1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2
3.. _func-mmap:
4
5***********
6V4L2 mmap()
7***********
8
9Name
10====
11
12v4l2-mmap - Map device memory into application address space
13
14
15Synopsis
16========
17
18.. code-block:: c
19
20    #include <unistd.h>
21    #include <sys/mman.h>
22
23
24.. c:function:: void *mmap( void *start, size_t length, int prot, int flags, int fd, off_t offset )
25    :name: v4l2-mmap
26
27Arguments
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    :c:type:`v4l2_buffer` ``length`` field for the
41    single-planar API, and the same value as returned by the driver in
42    the struct :c:type:`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.
51
52    .. note::
53
54      #. The Linux ``videobuf`` kernel module, which is used by some
55	 drivers supports only ``PROT_READ`` | ``PROT_WRITE``. When the
56	 driver does not support the desired protection, the
57	 :ref:`mmap() <func-mmap>` function fails.
58
59      #. Device memory accesses (e. g. the memory on a graphics card
60	 with video capturing hardware) may incur a performance penalty
61	 compared to main memory accesses, or reads may be significantly
62	 slower than writes or vice versa. Other I/O methods may be more
63	 efficient in such case.
64
65``flags``
66    The ``flags`` parameter specifies the type of the mapped object,
67    mapping options and whether modifications made to the mapped copy of
68    the page are private to the process or are to be shared with other
69    references.
70
71    ``MAP_FIXED`` requests that the driver selects no other address than
72    the one specified. If the specified address cannot be used,
73    :ref:`mmap() <func-mmap>` will fail. If ``MAP_FIXED`` is specified,
74    ``start`` must be a multiple of the pagesize. Use of this option is
75    discouraged.
76
77    One of the ``MAP_SHARED`` or ``MAP_PRIVATE`` flags must be set.
78    ``MAP_SHARED`` allows applications to share the mapped memory with
79    other (e. g. child-) processes.
80
81    .. note::
82
83       The Linux ``videobuf`` module  which is used by some
84       drivers supports only ``MAP_SHARED``. ``MAP_PRIVATE`` requests
85       copy-on-write semantics. V4L2 applications should not set the
86       ``MAP_PRIVATE``, ``MAP_DENYWRITE``, ``MAP_EXECUTABLE`` or ``MAP_ANON``
87       flags.
88
89``fd``
90    File descriptor returned by :ref:`open() <func-open>`.
91
92``offset``
93    Offset of the buffer in device memory. This must be the same value
94    as returned by the driver in the struct
95    :c:type:`v4l2_buffer` ``m`` union ``offset`` field for
96    the single-planar API, and the same value as returned by the driver
97    in the struct :c:type:`v4l2_plane` ``m`` union
98    ``mem_offset`` field for the multi-planar API.
99
100
101Description
102===========
103
104The :ref:`mmap() <func-mmap>` function asks to map ``length`` bytes starting at
105``offset`` in the memory of the device specified by ``fd`` into the
106application address space, preferably at address ``start``. This latter
107address is a hint only, and is usually specified as 0.
108
109Suitable length and offset parameters are queried with the
110:ref:`VIDIOC_QUERYBUF` ioctl. Buffers must be
111allocated with the :ref:`VIDIOC_REQBUFS` ioctl
112before they can be queried.
113
114To unmap buffers the :ref:`munmap() <func-munmap>` function is used.
115
116
117Return Value
118============
119
120On success :ref:`mmap() <func-mmap>` returns a pointer to the mapped buffer. On
121error ``MAP_FAILED`` (-1) is returned, and the ``errno`` variable is set
122appropriately. Possible error codes are:
123
124EBADF
125    ``fd`` is not a valid file descriptor.
126
127EACCES
128    ``fd`` is not open for reading and writing.
129
130EINVAL
131    The ``start`` or ``length`` or ``offset`` are not suitable. (E. g.
132    they are too large, or not aligned on a ``PAGESIZE`` boundary.)
133
134    The ``flags`` or ``prot`` value is not supported.
135
136    No buffers have been allocated with the
137    :ref:`VIDIOC_REQBUFS` ioctl.
138
139ENOMEM
140    Not enough physical or virtual memory was available to complete the
141    request.
142