# 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 .

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 ) 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.

# Read/Write

Input and output devices support the `read()` and `write()` function, respectively, when the `V4L2_CAP_READWRITE` flag in the capabilities field of &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 &func-read; function, to write the &func-write; function. Drivers must implement one I/O method if they exchange data with applications, but it need not be this. 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. When reading or writing is supported, the driver must also support the &func-select; and &func-poll; function. At the driver level `select()` and `poll()` are the same, and `select()` is too important to be optional.

# 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 &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. 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.

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 &func-mmap; function. The location of the buffers in device memory can be determined with the &VIDIOC-QUERYBUF; ioctl. In the single-planar API case, the m.offset and length returned in a &v4l2-buffer; are passed as sixth and second parameter to the `mmap()` function. When using the multi-planar API, struct &v4l2-buffer; contains an array of &v4l2-plane; structures, each containing its own m.offset and length. When using the multi-planar API, every plane of every buffer has to be mapped separately, so the number of calls to &func-mmap; should be equal to number of buffers times number of planes in each buffer. 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 &func-munmap; function.

## Mapping buffers in the single-planar API

    &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++) {
    	&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, /* recommended */
    				MAP_SHARED,             /* recommended */
    				fd, buffer.m.offset);

    	if (MAP_FAILED == buffers[i].start) {
    		/* If you do not exit here you should unmap() and free()
    		   the buffers mapped so far. */
    		perror("mmap");
    		exit(EXIT_FAILURE);
    	}
    }

    /* Cleanup. */

    for (i = 0; i < reqbuf.count; i++)
    	munmap(buffers[i].start, buffers[i].length);

## Mapping buffers in the multi-planar API

    &v4l2-requestbuffers; reqbuf;
    /* Our current format uses 3 planes per buffer */
    #define FMT_NUM_PLANES = 3

    struct {
    	void *start[FMT_NUM_PLANES];
    	size_t length[FMT_NUM_PLANES];
    } *buffers;
    unsigned int i, j;

    memset(&reqbuf, 0, sizeof(reqbuf));
    reqbuf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE;
    reqbuf.memory = V4L2_MEMORY_MMAP;
    reqbuf.count = 20;

    if (ioctl(fd, &VIDIOC-REQBUFS;, &reqbuf) < 0) {
    	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++) {
    	&v4l2-buffer; buffer;
    	&v4l2-plane; planes[FMT_NUM_PLANES];

    	memset(&buffer, 0, sizeof(buffer));
    	buffer.type = reqbuf.type;
    	buffer.memory = V4L2_MEMORY_MMAP;
    	buffer.index = i;
    	/* length in struct v4l2_buffer in multi-planar API stores the size
    	 * of planes array. */
    	buffer.length = FMT_NUM_PLANES;
    	buffer.m.planes = planes;

    	if (ioctl(fd, &VIDIOC-QUERYBUF;, &buffer) < 0) {
    		perror("VIDIOC_QUERYBUF");
    		exit(EXIT_FAILURE);
    	}

    	/* Every plane has to be mapped separately */
    	for (j = 0; j < FMT_NUM_PLANES; j++) {
    		buffers[i].length[j] = buffer.m.planes[j].length; /* remember for munmap() */

    		buffers[i].start[j] = mmap(NULL, buffer.m.planes[j].length,
    				 PROT_READ | PROT_WRITE, /* recommended */
    				 MAP_SHARED,             /* recommended */
    				 fd, buffer.m.planes[j].m.offset);

    		if (MAP_FAILED == buffers[i].start[j]) {
    			/* If you do not exit here you should unmap() and free()
    			   the buffers and planes mapped so far. */
    			perror("mmap");
    			exit(EXIT_FAILURE);
    		}
    	}
    }

    /* Cleanup. */

    for (i = 0; i < reqbuf.count; i++)
    	for (j = 0; j < FMT_NUM_PLANES; j++)
    		munmap(buffers[i].start[j], buffers[i].length[j]);

Conceptually 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. 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. The index number of a buffer (&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 &func-open; function, `VIDIOC_DQBUF` returns immediately with an &EAGAIN; when no buffer is available. The &func-select; or &func-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 &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. At the driver level `select()` and `poll()` are the same, and `select()` is too important to be optional. The rest should be evident.

[capture example]

# 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 &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 (planes) 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 &v4l2-buffer; (or in &v4l2-plane; in the multi-planar API case). The driver must be switched into user pointer I/O mode by calling the &VIDIOC-REQBUFS; with the desired buffer type. No buffers (planes) are allocated beforehand, consequently they are not indexed and cannot be queried like mapped buffers with the `VIDIOC_QUERYBUF` ioctl.

## Initiating streaming I/O with user pointers

    &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 (plane) 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. 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.

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 &func-open; function, `VIDIOC_DQBUF` returns immediately with an &EAGAIN; when no buffer is available. The &func-select; or &func-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 &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. At the driver level `select()` and `poll()` are the same, and `select()` is too important to be optional. The rest should be evident.

# Asynchronous I/O

This method is not defined yet.

# Buffers

A buffer contains data exchanged by application and driver using one of the Streaming I/O methods. In the multi-planar API, the data is held in planes, while the buffer structure acts as a container for the planes. Only pointers to buffers (planes) 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. In the multi-planar API, some plane-specific members of struct v4l2_buffer, such as pointers and sizes for each plane, are stored in struct v4l2_plane instead. In that case, struct v4l2_buffer contains an array of plane structures.

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 .

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. 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.

## struct v4l2_buffer

&cs-ustr;
__u32 | index |  | 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 (&v4l2-requestbuffers; count) minus one.
&v4l2-buf-type; | type |  | Type of the buffer, same as &v4l2-format; type or &v4l2-requestbuffers; type, set by the application.
__u32 | bytesused |  | 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.
__u32 | flags |  | Flags set by the application or driver, see .
&v4l2-field; | field |  | Indicates the field order of the image in the buffer, see . 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 timeval

timestamp

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.

&v4l2-timecode;

timecode

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.

__u32 | sequence |  | 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 .

&v4l2-memory; | memory |  | This field must be set by applications and/or drivers in accordance with the selected I/O method.
union | m
| __u32 | offset | For the single-planar API and when 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 &func-mmap; function not useful for applications. See for details
| unsigned long | userptr | For the single-planar API and when 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 for details.
| struct v4l2_plane | *planes | When using the multi-planar API, contains a userspace pointer to an array of &v4l2-plane;. The size of the array should be put in the length field of this v4l2_buffer structure.
__u32 | length |  | Size of the buffer (not the payload) in bytes for the single-planar API. For the multi-planar API should contain the number of elements in the planes array.
__u32 | input |  | 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 &v4l2-input; field index.
__u32 | reserved |  | A place holder for future extensions and custom (driver defined) buffer types `V4L2_BUF_TYPE_PRIVATE` and higher. Applications should set this to 0.

## struct v4l2_plane

&cs-ustr;
__u32 | bytesused |  | The number of bytes occupied by data in the plane (its payload).
__u32 | length |  | Size in bytes of the plane (not its payload).
union | m |  |
| __u32 | mem_offset | When the memory type in the containing &v4l2-buffer; is `V4L2_MEMORY_MMAP`, this is the value that should be passed to &func-mmap;, similar to the offset field in &v4l2-buffer;.
| __unsigned long | userptr | When the memory type in the containing &v4l2-buffer; is `V4L2_MEMORY_USERPTR`, this is a userspace pointer to the memory allocated for this plane by an application.
__u32 | data_offset |  | Offset in bytes to video data in the plane, if applicable.
__u32 | reserved[11] |  | Reserved for future use. Should be zeroed by an application.

## enum v4l2_buf_type

&cs-def;
`V4L2_BUF_TYPE_VIDEO_CAPTURE` | 1 | Buffer of a single-planar video capture stream, see .
`V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE` | 9 | Buffer of a multi-planar video capture stream, see .
`V4L2_BUF_TYPE_VIDEO_OUTPUT` | 2 | Buffer of a single-planar video output stream, see .
`V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE` | 10 | Buffer of a multi-planar video output stream, see .
`V4L2_BUF_TYPE_VIDEO_OVERLAY` | 3 | Buffer for video overlay, see .
`V4L2_BUF_TYPE_VBI_CAPTURE` | 4 | Buffer of a raw VBI capture stream, see .
`V4L2_BUF_TYPE_VBI_OUTPUT` | 5 | Buffer of a raw VBI output stream, see .
`V4L2_BUF_TYPE_SLICED_VBI_CAPTURE` | 6 | Buffer of a sliced VBI capture stream, see .
`V4L2_BUF_TYPE_SLICED_VBI_OUTPUT` | 7 | Buffer of a sliced VBI output stream, see .
`V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY` | 8 | Buffer for video output overlay (OSD), see . Status: Experimental.
`V4L2_BUF_TYPE_PRIVATE` | 0x80 | This and higher values are reserved for custom (driver defined) buffer types.

## Buffer Flags

&cs-def;
`V4L2_BUF_FLAG_MAPPED` | 0x0001 | The buffer resides in device memory and has been mapped into the application's address space, see 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_QUEUED`

0x0002

Internally 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_DONE`

0x0004

When 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_ERROR` | 0x0040 | When this flag is set, the buffer has been dequeued successfully, although the data might have been corrupted. This is recoverable, streaming may continue as normal and the buffer may be reused normally. Drivers set this flag when the `VIDIOC_DQBUF` ioctl is called.
`V4L2_BUF_FLAG_KEYFRAME` | 0x0008 | Drivers 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), &ie; can be decompressed on its own.
`V4L2_BUF_FLAG_PFRAME` | 0x0010 | Similar to `V4L2_BUF_FLAG_KEYFRAME` this flags predicted frames or fields which contain only differences to a previous key frame.
`V4L2_BUF_FLAG_BFRAME` | 0x0020 | Similar to `V4L2_BUF_FLAG_PFRAME` this is a bidirectional predicted frame or field. [ooc tbd]
`V4L2_BUF_FLAG_TIMECODE` | 0x0100 | The timecode field is valid. Drivers set or clear this flag when the `VIDIOC_DQBUF` ioctl is called.
`V4L2_BUF_FLAG_INPUT` | 0x0200 | The input field is valid. Applications set or clear this flag before calling the `VIDIOC_QBUF` ioctl.
`V4L2_BUF_FLAG_PREPARED` | 0x0400 | The buffer has been prepared for I/O and can be queued by the application. Drivers set or clear this flag when the VIDIOC_QUERYBUF, VIDIOC_PREPARE_BUF, VIDIOC_QBUF or VIDIOC_DQBUF ioctl is called.
`V4L2_BUF_FLAG_NO_CACHE_INVALIDATE` | 0x0400 | Caches do not have to be invalidated for this buffer. Typically applications shall use this flag if the data captured in the buffer is not going to be touched by the CPU, instead the buffer will, probably, be passed on to a DMA-capable hardware unit for further processing or output.
`V4L2_BUF_FLAG_NO_CACHE_CLEAN` | 0x0800 | Caches do not have to be cleaned for this buffer. Typically applications shall use this flag for output buffers if the data in this buffer has not been created by the CPU but by some DMA-capable unit, in which case caches have not been used.

## enum v4l2_memory

&cs-def;
`V4L2_MEMORY_MMAP` | 1 | The buffer is used for memory mapping I/O.
`V4L2_MEMORY_USERPTR` | 2 | The buffer is used for user pointer I/O.
`V4L2_MEMORY_OVERLAY` | 3 | [to do]

## Timecodes

The v4l2_timecode structure is designed to hold a or similar timecode. (struct timeval timestamps are stored in &v4l2-buffer; field timestamp.)

### struct v4l2_timecode

&cs-str;
__u32 | type | Frame rate the timecodes are based on, see .
__u32 | flags | Timecode flags, see .
__u8 | frames | Frame count, 0 ... 23/24/29/49/59, depending on the type of timecode.
__u8 | seconds | Seconds count, 0 ... 59. This is a binary, not BCD number.
__u8 | minutes | Minutes count, 0 ... 59. This is a binary, not BCD number.
__u8 | hours | Hours count, 0 ... 29. This is a binary, not BCD number.
__u8 | userbits[4] | The "user group" bits from the timecode.

### Timecode Types

&cs-def;
`V4L2_TC_TYPE_24FPS` | 1 | 24 frames per second, i. e. film.
`V4L2_TC_TYPE_25FPS` | 2 | 25 frames per second, &ie; PAL or SECAM video.
`V4L2_TC_TYPE_30FPS` | 3 | 30 frames per second, &ie; NTSC video.
`V4L2_TC_TYPE_50FPS` | 4 |
`V4L2_TC_TYPE_60FPS` | 5 |

### Timecode Flags

&cs-def;
`V4L2_TC_FLAG_DROPFRAME` | 0x0001 | Indicates "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_COLORFRAME` | 0x0002 | The "color frame" flag.
`V4L2_TC_USERBITS_field` | 0x000C | Field mask for the "binary group flags".
`V4L2_TC_USERBITS_USERDEFINED` | 0x0000 | Unspecified format.
`V4L2_TC_USERBITS_8BITCHARS` | 0x0008 | 8-bit ISO characters.

# 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 (aka odd) and bottom (aka even) 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, &eg; 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 &v4l2-pix-format; before calling the &VIDIOC-S-FMT; ioctl. If this is not desired it should have the value `V4L2_FIELD_ANY` (0).

## enum v4l2_field

&cs-def;
`V4L2_FIELD_ANY` | 0 | Applications 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. &v4l2-buffer; field can never be `V4L2_FIELD_ANY`.
`V4L2_FIELD_NONE` | 1 | Images 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_TOP` | 2 | Images consist of the top (aka odd) field only.
`V4L2_FIELD_BOTTOM` | 3 | Images consist of the bottom (aka even) 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_INTERLACED` | 4 | Images 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_TB` | 5 | Images 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_BT` | 6 | Images 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_ALTERNATE`

7

The two fields of a frame are passed in separate buffers, in temporal order, &ie; 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 &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 &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.

`V4L2_FIELD_INTERLACED_TB` | 8 | Images contain both fields, interleaved line by line, top field first. The top field is transmitted first.
`V4L2_FIELD_INTERLACED_BT` | 9 | Images contain both fields, interleaved line by line, top field first. The bottom field is transmitted first.

## Field Order, Top Field First Transmitted

## Field Order, Bottom Field First Transmitted