Video for Linux Two API Specification: Revision 0.24
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# ioctl VIDIOC_REQBUFS

## Name

VIDIOC_REQBUFS -- Initiate Memory Mapping or User Pointer I/O

## Synopsis

``int ioctl`(int fd, int request, struct v4l2_requestbuffers *argp);`

## Arguments

`fd`

  File descriptor returned by `open()`.

`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.[1] When memory mapping I/O 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`.

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 error code.

Table 1. struct `v4l2_requestbuffers`

__u32 | `count` | The number of buffers requested or granted. This field is only used when `memory` is set to `V4L2_MEMORY_MMAP`.
enum v4l2_buf_type | `type` | Type of the stream or buffers, this is the same as the struct v4l2_format `type` field. See Table 3-2 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.

## Return Value

On success 0 is returned, on error -1 and the `errno` variable is set appropriately:

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.

## Notes

[1] | For example video output requires at least two buffers, one displayed and one filled by the application.

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