V4L2 mmap()

&manvol;

v4l2-mmap

Map device memory into application address space

    #include <unistd.h>
    #include <sys/mman.h>

    	void *mmap
    	void *start
    	size_t length
    	int prot
    	int flags
    	int fd
    	off_t offset

## Arguments

`start`

- Map the buffer to this address in the application's address space. When the `MAP_FIXED` flag is specified, `start` must be a multiple of the pagesize and mmap will fail when the specified address cannot be used. Use of this option is discouraged; applications should just specify a `NULL` pointer here.

`length`

- Length of the memory area to map. This must be the same value as returned by the driver in the &v4l2-buffer; length field for the single-planar API, and the same value as returned by the driver in the &v4l2-plane; length field for the multi-planar API.

`prot`

- The `prot` argument describes the desired memory protection. Regardless of the device type and the direction of data exchange it should be set to `PROT_READ` | `PROT_WRITE`, permitting read and write access to image buffers. Drivers should support at least this combination of flags. Note the Linux `video-buf` kernel module, which is used by the bttv, saa7134, saa7146, cx88 and vivi driver supports only `PROT_READ` | `PROT_WRITE`. When the driver does not support the desired protection the `mmap()` function fails.

  Note device memory accesses (⪚ the memory on a graphics card with video capturing hardware) may incur a performance penalty compared to main memory accesses, or reads may be significantly slower than writes or vice versa. Other I/O methods may be more efficient in this case.

`flags`

- The `flags` parameter specifies the type of the mapped object, mapping options and whether modifications made to the mapped copy of the page are private to the process or are to be shared with other references.

  `MAP_FIXED` requests that the driver selects no other address than the one specified. If the specified address cannot be used, `mmap()` will fail. If `MAP_FIXED` is specified, `start` must be a multiple of the pagesize. Use of this option is discouraged.

  One of the `MAP_SHARED` or `MAP_PRIVATE` flags must be set. `MAP_SHARED` allows applications to share the mapped memory with other (⪚ child-) processes. Note the Linux `video-buf` module which is used by the bttv, saa7134, saa7146, cx88 and vivi driver supports only `MAP_SHARED`. `MAP_PRIVATE` requests copy-on-write semantics. V4L2 applications should not set the `MAP_PRIVATE`, `MAP_DENYWRITE`, `MAP_EXECUTABLE` or `MAP_ANON` flag.

`fd`

- &fd;

`offset`

- Offset of the buffer in device memory. This must be the same value as returned by the driver in the &v4l2-buffer; m union offset field for the single-planar API, and the same value as returned by the driver in the &v4l2-plane; m union mem_offset field for the multi-planar API.

## Description

The `mmap()` function asks to map `length` bytes starting at `offset` in the memory of the device specified by `fd` into the application address space, preferably at address `start`. This latter address is a hint only, and is usually specified as 0.

Suitable length and offset parameters are queried with the &VIDIOC-QUERYBUF; ioctl. Buffers must be allocated with the &VIDIOC-REQBUFS; ioctl before they can be queried.

To unmap buffers the &func-munmap; function is used.

## Return Value

On success `mmap()` returns a pointer to the mapped buffer. On error `MAP_FAILED` (-1) is returned, and the `errno` variable is set appropriately. Possible error codes are:

EBADF

- `fd` is not a valid file descriptor.

EACCES

- `fd` is not open for reading and writing.

EINVAL

- The `start` or `length` or `offset` are not suitable. (E. g. they are too large, or not aligned on a `PAGESIZE` boundary.)

  The `flags` or `prot` value is not supported.

  No buffers have been allocated with the &VIDIOC-REQBUFS; ioctl.

ENOMEM

- Not enough physical or virtual memory was available to complete the request.