Name

v4l2-mmap - Map device memory into application address space

Synopsis

.. code-block:: c

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

.. c:function:: 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 struct :c:type:v4l2_buffer length field for the single-planar API, and the same value as returned by the driver in the struct :c:type: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 ``videobuf`` kernel module, which is used by some
 drivers supports only ``PROT_READ`` | ``PROT_WRITE``. When the
 driver does not support the desired protection, the
 :c:func:`mmap()` function fails.

  #. Device memory accesses (e. g. 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 such 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,
:c:func:`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 (e. g. child-) processes.

.. note::

   The Linux ``videobuf`` module  which is used by some
   drivers 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``
   flags.

fd File descriptor returned by :c:func:open().

offset Offset of the buffer in device memory. This must be the same value as returned by the driver in the struct :c:type:v4l2_buffer m union offset field for the single-planar API, and the same value as returned by the driver in the struct :c:type:v4l2_plane m union mem_offset field for the multi-planar API.

Description

The :c:func: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 :ref:VIDIOC_QUERYBUF ioctl. Buffers must be allocated with the :ref:VIDIOC_REQBUFS ioctl before they can be queried.

To unmap buffers the :c:func:munmap() function is used.

Return Value

On success :c:func: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
:ref:`VIDIOC_REQBUFS` ioctl.

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