Documentation/media/uapi/v4l/func-mmap.rst

   1 .. -*- coding: utf-8; mode: rst -*-
   2
   3 .. _func-mmap:
   4
   5 ***********
   6 V4L2 mmap()
   7 ***********
   8
   9 Name
  10 ====
  11
  12 v4l2-mmap - Map device memory into application address space
  13
  14
  15 Synopsis
  16 ========
  17
  18 .. code-block:: c
  19
  20     #include <unistd.h>
  21     #include <sys/mman.h>
  22
  23
  24 .. cpp:function:: void *mmap( void *start, size_t length, int prot, int flags, int fd, off_t offset )
  25
  26
  27 Arguments
  28 =========
  29
  30 ``start``
  31     Map the buffer to this address in the application's address space.
  32     When the ``MAP_FIXED`` flag is specified, ``start`` must be a
  33     multiple of the pagesize and mmap will fail when the specified
  34     address cannot be used. Use of this option is discouraged;
  35     applications should just specify a ``NULL`` pointer here.
  36
  37 ``length``
  38     Length of the memory area to map. This must be the same value as
  39     returned by the driver in the struct
  40     :ref:`v4l2_buffer <v4l2-buffer>` ``length`` field for the
  41     single-planar API, and the same value as returned by the driver in
  42     the struct :ref:`v4l2_plane <v4l2-plane>` ``length`` field for
  43     the multi-planar API.
  44
  45 ``prot``
  46     The ``prot`` argument describes the desired memory protection.
  47     Regardless of the device type and the direction of data exchange it
  48     should be set to ``PROT_READ`` | ``PROT_WRITE``, permitting read
  49     and write access to image buffers. Drivers should support at least
  50     this combination of flags. Note the Linux ``video-buf`` kernel
  51     module, which is used by the bttv, saa7134, saa7146, cx88 and vivi
  52     driver supports only ``PROT_READ`` | ``PROT_WRITE``. When the
  53     driver does not support the desired protection the
  54     :ref:`mmap() <func-mmap>` function fails.
  55
  56     Note device memory accesses (e. g. the memory on a graphics card
  57     with video capturing hardware) may incur a performance penalty
  58     compared to main memory accesses, or reads may be significantly
  59     slower than writes or vice versa. Other I/O methods may be more
  60     efficient in this case.
  61
  62 ``flags``
  63     The ``flags`` parameter specifies the type of the mapped object,
  64     mapping options and whether modifications made to the mapped copy of
  65     the page are private to the process or are to be shared with other
  66     references.
  67
  68     ``MAP_FIXED`` requests that the driver selects no other address than
  69     the one specified. If the specified address cannot be used,
  70     :ref:`mmap() <func-mmap>` will fail. If ``MAP_FIXED`` is specified,
  71     ``start`` must be a multiple of the pagesize. Use of this option is
  72     discouraged.
  73
  74     One of the ``MAP_SHARED`` or ``MAP_PRIVATE`` flags must be set.
  75     ``MAP_SHARED`` allows applications to share the mapped memory with
  76     other (e. g. child-) processes. Note the Linux ``video-buf`` module
  77     which is used by the bttv, saa7134, saa7146, cx88 and vivi driver
  78     supports only ``MAP_SHARED``. ``MAP_PRIVATE`` requests copy-on-write
  79     semantics. V4L2 applications should not set the ``MAP_PRIVATE``,
  80     ``MAP_DENYWRITE``, ``MAP_EXECUTABLE`` or ``MAP_ANON`` flag.
  81
  82 ``fd``
  83     File descriptor returned by :ref:`open() <func-open>`.
  84
  85 ``offset``
  86     Offset of the buffer in device memory. This must be the same value
  87     as returned by the driver in the struct
  88     :ref:`v4l2_buffer <v4l2-buffer>` ``m`` union ``offset`` field for
  89     the single-planar API, and the same value as returned by the driver
  90     in the struct :ref:`v4l2_plane <v4l2-plane>` ``m`` union
  91     ``mem_offset`` field for the multi-planar API.
  92
  93
  94 Description
  95 ===========
  96
  97 The :ref:`mmap() <func-mmap>` function asks to map ``length`` bytes starting at
  98 ``offset`` in the memory of the device specified by ``fd`` into the
  99 application address space, preferably at address ``start``. This latter
 100 address is a hint only, and is usually specified as 0.
 101
 102 Suitable length and offset parameters are queried with the
 103 :ref:`VIDIOC_QUERYBUF` ioctl. Buffers must be
 104 allocated with the :ref:`VIDIOC_REQBUFS` ioctl
 105 before they can be queried.
 106
 107 To unmap buffers the :ref:`munmap() <func-munmap>` function is used.
 108
 109
 110 Return Value
 111 ============
 112
 113 On success :ref:`mmap() <func-mmap>` returns a pointer to the mapped buffer. On
 114 error ``MAP_FAILED`` (-1) is returned, and the ``errno`` variable is set
 115 appropriately. Possible error codes are:
 116
 117 EBADF
 118     ``fd`` is not a valid file descriptor.
 119
 120 EACCES
 121     ``fd`` is not open for reading and writing.
 122
 123 EINVAL
 124     The ``start`` or ``length`` or ``offset`` are not suitable. (E. g.
 125     they are too large, or not aligned on a ``PAGESIZE`` boundary.)
 126
 127     The ``flags`` or ``prot`` value is not supported.
 128
 129     No buffers have been allocated with the
 130     :ref:`VIDIOC_REQBUFS` ioctl.
 131
 132 ENOMEM
 133     Not enough physical or virtual memory was available to complete the
 134     request.