ioctl VIDIOC_QBUF, VIDIOC_DQBUF
&manvol;

# VIDIOC_QBUF

# VIDIOC_DQBUF

Exchange a buffer with the driver

    	int ioctl
    	int fd
    	int request
    	struct v4l2_buffer *argp

## Arguments

`fd`

  &fd;

`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 &v4l2-buffer; to the same buffer type as was previously used with &v4l2-format; type and &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; (&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 for details. Applications must also set flags to 0. If a driver supports capturing from specific video inputs and you want to specify a video input, then flags should be set to `V4L2_BUF_FLAG_INPUT` and the field input must be initialized to the desired input. The reserved field must be set to 0.

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;.

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 `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.

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 &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.

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 v4l2_buffer structure is specified in .

&return-value;

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

  Not enough physical or virtual memory was available 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. 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.