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.