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