Manual browser: video(4)

VIDEO(4) Kernel Interfaces Manual VIDEO(4)


videodevice-independent video driver layer


#include <sys/videoio.h>


The video driver provides support for various video peripherals. It provides a uniform programming interface layer above different underlying video hardware drivers. The video layer provides a Video4Linux2 compatible API. A number of ioctl(2) commands are supported controlling the device.

The device file for video operation is /dev/video.


Video data is separated into logical video samples which will typically be one complete video frame. With compressed formats, a video sample may be one logical chunk and not one complete frame depending on the compression format. Video samples may be read from /dev/video in one of several different modes.

In read mode, calls to read(2) will return at most the data of one video sample. If the entire sample is not read, then subsequent reads will return at most the remaining data in that video sample.

Video samples may be mapped into memory with mmap(2). The driver allocates internal buffers for a number of video samples which are mapped into memory. Initiating this mode requires several ioctl(2) commands: VIDIOC_REQBUFS to request the driver reserve buffers, VIDIOC_QUERYBUF to query the details of each buffer, mmap(2) to map each buffer into memory, VIDIOC_QBUF to queue the buffers for receiving video data, VIDIOC_STREAMON to begin streaming of video data, and VIDIOC_DQBUF to remove a filled buffer from the queue. At this point the video data from the dequeued buffer is valid.


VIDIOC_QUERYCAP (struct v4l2_capability)
This command queries the capabilities of the device. The first three fields are informational NULL terminated strings filled by the driver: driver describes the driver used by this device, card describes the video capture card or camera, and buf_info represents the bus to which the hardware device is attached.

The capabilities field contains a number of flags indicating various features supported by the driver or hardware:

support video capturing
supports the read(2) and/or write(2) mode
supports mmap(2) mode

struct v4l2_capability { 
	uint8_t		driver[16]; 
	uint8_t		card[32]; 
	uint8_t		bus_info[32]; 
	uint32_t	version; 
	uint32_t	capabilities; 
	uint32_t	reserved[4]; 


VIDIOC_REQBUFS (struct v4l2_requestbuffers)
This command requests that the driver reserve space for count samples. type must be set to V4L2_BUF_TYPE_VIDEO_CAPTURE and memory to V4L2_MEMORY_MMAP. The returned count represents the actual number of samples reserved which may be more or fewer than requested.

struct v4l2_requestbuffers { 
	uint32_t		count; 
	enum v4l2_buf_type	type; 
	enum v4l2_memory	memory; 
	uint32_t		reserved[2]; 
VIDIOC_QUERYBUF (struct v4l2_buffer)
This command should be called for each buffer in count above. The fields index, type, and memory must be set to a valid index from 0 to count-1, and the same type and memory as used in VIDIOC_QUERYBUF. The driver returns m.offset and length.

struct v4l2_buffer { 
	uint32_t		index; 
	enum v4l2_buf_type	type; 
	uint32_t		bytesused; 
	uint32_t		flags; 
	enum v4l2_field		field; 
	struct timeval		timestamp; 
	struct v4l2_timecode	timecode; 
	uint32_t		sequence; 
	enum v4l2_memory	memory; 
	union { 
		uint32_t	offset; 
		unsigned long	userptr; 
	} m; 
	uint32_t		length; 
	uint32_t		input; 
	uint32_t		reserved; 
Each buffer must be mapped with a call to mmap(2), passing the length and m.offset values obtained above. The prot PROT_READ|PROT_WRITE and flags MAP_SHARED are recommended.
VIDIOC_QBUF (struct v4l2_buffer)
This command indicates to the driver that the buffer is ready to receive a video sample. The following fields must be set: index, set to a valid buffer index from 0 to count - 1; type, set to the same type used above; and memory, set to the same memory used above. Each buffer should be queued with this command. Order is not important.
This command starts streaming. Queued buffers will be filled with data. select(2) will indicate that a buffer is done and available for reading.
VIDIOC_DQBUF (struct v4l2_buffer)
This command dequeues an available buffer from the driver. If no buffer is available, it blocks until one is, unless O_NONBLOCK was specified to open(2), in which case it returns EAGAIN. select(2), or poll(2) prior to initiating any other mode will begin streaming of video for reading with read(2). In this streaming mode select(2) or poll(2) indicate the availability of a video frame. Calls to read(2) will return at most the video data of one video sample. If the entire sample is not read, then subsequent reads will return at most the remaining data in that video sample.




The video device driver first appeared in NetBSD 5.0.


Patrick Mahoney <>


Does not support the complete V4L2 API. Only supports the capture interface. Does not support writing, overlay, VBI, tuner, audio, radio, or asyncio.
March 5, 2011 NetBSD 7.0