ioctl VIDIOC_QBUF, VIDIOC_DQBUF
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## 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 buffer applications set the `type` field of a struct v4l2_buffer to the same buffer type as was previously used with struct v4l2_format `type` and struct v4l2_requestbuffers `type`. Applications must also set 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`, `V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE`, or `V4L2_BUF_TYPE_VBI_OUTPUT`) applications must also initialize the `bytesused`, `field` and `timestamp` fields, see the section called “Buffers” for details. Applications must also set `flags` to 0. The `reserved2` and `reserved` fields must be set to 0. When using the multi-planar API, the `m.planes` field must contain a userspace pointer to a filled-in array of struct v4l2_plane and the `length` field must be set to the number of elements in that array.

To enqueue a memory mapped buffer applications set the `memory` field to `V4L2_MEMORY_MMAP`. 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 `memory` field to `V4L2_MEMORY_USERPTR`, the `m.userptr` field to the address of the buffer and `length` to its size. When the multi-planar API is used, `m.userptr` and `length` members of the passed array of struct v4l2_plane have to be used instead. 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 is called, or until the device is closed.

To enqueue a DMABUF buffer applications set the `memory` field to `V4L2_MEMORY_DMABUF` and the `m.fd` field to a file descriptor associated with a DMABUF buffer. When the multi-planar API is used the `m.fd` fields of the passed array of struct v4l2_plane have to be used instead. 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 buffer. Locking a buffer means passing it to a driver for a hardware access (usually DMA). If an application accesses (reads/writes) a locked buffer then the result is undefined. Buffers remain locked until dequeued, until the `VIDIOC_STREAMOFF` or `VIDIOC_REQBUFS` ioctl is 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`, `memory` and `reserved` 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. The driver may also set `V4L2_BUF_FLAG_ERROR` in the `flags` field. It indicates a non-critical (recoverable) streaming error. In such case the application may continue as normal, but should be aware that data in the dequeued buffer might be corrupted. When using the multi-planar API, the planes array must be passed in as well.

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 the section called “Buffers”.

## Return Value

On success 0 is returned, on error -1 and the `errno` variable is set appropriately. The generic error codes are described at the Generic Error Codes chapter.

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.

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. Reusing such buffer may be unsafe though and its details (e.g. `index`) may not be returned either. It is recommended that drivers indicate recoverable errors by setting the `V4L2_BUF_FLAG_ERROR` and returning 0 instead. In that case the application should be able to safely reuse the buffer and continue streaming.

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