VIDIOC_REQBUFS — Initiate Memory Mapping or User Pointer I/O
int ioctl( | int fd, |
| int request, | |
struct v4l2_requestbuffers *argp); |
This ioctl is used to initiate memory mapped or user pointer I/O. Memory mapped buffers are located in device memory and must be allocated with this ioctl before they can be mapped into the application's address space. User buffers are allocated by applications themselves, and this ioctl is merely used to switch the driver into user pointer I/O mode and to setup some internal structures.
To allocate device buffers applications initialize all
fields of the v4l2_requestbuffers structure.
They set the type field to the respective
stream or buffer type, the count field to
the desired number of buffers, memory
must be set to the requested I/O method and the reserved array
must be zeroed. When the ioctl
is called with a pointer to this structure the driver will attempt to allocate
the requested number of buffers and it stores the actual number
allocated in the count field. It can be
smaller than the number requested, even zero, when the driver runs out
of free memory. A larger number is also possible when the driver requires
more buffers to function correctly. For example video output requires at least two buffers,
one displayed and one filled by the application.
When the I/O method is not supported the ioctl returns an EINVAL error code.
Applications can call VIDIOC_REQBUFS
again to change the number of buffers, however this cannot succeed
when any buffers are still mapped. A count
value of zero frees all buffers, after aborting or finishing any DMA
in progress, an implicit VIDIOC_STREAMOFF.
Table A.81. struct v4l2_requestbuffers
| __u32 | count | The number of buffers requested or granted. |
| enum v4l2_buf_type | type | Type of the stream or buffers, this is the same
as the struct v4l2_format type field. See Table 3.3, “enum v4l2_buf_type” for valid values. |
| enum v4l2_memory | memory | Applications set this field to
V4L2_MEMORY_MMAP or
V4L2_MEMORY_USERPTR. |
| __u32 | reserved[2] | A place holder for future extensions and custom
(driver defined) buffer types V4L2_BUF_TYPE_PRIVATE and
higher. This array should be zeroed by applications. |
On success 0 is returned, on error -1 and the errno variable is set appropriately:
The driver supports multiple opens and I/O is already in progress, or reallocation of buffers was attempted although one or more are still mapped.
The buffer type (type field) or the
requested I/O method (memory) is not
supported.