ioctl VIDIOC_REQBUFS

&manvol;

VIDIOC_REQBUFS

Initiate Memory Mapping or User Pointer I/O

    	int ioctl
    	int fd
    	int request
    	struct v4l2_requestbuffers *argp

## Arguments

`fd`

- &fd;

`request`

- VIDIOC_REQBUFS

`argp`

## Description

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 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;.

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;.

## struct v4l2_requestbuffers

&cs-str;
__u32 | count | The number of buffers requested or granted.
__u32 | type | Type of the stream or buffers, this is the same as the &v4l2-format; type field. See v4l2-buf-type for valid values.
__u32 | memory | Applications set this field to `V4L2_MEMORY_MMAP`, `V4L2_MEMORY_DMABUF` or `V4L2_MEMORY_USERPTR`. See v4l2-memory.
__u32 | reserved[2] | A place holder for future extensions. Drivers and applications must set the array to zero.

&return-value;

EINVAL

- The buffer type (type field) or the requested I/O method (memory) is not supported.