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.
On success 0 is returned, on error -1 and the errno variable is set appropriately:
Non-blocking I/O has been selected using
O_NONBLOCK and no buffer was in the outgoing
queue.
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.
Not enough physical or virtual memory was available to enqueue a user pointer buffer.
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.