func-mmap.rst

  1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
  2.. c:namespace:: V4L
  3
  4.. _func-mmap:
  5
  6***********
  7V4L2 mmap()
  8***********
  9
 10Name
 11====
 12
 13v4l2-mmap - Map device memory into application address space
 14
 15Synopsis
 16========
 17
 18.. code-block:: c
 19
 20    #include <unistd.h>
 21    #include <sys/mman.h>
 22
 23.. c:function:: void *mmap( void *start, size_t length, int prot, int flags, int fd, off_t offset )
 24
 25Arguments
 26=========
 27
 28``start``
 29    Map the buffer to this address in the application's address space.
 30    When the ``MAP_FIXED`` flag is specified, ``start`` must be a
 31    multiple of the pagesize and mmap will fail when the specified
 32    address cannot be used. Use of this option is discouraged;
 33    applications should just specify a ``NULL`` pointer here.
 34
 35``length``
 36    Length of the memory area to map. This must be the same value as
 37    returned by the driver in the struct
 38    :c:type:`v4l2_buffer` ``length`` field for the
 39    single-planar API, and the same value as returned by the driver in
 40    the struct :c:type:`v4l2_plane` ``length`` field for
 41    the multi-planar API.
 42
 43``prot``
 44    The ``prot`` argument describes the desired memory protection.
 45    Regardless of the device type and the direction of data exchange it
 46    should be set to ``PROT_READ`` | ``PROT_WRITE``, permitting read
 47    and write access to image buffers. Drivers should support at least
 48    this combination of flags.
 49
 50    .. note::
 51
 52      #. The Linux ``videobuf`` kernel module, which is used by some
 53	 drivers supports only ``PROT_READ`` | ``PROT_WRITE``. When the
 54	 driver does not support the desired protection, the
 55	 :c:func:`mmap()` function fails.
 56
 57      #. Device memory accesses (e. g. the memory on a graphics card
 58	 with video capturing hardware) may incur a performance penalty
 59	 compared to main memory accesses, or reads may be significantly
 60	 slower than writes or vice versa. Other I/O methods may be more
 61	 efficient in such case.
 62
 63``flags``
 64    The ``flags`` parameter specifies the type of the mapped object,
 65    mapping options and whether modifications made to the mapped copy of
 66    the page are private to the process or are to be shared with other
 67    references.
 68
 69    ``MAP_FIXED`` requests that the driver selects no other address than
 70    the one specified. If the specified address cannot be used,
 71    :c:func:`mmap()` will fail. If ``MAP_FIXED`` is specified,
 72    ``start`` must be a multiple of the pagesize. Use of this option is
 73    discouraged.
 74
 75    One of the ``MAP_SHARED`` or ``MAP_PRIVATE`` flags must be set.
 76    ``MAP_SHARED`` allows applications to share the mapped memory with
 77    other (e. g. child-) processes.
 78
 79    .. note::
 80
 81       The Linux ``videobuf`` module  which is used by some
 82       drivers supports only ``MAP_SHARED``. ``MAP_PRIVATE`` requests
 83       copy-on-write semantics. V4L2 applications should not set the
 84       ``MAP_PRIVATE``, ``MAP_DENYWRITE``, ``MAP_EXECUTABLE`` or ``MAP_ANON``
 85       flags.
 86
 87``fd``
 88    File descriptor returned by :c:func:`open()`.
 89
 90``offset``
 91    Offset of the buffer in device memory. This must be the same value
 92    as returned by the driver in the struct
 93    :c:type:`v4l2_buffer` ``m`` union ``offset`` field for
 94    the single-planar API, and the same value as returned by the driver
 95    in the struct :c:type:`v4l2_plane` ``m`` union
 96    ``mem_offset`` field for the multi-planar API.
 97
 98Description
 99===========
100
101The :c:func:`mmap()` function asks to map ``length`` bytes starting at
102``offset`` in the memory of the device specified by ``fd`` into the
103application address space, preferably at address ``start``. This latter
104address is a hint only, and is usually specified as 0.
105
106Suitable length and offset parameters are queried with the
107:ref:`VIDIOC_QUERYBUF` ioctl. Buffers must be
108allocated with the :ref:`VIDIOC_REQBUFS` ioctl
109before they can be queried.
110
111To unmap buffers the :c:func:`munmap()` function is used.
112
113Return Value
114============
115
116On success :c:func:`mmap()` returns a pointer to the mapped buffer. On
117error ``MAP_FAILED`` (-1) is returned, and the ``errno`` variable is set
118appropriately. Possible error codes are:
119
120EBADF
121    ``fd`` is not a valid file descriptor.
122
123EACCES
124    ``fd`` is not open for reading and writing.
125
126EINVAL
127    The ``start`` or ``length`` or ``offset`` are not suitable. (E. g.
128    they are too large, or not aligned on a ``PAGESIZE`` boundary.)
129
130    The ``flags`` or ``prot`` value is not supported.
131
132    No buffers have been allocated with the
133    :ref:`VIDIOC_REQBUFS` ioctl.
134
135ENOMEM
136    Not enough physical or virtual memory was available to complete the
137    request.