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1 | .. -*- coding: utf-8; mode: rst -*- |
2 | ||
3 | .. _func-mmap: | |
4 | ||
5 | *********** | |
6 | V4L2 mmap() | |
7 | *********** | |
8 | ||
15e7d615 | 9 | Name |
586027ce | 10 | ==== |
5377d91f | 11 | |
586027ce | 12 | v4l2-mmap - Map device memory into application address space |
5377d91f | 13 | |
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14 | |
15 | Synopsis | |
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16 | ======== |
17 | ||
18 | .. code-block:: c | |
19 | ||
20 | #include <unistd.h> | |
21 | #include <sys/mman.h> | |
22 | ||
23 | ||
b7e67f6c | 24 | .. cpp:function:: void *mmap( void *start, size_t length, int prot, int flags, int fd, off_t offset ) |
5377d91f | 25 | |
586027ce | 26 | |
15e7d615 | 27 | Arguments |
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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 | |
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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. | |
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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, | |
760c7010 | 73 | :ref:`mmap() <func-mmap>` will fail. If ``MAP_FIXED`` is specified, |
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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 | |
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79 | other (e. g. child-) processes. |
80 | ||
81 | .. note:: 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. | |
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86 | |
87 | ``fd`` | |
88 | File descriptor returned by :ref:`open() <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 | :ref:`v4l2_buffer <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 :ref:`v4l2_plane <v4l2-plane>` ``m`` union | |
96 | ``mem_offset`` field for the multi-planar API. | |
97 | ||
98 | ||
15e7d615 | 99 | Description |
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100 | =========== |
101 | ||
760c7010 | 102 | The :ref:`mmap() <func-mmap>` function asks to map ``length`` bytes starting at |
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103 | ``offset`` in the memory of the device specified by ``fd`` into the |
104 | application address space, preferably at address ``start``. This latter | |
105 | address is a hint only, and is usually specified as 0. | |
106 | ||
107 | Suitable length and offset parameters are queried with the | |
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108 | :ref:`VIDIOC_QUERYBUF` ioctl. Buffers must be |
109 | allocated with the :ref:`VIDIOC_REQBUFS` ioctl | |
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110 | before they can be queried. |
111 | ||
112 | To unmap buffers the :ref:`munmap() <func-munmap>` function is used. | |
113 | ||
114 | ||
15e7d615 | 115 | Return Value |
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116 | ============ |
117 | ||
760c7010 | 118 | On success :ref:`mmap() <func-mmap>` returns a pointer to the mapped buffer. On |
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119 | error ``MAP_FAILED`` (-1) is returned, and the ``errno`` variable is set |
120 | appropriately. Possible error codes are: | |
121 | ||
122 | EBADF | |
123 | ``fd`` is not a valid file descriptor. | |
124 | ||
125 | EACCES | |
126 | ``fd`` is not open for reading and writing. | |
127 | ||
128 | EINVAL | |
129 | The ``start`` or ``length`` or ``offset`` are not suitable. (E. g. | |
130 | they are too large, or not aligned on a ``PAGESIZE`` boundary.) | |
131 | ||
132 | The ``flags`` or ``prot`` value is not supported. | |
133 | ||
134 | No buffers have been allocated with the | |
7347081e | 135 | :ref:`VIDIOC_REQBUFS` ioctl. |
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136 | |
137 | ENOMEM | |
138 | Not enough physical or virtual memory was available to complete the | |
139 | request. |