7.3. ioctl VIDIOC_CREATE_BUFS¶
VIDIOC_CREATE_BUFS - Create buffers for Memory Mapped or User Pointer or DMA Buffer I/O
ioctl(int fd, VIDIOC_CREATE_BUFS, struct v4l2_create_buffers *argp)¶
- File descriptor returned by open().
- Pointer to struct
This ioctl is used to create buffers for memory mapped or user pointer or DMA buffer I/O. It can be used as an alternative or in addition to the ioctl VIDIOC_REQBUFS ioctl, when a tighter control over buffers is required. This ioctl can be called multiple times to create buffers of different sizes.
To allocate the device buffers applications must initialize the relevant
fields of the struct
v4l2_create_buffers structure. The
count field must be set to the number of requested buffers, the
memory field specifies the requested I/O method and the
array must be zeroed.
format field specifies the image format that the buffers must be
able to handle. The application has to fill in this struct
v4l2_format. Usually this will be done using the
VIDIOC_G_FMT ioctls to ensure that the
requested format is supported by the driver. Based on the format’s
type field the requested buffer size (for single-planar) or plane
sizes (for multi-planar formats) will be used for the allocated buffers.
The driver may return an error if the size(s) are not supported by the
hardware (usually because they are too small).
The buffers created by this ioctl will have as minimum size the size
defined by the
format.pix.sizeimage field (or the corresponding
fields for other format types). Usually if the
field is less than the minimum required for the given format, then an
error will be returned since drivers will typically not allow this. If
it is larger, then the value will be used as-is. In other words, the
driver may reject the requested size, but if it is accepted the driver
will use it unchanged.
When the ioctl is called with a pointer to this structure the driver
will attempt to allocate up to the requested number of buffers and store
the actual number allocated and the starting index in the
index fields respectively. On return
count can be smaller
than the number requested.
||The starting buffer index, returned by the driver.|
||The number of buffers requested or granted. If count == 0, then
ioctl VIDIOC_CREATE_BUFS will set
||Applications set this field to
||Filled in by the application, preserved by the driver.|
Set by the driver. If 0, then the driver doesn’t support
capabilities. In that case all you know is that the driver is
guaranteed to support
If you want to just query the capabilities without making any
other changes, then set
||A place holder for future extensions. Drivers and applications must set the array to zero.|
7.3.5. 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.
- No memory to allocate buffers for memory mapped I/O.
- The buffer type (
format.typefield), requested I/O method (
memory) or format (
formatfield) is not valid.