7.46. ioctl VIDIOC_QBUF, VIDIOC_DQBUF¶
7.46.1. Name¶
VIDIOC_QBUF - VIDIOC_DQBUF - Exchange a buffer with the driver
7.46.2. Synopsis¶
-
VIDIOC_QBUF¶
int ioctl(int fd, VIDIOC_QBUF, struct v4l2_buffer *argp)
-
VIDIOC_DQBUF¶
int ioctl(int fd, VIDIOC_DQBUF, struct v4l2_buffer *argp)
7.46.3. Arguments¶
fdFile descriptor returned by
open().argpPointer to struct
v4l2_buffer.
7.46.4. 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
ioctl VIDIOC_REQBUFS (struct
v4l2_requestbuffers count) minus
one. The contents of the struct v4l2_buffer returned
by a ioctl 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 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
ioctl 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
ioctl VIDIOC_REQBUFS ioctl is called, or until the
device is closed.
The request_fd field can be used with the VIDIOC_QBUF ioctl to specify
the file descriptor of a request, if requests are
in use. Setting it means that the buffer will not be passed to the driver
until the request itself is queued. Also, the driver will apply any
settings associated with the request for this buffer. This field will
be ignored unless the V4L2_BUF_FLAG_REQUEST_FD flag is set.
If the device does not support requests, then EBADR will be returned.
If requests are supported but an invalid request file descriptor is given,
then EINVAL will be returned.
Caution
It is not allowed to mix queuing requests with queuing buffers directly.
EBUSY will be returned if the first buffer was queued directly and
then the application tries to queue a request, or vice versa. After
closing the file descriptor, calling
VIDIOC_STREAMOFF or calling ioctl VIDIOC_REQBUFS
the check for this will be reset.
For memory-to-memory devices you can specify the
request_fd only for output buffers, not for capture buffers. Attempting
to specify this for a capture buffer will result in an EBADR error.
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 all 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.
If the application sets the memory field to V4L2_MEMORY_DMABUF to
dequeue a DMABUF buffer, the driver fills the m.fd field
with a file descriptor numerically the same as the one given to VIDIOC_QBUF
when the buffer was enqueued. No new file descriptor is created at dequeue time
and the value is only for the application convenience. When the multi-planar
API is used the m.fd fields of the passed array of struct
v4l2_plane are filled instead.
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 struct v4l2_buffer structure is specified in
Buffers.
7.46.5. 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_NONBLOCKand no buffer was in the outgoing queue.- EINVAL
The buffer
typeis not supported, or theindexis out of bounds, or no buffers have been allocated yet, or theuserptrorlengthare invalid, or theV4L2_BUF_FLAG_REQUEST_FDflag was set but the givenrequest_fdwas invalid, orm.fdwas an invalid DMABUF file descriptor.- EIO
VIDIOC_DQBUFfailed 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 theV4L2_BUF_FLAG_ERRORand returning 0 instead. In that case the application should be able to safely reuse the buffer and continue streaming.- EPIPE
VIDIOC_DQBUFreturns this on an empty capture queue for mem2mem codecs if a buffer with theV4L2_BUF_FLAG_LASTwas already dequeued and no new buffers are expected to become available.- EBADR
The
V4L2_BUF_FLAG_REQUEST_FDflag was set but the device does not support requests for the given buffer type, or theV4L2_BUF_FLAG_REQUEST_FDflag was not set but the device requires that the buffer is part of a request.- EBUSY
The first buffer was queued via a request, but the application now tries to queue it directly, or vice versa (it is not permitted to mix the two APIs).