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 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.

To allocate device buffers applications initialize three fields of a v4l2_requestbuffers structure. They set the type field to the respective stream or buffer type, the count field to the desired number of buffers, and memory must be set to `V4L2_MEMORY_MMAP`. When the ioctl is called with a pointer to this structure the driver attempts to allocate the requested number of buffers and 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 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 memory mapping I/O 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;.

To negotiate user pointer I/O, applications initialize only the type field and set memory to `V4L2_MEMORY_USERPTR`. When the ioctl is called with a pointer to this structure the driver prepares for user pointer I/O, when this I/O method is not supported the ioctl returns an &EINVAL;.

### struct v4l2_requestbuffers

&cs-str;
__u32 | count | The number of buffers requested or granted. This field is only used when memory is set to `V4L2_MEMORY_MMAP`.
&v4l2-buf-type; | type | Type of the stream or buffers, this is the same as the &v4l2-format; type field. See for valid values.
&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.

&return-value;

EBUSY

  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.

EINVAL

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