8.46. ioctl VIDIOC_QBUF, VIDIOC_DQBUF¶
8.46.1. Name¶
VIDIOC_QBUF - VIDIOC_DQBUF - Exchange a buffer with the driver
8.46.4. 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 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.
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 struct v4l2_buffer structure is specified in Buffers.
8.46.5. 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_NONBLOCK
and no buffer was in the outgoing queue. - EINVAL
- The buffer
type
is not supported, or theindex
is out of bounds, or no buffers have been allocated yet, or theuserptr
orlength
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 theV4L2_BUF_FLAG_ERROR
and returning 0 instead. In that case the application should be able to safely reuse the buffer and continue streaming.- EPIPE
VIDIOC_DQBUF
returns this on an empty capture queue for mem2mem codecs if a buffer with theV4L2_BUF_FLAG_LAST
was already dequeued and no new buffers are expected to become available.