1 .. Permission is granted to copy, distribute and/or modify this
2 .. document under the terms of the GNU Free Documentation License,
3 .. Version 1.1 or any later version published by the Free Software
4 .. Foundation, with no Invariant Sections, no Front-Cover Texts
5 .. and no Back-Cover Texts. A copy of the license is included at
6 .. Documentation/media/uapi/fdl-appendix.rst.
8 .. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections
19 v4l2-mmap - Map device memory into application address space
31 .. c:function:: void *mmap( void *start, size_t length, int prot, int flags, int fd, off_t offset )
38 Map the buffer to this address in the application's address space.
39 When the ``MAP_FIXED`` flag is specified, ``start`` must be a
40 multiple of the pagesize and mmap will fail when the specified
41 address cannot be used. Use of this option is discouraged;
42 applications should just specify a ``NULL`` pointer here.
45 Length of the memory area to map. This must be the same value as
46 returned by the driver in the struct
47 :c:type:`v4l2_buffer` ``length`` field for the
48 single-planar API, and the same value as returned by the driver in
49 the struct :c:type:`v4l2_plane` ``length`` field for
53 The ``prot`` argument describes the desired memory protection.
54 Regardless of the device type and the direction of data exchange it
55 should be set to ``PROT_READ`` | ``PROT_WRITE``, permitting read
56 and write access to image buffers. Drivers should support at least
57 this combination of flags.
61 #. The Linux ``videobuf`` kernel module, which is used by some
62 drivers supports only ``PROT_READ`` | ``PROT_WRITE``. When the
63 driver does not support the desired protection, the
64 :ref:`mmap() <func-mmap>` function fails.
66 #. Device memory accesses (e. g. the memory on a graphics card
67 with video capturing hardware) may incur a performance penalty
68 compared to main memory accesses, or reads may be significantly
69 slower than writes or vice versa. Other I/O methods may be more
70 efficient in such case.
73 The ``flags`` parameter specifies the type of the mapped object,
74 mapping options and whether modifications made to the mapped copy of
75 the page are private to the process or are to be shared with other
78 ``MAP_FIXED`` requests that the driver selects no other address than
79 the one specified. If the specified address cannot be used,
80 :ref:`mmap() <func-mmap>` will fail. If ``MAP_FIXED`` is specified,
81 ``start`` must be a multiple of the pagesize. Use of this option is
84 One of the ``MAP_SHARED`` or ``MAP_PRIVATE`` flags must be set.
85 ``MAP_SHARED`` allows applications to share the mapped memory with
86 other (e. g. child-) processes.
90 The Linux ``videobuf`` module which is used by some
91 drivers supports only ``MAP_SHARED``. ``MAP_PRIVATE`` requests
92 copy-on-write semantics. V4L2 applications should not set the
93 ``MAP_PRIVATE``, ``MAP_DENYWRITE``, ``MAP_EXECUTABLE`` or ``MAP_ANON``
97 File descriptor returned by :ref:`open() <func-open>`.
100 Offset of the buffer in device memory. This must be the same value
101 as returned by the driver in the struct
102 :c:type:`v4l2_buffer` ``m`` union ``offset`` field for
103 the single-planar API, and the same value as returned by the driver
104 in the struct :c:type:`v4l2_plane` ``m`` union
105 ``mem_offset`` field for the multi-planar API.
111 The :ref:`mmap() <func-mmap>` function asks to map ``length`` bytes starting at
112 ``offset`` in the memory of the device specified by ``fd`` into the
113 application address space, preferably at address ``start``. This latter
114 address is a hint only, and is usually specified as 0.
116 Suitable length and offset parameters are queried with the
117 :ref:`VIDIOC_QUERYBUF` ioctl. Buffers must be
118 allocated with the :ref:`VIDIOC_REQBUFS` ioctl
119 before they can be queried.
121 To unmap buffers the :ref:`munmap() <func-munmap>` function is used.
127 On success :ref:`mmap() <func-mmap>` returns a pointer to the mapped buffer. On
128 error ``MAP_FAILED`` (-1) is returned, and the ``errno`` variable is set
129 appropriately. Possible error codes are:
132 ``fd`` is not a valid file descriptor.
135 ``fd`` is not open for reading and writing.
138 The ``start`` or ``length`` or ``offset`` are not suitable. (E. g.
139 they are too large, or not aligned on a ``PAGESIZE`` boundary.)
141 The ``flags`` or ``prot`` value is not supported.
143 No buffers have been allocated with the
144 :ref:`VIDIOC_REQBUFS` ioctl.
147 Not enough physical or virtual memory was available to complete the