VIDIOC_QUERYBUF — Query the status of a buffer
int ioctl( | int fd, |
| int request, | |
struct v4l2_buffer *argp); |
This ioctl is part of the streaming
I/O method. It can be used to query the status of a
buffer at any time after buffers have been allocated with the
VIDIOC_REQBUFS ioctl.
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, 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 reserved field should to set to 0.
When using the multi-planar API, the
m.planes field must contain a userspace pointer to an
array of struct v4l2_plane and the length field has
to be set to the number of elements in that array.
After calling VIDIOC_QUERYBUF with a pointer to
this structure drivers return an error code or fill the rest of
the structure.
In the flags field the
V4L2_BUF_FLAG_MAPPED,
V4L2_BUF_FLAG_PREPARED,
V4L2_BUF_FLAG_QUEUED and
V4L2_BUF_FLAG_DONE flags will be valid. The
memory field will be set to the current
I/O method. For the single-planar API, the m.offset
contains the offset of the buffer from the start of the device memory,
the length field its size. For the multi-planar API,
fields m.mem_offset and
length in the m.planes
array elements will be used instead and the length
field of struct v4l2_buffer is set to the number of filled-in array elements.
The driver may or may not set the remaining fields and flags, they are
meaningless in this context.
The v4l2_buffer structure is specified in the section called “Buffers”.
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.
The buffer type is not
supported, or the index is out of bounds.