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¶
-
int
ioctl
(int fd, VIDIOC_QBUF, struct v4l2_buffer *argp)¶
-
int
ioctl
(int fd, VIDIOC_DQBUF, struct v4l2_buffer *argp)¶
7.46.3. Arguments¶
fd
- File descriptor returned by open().
argp
- Pointer 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 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.
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_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, or theV4L2_BUF_FLAG_REQUEST_FD
flag was set but the the givenrequest_fd
was invalid, orm.fd
was an invalid DMABUF file descriptor. - 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.- EBADR
- The
V4L2_BUF_FLAG_REQUEST_FD
flag was set but the device does not support requests for the given buffer type, or theV4L2_BUF_FLAG_REQUEST_FD
flag 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).