7.52. ioctl VIDIOC_REQBUFS¶
VIDIOC_REQBUFS - Initiate Memory Mapping, User Pointer I/O or DMA buffer I/O
ioctl(int fd, VIDIOC_REQBUFS, struct v4l2_requestbuffers *argp)¶
- File descriptor returned by open().
- Pointer to struct
This ioctl is used to initiate memory mapped, user pointer or DMABUF based 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. Similarly, DMABUF buffers are allocated by applications through a device driver, and this ioctl only configures the driver into DMABUF I/O mode without performing any direct allocation.
To allocate device buffers applications initialize all fields of the
v4l2_requestbuffers structure. They set the
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
Applications can call ioctl VIDIOC_REQBUFS again to change the number of
buffers, however this cannot succeed when any buffers are still mapped.
count value of zero frees all buffers, after aborting or finishing
any DMA in progress, an implicit
||The number of buffers requested or granted.|
||Type of the stream or buffers, this is the same as the struct
||Applications set this field to
||A place holder for future extensions. Drivers and applications must set the array to zero.|
7.52.5. 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.
- The buffer type (
typefield) or the requested I/O method (
memory) is not supported.