, &buf))
errno_exit ("VIDIOC_QBUF");
break;
}
return 1;
}
static void
mainloop (void)
{
unsigned int count;
count = 100;
while (count-- > 0) {
for (;;) {
fd_set fds;
struct timeval tv;
int r;
FD_ZERO (&fds);
FD_SET (fd, &fds);
/* Timeout. */
tv.tv_sec = 2;
tv.tv_usec = 0;
r = select (fd + 1, &fds, NULL, NULL, &tv);
if (-1 == r) {
if (EINTR == errno)
continue;
errno_exit ("select");
}
if (0 == r) {
fprintf (stderr, "select timeout\n");
exit (EXIT_FAILURE);
}
if (read_frame ())
break;
/* EAGAIN - continue select loop. */
}
}
}
static void
stop_capturing (void)
{
enum v4l2_buf_type type;
switch (io) {
case IO_METHOD_READ:
/* Nothing to do. */
break;
case IO_METHOD_MMAP:
case IO_METHOD_USERPTR:
type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (-1 == xioctl (fd, VIDIOC_STREAMOFF, &type))
errno_exit ("VIDIOC_STREAMOFF");
break;
}
}
static void
start_capturing (void)
{
unsigned int i;
enum v4l2_buf_type type;
switch (io) {
case IO_METHOD_READ:
/* Nothing to do. */
break;
case IO_METHOD_MMAP:
for (i = 0; i < n_buffers; ++i) {
struct v4l2_buffer buf;
CLEAR (buf);
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP;
buf.index = i;
if (-1 == xioctl (fd, VIDIOC_QBUF, &buf))
errno_exit ("VIDIOC_QBUF");
}
type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (-1 == xioctl (fd, VIDIOC_STREAMON, &type))
errno_exit ("VIDIOC_STREAMON");
break;
case IO_METHOD_USERPTR:
for (i = 0; i < n_buffers; ++i) {
struct v4l2_buffer buf;
CLEAR (buf);
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_USERPTR;
buf.m.userptr = (unsigned long) buffers[i].start;
buf.length = buffers[i].length;
if (-1 == xioctl (fd, VIDIOC_QBUF, &buf))
errno_exit ("VIDIOC_QBUF");
}
type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (-1 == xioctl (fd, VIDIOC_STREAMON, &type))
errno_exit ("VIDIOC_STREAMON");
break;
}
}
static void
uninit_device (void)
{
unsigned int i;
switch (io) {
case IO_METHOD_READ:
free (buffers[0].start);
break;
case IO_METHOD_MMAP:
for (i = 0; i < n_buffers; ++i)
if (-1 == munmap (buffers[i].start, buffers[i].length))
errno_exit ("munmap");
break;
case IO_METHOD_USERPTR:
for (i = 0; i < n_buffers; ++i)
free (buffers[i].start);
break;
}
free (buffers);
}
static void
init_read (unsigned int buffer_size)
{
buffers = calloc (1, sizeof (*buffers));
if (!buffers) {
fprintf (stderr, "Out of memory\n");
exit (EXIT_FAILURE);
}
buffers[0].length = buffer_size;
buffers[0].start = malloc (buffer_size);
if (!buffers[0].start) {
fprintf (stderr, "Out of memory\n");
exit (EXIT_FAILURE);
}
}
static void
init_mmap (void)
{
struct v4l2_requestbuffers req;
CLEAR (req);
req.count = 4;
req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
req.memory = V4L2_MEMORY_MMAP;
if (-1 == xioctl (fd, VIDIOC_REQBUFS, &req)) {
if (EINVAL == errno) {
fprintf (stderr, "%s does not support "
"memory mapping\n", dev_name);
exit (EXIT_FAILURE);
} else {
errno_exit ("VIDIOC_REQBUFS");
}
}
if (req.count < 2) {
fprintf (stderr, "Insufficient buffer memory on %s\n",
dev_name);
exit (EXIT_FAILURE);
}
buffers = calloc (req.count, sizeof (*buffers));
if (!buffers) {
fprintf (stderr, "Out of memory\n");
exit (EXIT_FAILURE);
}
for (n_buffers = 0; n_buffers < req.count; ++n_buffers) {
struct v4l2_buffer buf;
CLEAR (buf);
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP;
buf.index = n_buffers;
if (-1 == xioctl (fd, VIDIOC_QUERYBUF, &buf))
errno_exit ("VIDIOC_QUERYBUF");
buffers[n_buffers].length = buf.length;
buffers[n_buffers].start =
mmap (NULL /* start anywhere */,
buf.length,
PROT_READ | PROT_WRITE /* required */,
MAP_SHARED /* recommended */,
fd, buf.m.offset);
if (MAP_FAILED == buffers[n_buffers].start)
errno_exit ("mmap");
}
}
static void
init_userp (unsigned int buffer_size)
{
struct v4l2_requestbuffers req;
CLEAR (req);
req.count = 4;
req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
req.memory = V4L2_MEMORY_USERPTR;
if (-1 == xioctl (fd, VIDIOC_REQBUFS, &req)) {
if (EINVAL == errno) {
fprintf (stderr, "%s does not support "
"user pointer i/o\n", dev_name);
exit (EXIT_FAILURE);
} else {
errno_exit ("VIDIOC_REQBUFS");
}
}
buffers = calloc (4, sizeof (*buffers));
if (!buffers) {
fprintf (stderr, "Out of memory\n");
exit (EXIT_FAILURE);
}
for (n_buffers = 0; n_buffers < 4; ++n_buffers) {
buffers[n_buffers].length = buffer_size;
buffers[n_buffers].start = malloc (buffer_size);
if (!buffers[n_buffers].start) {
fprintf (stderr, "Out of memory\n");
exit (EXIT_FAILURE);
}
}
}
static void
init_device (void)
{
struct v4l2_capability cap;
struct v4l2_cropcap cropcap;
struct v4l2_crop crop;
struct v4l2_format fmt;
unsigned int min;
if (-1 == xioctl (fd, VIDIOC_QUERYCAP, &cap)) {
if (EINVAL == errno) {
fprintf (stderr, "%s is no V4L2 device\n",
dev_name);
exit (EXIT_FAILURE);
} else {
errno_exit ("VIDIOC_QUERYCAP");
}
}
if (!(cap.capabilities & V4L2_CAP_VIDEO_CAPTURE)) {
fprintf (stderr, "%s is no video capture device\n",
dev_name);
exit (EXIT_FAILURE);
}
switch (io) {
case IO_METHOD_READ:
if (!(cap.capabilities & V4L2_CAP_READWRITE)) {
fprintf (stderr, "%s does not support read i/o\n",
dev_name);
exit (EXIT_FAILURE);
}
break;
case IO_METHOD_MMAP:
case IO_METHOD_USERPTR:
if (!(cap.capabilities & V4L2_CAP_STREAMING)) {
fprintf (stderr, "%s does not support streaming i/o\n",
dev_name);
exit (EXIT_FAILURE);
}
break;
}
/* Select video input, video standard and tune here. */
CLEAR (cropcap);
cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (0 == xioctl (fd, VIDIOC_CROPCAP, &cropcap)) {
crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
crop.c = cropcap.defrect; /* reset to default */
if (-1 == xioctl (fd, VIDIOC_S_CROP, &crop)) {
switch (errno) {
case EINVAL:
/* Cropping not supported. */
break;
default:
/* Errors ignored. */
break;
}
}
} else {
/* Errors ignored. */
}
CLEAR (fmt);
fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
fmt.fmt.pix.width = 640;
fmt.fmt.pix.height = 480;
fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
fmt.fmt.pix.field = V4L2_FIELD_INTERLACED;
if (-1 == xioctl (fd, VIDIOC_S_FMT, &fmt))
errno_exit ("VIDIOC_S_FMT");
/* Note VIDIOC_S_FMT may change width and height. */
/* Buggy driver paranoia. */
min = fmt.fmt.pix.width * 2;
if (fmt.fmt.pix.bytesperline < min)
fmt.fmt.pix.bytesperline = min;
min = fmt.fmt.pix.bytesperline * fmt.fmt.pix.height;
if (fmt.fmt.pix.sizeimage < min)
fmt.fmt.pix.sizeimage = min;
switch (io) {
case IO_METHOD_READ:
init_read (fmt.fmt.pix.sizeimage);
break;
case IO_METHOD_MMAP:
init_mmap ();
break;
case IO_METHOD_USERPTR:
init_userp (fmt.fmt.pix.sizeimage);
break;
}
}
static void
close_device (void)
{
if (-1 == close (fd))
errno_exit ("close");
fd = -1;
}
static void
open_device (void)
{
struct stat st;
if (-1 == stat (dev_name, &st)) {
fprintf (stderr, "Cannot identify '%s': %d, %s\n",
dev_name, errno, strerror (errno));
exit (EXIT_FAILURE);
}
if (!S_ISCHR (st.st_mode)) {
fprintf (stderr, "%s is no device\n", dev_name);
exit (EXIT_FAILURE);
}
fd = open (dev_name, O_RDWR /* required */ | O_NONBLOCK, 0);
if (-1 == fd) {
fprintf (stderr, "Cannot open '%s': %d, %s\n",
dev_name, errno, strerror (errno));
exit (EXIT_FAILURE);
}
}
static void
usage (FILE * fp,
int argc,
char ** argv)
{
fprintf (fp,
"Usage: %s [options]\n\n"
"Options:\n"
"-d | --device name Video device name [/dev/video]\n"
"-h | --help Print this message\n"
"-m | --mmap Use memory mapped buffers\n"
"-r | --read Use read() calls\n"
"-u | --userp Use application allocated buffers\n"
"",
argv[0]);
}
static const char short_options [] = "d:hmru";
static const struct option
long_options [] = {
{ "device", required_argument, NULL, 'd' },
{ "help", no_argument, NULL, 'h' },
{ "mmap", no_argument, NULL, 'm' },
{ "read", no_argument, NULL, 'r' },
{ "userp", no_argument, NULL, 'u' },
{ 0, 0, 0, 0 }
};
int
main (int argc,
char ** argv)
{
dev_name = "/dev/video";
for (;;) {
int index;
int c;
c = getopt_long (argc, argv,
short_options, long_options,
&index);
if (-1 == c)
break;
switch (c) {
case 0: /* getopt_long() flag */
break;
case 'd':
dev_name = optarg;
break;
case 'h':
usage (stdout, argc, argv);
exit (EXIT_SUCCESS);
case 'm':
io = IO_METHOD_MMAP;
break;
case 'r':
io = IO_METHOD_READ;
break;
case 'u':
io = IO_METHOD_USERPTR;
break;
default:
usage (stderr, argc, argv);
exit (EXIT_FAILURE);
}
}
open_device ();
init_device ();
start_capturing ();
mainloop ();
stop_capturing ();
uninit_device ();
close_device ();
exit (EXIT_SUCCESS);
return 0;
}

Appendix C. GNU Free Documentation License

C.1. 0. PREAMBLE

The purpose of this License is to make a manual, textbook, or other written document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

This License is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.


C.2. 1. APPLICABILITY AND DEFINITIONS

This License applies to any manual or other work that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. The "Document", below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as "you".

A "Modified Version" of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.

A "Secondary Section" is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (For example, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.

The "Invariant Sections" are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License.

The "Cover Texts" are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License.

A "Transparent" copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, whose contents can be viewed and edited directly and straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup has been designed to thwart or discourage subsequent modification by readers is not Transparent. A copy that is not "Transparent" is called "Opaque".

Examples of suitable formats for Transparent copies include plain ASCII without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML designed for human modification. Opaque formats include PostScript, PDF, proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML produced by some word processors for output purposes only.

The "Title Page" means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, "Title Page" means the text near the most prominent appearance of the work's title, preceding the beginning of the body of the text.


C.3. 2. VERBATIM COPYING

You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.

You may also lend copies, under the same conditions stated above, and you may publicly display copies.


C.4. 3. COPYING IN QUANTITY

If you publish printed copies of the Document numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.

If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.

If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a publicly-accessible computer-network location containing a complete Transparent copy of the Document, free of added material, which the general network-using public has access to download anonymously at no charge using public-standard network protocols. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.

It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.


C.5. 4. MODIFICATIONS

You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:

  • A. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.

  • B. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has less than five).

  • C. State on the Title Page the name of the publisher of the Modified Version, as the publisher.

  • D. Preserve all the copyright notices of the Document.

  • E. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.

  • F. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below.

  • G. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document's license notice.

  • H. Include an unaltered copy of this License.

  • I. Preserve the section entitled "History", and its title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section entitled "History" in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.

  • J. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the "History" section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.

  • K. In any section entitled "Acknowledgements" or "Dedications", preserve the section's title, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.

  • L. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.

  • M. Delete any section entitled "Endorsements". Such a section may not be included in the Modified Version.

  • N. Do not retitle any existing section as "Endorsements" or to conflict in title with any Invariant Section.

If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles.

You may add a section entitled "Endorsements", provided it contains nothing but endorsements of your Modified Version by various parties--for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.

You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.

The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version .


C.6. 5. COMBINING DOCUMENTS

You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice.

The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.

In the combination, you must combine any sections entitled "History" in the various original documents, forming one section entitled "History"; likewise combine any sections entitled "Acknowledgements", and any sections entitled "Dedications". You must delete all sections entitled "Endorsements."


C.7. 6. COLLECTIONS OF DOCUMENTS

You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

You may extract a single document from such a collection, and dispbibute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.


C.8. 7. AGGREGATION WITH INDEPENDENT WORKS

A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, does not as a whole count as a Modified Version of the Document, provided no compilation copyright is claimed for the compilation. Such a compilation is called an "aggregate", and this License does not apply to the other self-contained works thus compiled with the Document , on account of their being thus compiled, if they are not themselves derivative works of the Document. If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one quarter of the entire aggregate, the Document's Cover Texts may be placed on covers that surround only the Document within the aggregate. Otherwise they must appear on covers around the whole aggregate.


C.9. 8. TRANSLATION

Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License provided that you also include the original English version of this License. In case of a disagreement between the translation and the original English version of this License, the original English version will prevail.


C.10. 9. TERMINATION

You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.


C.11. 10. FUTURE REVISIONS OF THIS LICENSE

The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License "or any later version" applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.


C.12. Addendum

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:

Copyright © YEAR YOUR NAME.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.1 or any later version published by the Free Software Foundation; with the Invariant Sections being LIST THEIR TITLES, with the Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST. A copy of the license is included in the section entitled "GNU Free Documentation License".

If you have no Invariant Sections, write "with no Invariant Sections" instead of saying which ones are invariant. If you have no Front-Cover Texts, write "no Front-Cover Texts" instead of "Front-Cover Texts being LIST"; likewise for Back-Cover Texts.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.


Bibliography

[EIA608] Electronic Industries Alliance (http://www.eia.org), EIA 608 "Recommended Practice for Line 21 Data Service".

[ITU470] International Telecommunication Union (http://www.itu.ch), ITU-R Recommendation BT.470-6 "Conventional Television Systems".

[ITU601] International Telecommunication Union (http://www.itu.ch), ITU-R Recommendation BT.601-5 "Studio Encoding Parameters of Digital Television for Standard 4:3 and Wide-Screen 16:9 Aspect Ratios".

[ITU709] International Telecommunication Union (http://www.itu.ch), ITU-R Recommendation BT.709-5 "Parameter values for the HDTV standards for production and international programme exchange".

[JFIF] Independent JPEG Group (http://www.ijg.org), JPEG File Interchange Format: Version 1.02.

[SMPTE12M] Society of Motion Picture and Television Engineers (http://www.smpte.org), SMPTE 12M-1999 "Television, Audio and Film - Time and Control Code".

[SMPTE170M] Society of Motion Picture and Television Engineers (http://www.smpte.org), SMPTE 170M-1999 "Television - Composite Analog Video Signal - NTSC for Studio Applications".

[SMPTE240M] Society of Motion Picture and Television Engineers (http://www.smpte.org), SMPTE 240M-1999 "Television - Signal Parameters - 1125-Line High-Definition Production".

[TELETEXT] European Telecommunication Standards Institute (http://www.etsi.org), ETS 300 706 "Enhanced Teletext specification".

[V4L] Alan Cox, Video4Linux API Specification.

This file is part of the Linux kernel sources under Documentation/video4linux.

[V4LPROG] Alan Cox, Video4Linux Programming (a.k.a. The Video4Linux Book), 2000.

About V4L driver programming. This book is part of the Linux kernel DocBook documentation, for example at http://kernelnewbies.org/documents/. SGML sources are included in the kernel sources.

[VPS] European Telecommunication Standards Institute (http://www.etsi.org), ETS 300 231 "Specification of the domestic video Programme Delivery Control system (PDC)".

[WSS] International Telecommunication Union (http://www.itu.ch), European Telecommunication Standards Institute (http://www.etsi.org), ITU-R Recommendation BT.1119, EN 300 294 "625-line television Wide Screen Signalling (WSS)".

Notes

[1]

Access permissions are associated with character device special files, we must ensure device numbers cannot change with load order. To this end minor numbers are no longer automatically assigned by the "videodev" module as in V4L but requested by the driver. The defaults will suffice for most people, unless two drivers are used which compete for the same minor numbers.

[2]

In earlier versions of the V4L2 API the module options where named after the device special file with a "unit_" prefix, expressing the minor number itself, not an offset. Rationale for this change is unknown. Lastly the naming and semantics are just a convention among driver writers, the point to note is that minor numbers are not supposed to be hardcoded into drivers.

[3]

Given a device file name one cannot reliable find related devices. For once names are arbitrary, they can be chosen freely by the system administrator. Also when there are multiple devices and only some support VBI capturing, say, /dev/video2 is not necessarily related to /dev/vbi2. We already noted finding devices by name or minor number is unreliable, accordingly useful is the ioctl offered by V4L to query the minor numbers of related devices.

[4]

Drivers could recognize the O_EXCL open flag. Presently this is not required, so application cannot know if it really works.

[5]

Actually struct v4l2_audio ought to have a tuner field like struct v4l2_input, not only making the API more consistent but also permitting radio devices with multiple tuners.

[6]

Some users are already confused by technical terms PAL, NTSC and SECAM. There is no point asking them to distinguish between B, G, D, or K when the software or hardware can do that automatically.

[7]

An alternative to the current scheme is to use pointers to indices as arguments of VIDIOC_G_STD and VIDIOC_S_STD, the struct v4l2_input and struct v4l2_output std field would be a set of indices like audioset.

Indices are consistent with the rest of the API and identify the standard unambiguously. In the present scheme of things an enumerated standard is looked up by v4l2_std_id. Now the standards supported by the inputs of a device can overlap. Just assume the tuner and composite input in the example above both exist on a device. An enumeration of "PAL-B/G", "PAL-H/I" suggests a choice which does not exist. We cannot merge or omit sets, because applications would be unable to find the standards reported by VIDIOC_G_STD. That leaves separate enumerations for each input. Also selecting a standard by v4l2_std_id can be ambiguous. Advantage of this method is that applications need not identify the standard indirectly, after enumerating.

So in summary, the lookup itself is unavoidable. The difference is only whether the lookup is necessary to find an enumerated standard or to switch to a standard by v4l2_std_id.

[8]

See Section 3.5> for a rationale. Probably even USB cameras follow some well known video standard. It might have been better to explicitly indicate elsewhere if a device cannot live up to normal expectations, instead of this exception.

[9]

It will be more convenient for applications if drivers make use of the V4L2_CTRL_FLAG_DISABLED flag, but that was never required.

[10]

Applications could call an ioctl to request events. After another process called VIDIOC_S_CTRL or another ioctl changing shared properties the select() function would indicate readability until any ioctl (querying the properties) is called.

[11]

Enumerating formats an application has no a-priori knowledge of (otherwise it could explicitely ask for them and need not enumerate) seems useless, but there are applications serving as proxy between drivers and the actual video applications for which this is useful.

[12]

It may be desirable to refer to the cropping area in terms of sampling frequency and scanning system lines, but in order to support a wide range of hardware we better make as few assumptions as possible.

[13]

It would be desirable if applications could depend on drivers supporting all I/O interfaces, but as much as the complex memory mapping I/O can be inadequate for some devices we have no reason to require this interface, which is most useful for simple applications capturing still images.

[14]

At the driver level select() and poll() are the same, and select() is too important to be optional.

[15]

One could use one file descriptor and set the buffer type field accordingly when calling VIDIOC_QBUF etc., but it makes the select() function ambiguous. We also like the clean approach of one file descriptor per logical stream. Video overlay for example is also a logical stream, although the CPU is not needed for continuous operation.

[16]

Random enqueue order permits applications processing images out of order (such as video codecs) to return buffers earlier, reducing the probability of data loss. Random fill order allows drivers to reuse buffers on a LIFO-basis, taking advantage of caches holding scatter-gather lists and the like.

[17]

At the driver level select() and poll() are the same, and select() is too important to be optional. The rest should be evident.

[18]

We expect that frequently used buffers are typically not swapped out. Anyway, the process of swapping, locking or generating scatter-gather lists may be time consuming. The delay can be masked by the depth of the incoming buffer queue, and perhaps by maintaining caches assuming a buffer will be soon enqueued again. On the other hand, to optimize memory usage drivers can limit the number of buffers locked in advance and recycle the most recently used buffers first. Of course, the pages of empty buffers in the incoming queue need not be saved to disk. Output buffers must be saved on the incoming and outgoing queue because an application may share them with other processes.

[19]

At the driver level select() and poll() are the same, and select() is too important to be optional. The rest should be evident.

[20]

Since no other Linux multimedia API supports unadjusted time it would be foolish to introduce here. We must use a universally supported clock to synchronize different media, hence time of day.

[21]

A common application of two file descriptors is the XFree86 Xv/V4L interface driver and a V4L2 application. While the X server controls video overlay, the application can take advantage of memory mapping and DMA.

In the opinion of the designers of this API, no driver writer taking the efforts to support simultaneous capturing and overlay will restrict this ability by requiring a single file descriptor, as in V4L and earlier versions of V4L2. Making this optional means applications depending on two file descriptors need backup routines to be compatible with all drivers, which is considerable more work than using two fds in applications which do not. Also two fd's fit the general concept of one file descriptor for each logical stream. Hence as a complexity trade-off drivers must support two file descriptors and may support single fd operation.

[22]

The X Window system defines "regions" which are vectors of struct BoxRec { short x1, y1, x2, y2; } with width = x2 - x1 and height = y2 - y1, so one cannot pass X11 clip lists directly.

[23]

ASK: Amplitude-Shift Keying. A high signal level represents a '1' bit, a low level a '0' bit.

[24]

The supported standards may overlap and we need an unambiguous set to find the current standard returned by VIDIOC_G_STD.

[25]

V4L2_CTRL_FLAG_DISABLED was intended for two purposes: Drivers can skip predefined controls not supported by the hardware (although returning EINVAL would do as well), or disable predefined and custom controls after hardware detection without the trouble of reordering control arrays and indices.

[26]

For example video output requires at least two buffers, one displayed and one filled by the application.

[27]

This is not implemented in XFree86.