0001 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
0002 .. c:namespace:: V4L
0003
0004 .. _func-mmap:
0005
0006 ***********
0007 V4L2 mmap()
0008 ***********
0009
0010 Name
0011 ====
0012
0013 v4l2-mmap - Map device memory into application address space
0014
0015 Synopsis
0016 ========
0017
0018 .. code-block:: c
0019
0020 #include <unistd.h>
0021 #include <sys/mman.h>
0022
0023 .. c:function:: void *mmap( void *start, size_t length, int prot, int flags, int fd, off_t offset )
0024
0025 Arguments
0026 =========
0027
0028 ``start``
0029 Map the buffer to this address in the application's address space.
0030 When the ``MAP_FIXED`` flag is specified, ``start`` must be a
0031 multiple of the pagesize and mmap will fail when the specified
0032 address cannot be used. Use of this option is discouraged;
0033 applications should just specify a ``NULL`` pointer here.
0034
0035 ``length``
0036 Length of the memory area to map. This must be the same value as
0037 returned by the driver in the struct
0038 :c:type:`v4l2_buffer` ``length`` field for the
0039 single-planar API, and the same value as returned by the driver in
0040 the struct :c:type:`v4l2_plane` ``length`` field for
0041 the multi-planar API.
0042
0043 ``prot``
0044 The ``prot`` argument describes the desired memory protection.
0045 Regardless of the device type and the direction of data exchange it
0046 should be set to ``PROT_READ`` | ``PROT_WRITE``, permitting read
0047 and write access to image buffers. Drivers should support at least
0048 this combination of flags.
0049
0050 .. note::
0051
0052 #. The Linux ``videobuf`` kernel module, which is used by some
0053 drivers supports only ``PROT_READ`` | ``PROT_WRITE``. When the
0054 driver does not support the desired protection, the
0055 :c:func:`mmap()` function fails.
0056
0057 #. Device memory accesses (e. g. the memory on a graphics card
0058 with video capturing hardware) may incur a performance penalty
0059 compared to main memory accesses, or reads may be significantly
0060 slower than writes or vice versa. Other I/O methods may be more
0061 efficient in such case.
0062
0063 ``flags``
0064 The ``flags`` parameter specifies the type of the mapped object,
0065 mapping options and whether modifications made to the mapped copy of
0066 the page are private to the process or are to be shared with other
0067 references.
0068
0069 ``MAP_FIXED`` requests that the driver selects no other address than
0070 the one specified. If the specified address cannot be used,
0071 :c:func:`mmap()` will fail. If ``MAP_FIXED`` is specified,
0072 ``start`` must be a multiple of the pagesize. Use of this option is
0073 discouraged.
0074
0075 One of the ``MAP_SHARED`` or ``MAP_PRIVATE`` flags must be set.
0076 ``MAP_SHARED`` allows applications to share the mapped memory with
0077 other (e. g. child-) processes.
0078
0079 .. note::
0080
0081 The Linux ``videobuf`` module which is used by some
0082 drivers supports only ``MAP_SHARED``. ``MAP_PRIVATE`` requests
0083 copy-on-write semantics. V4L2 applications should not set the
0084 ``MAP_PRIVATE``, ``MAP_DENYWRITE``, ``MAP_EXECUTABLE`` or ``MAP_ANON``
0085 flags.
0086
0087 ``fd``
0088 File descriptor returned by :c:func:`open()`.
0089
0090 ``offset``
0091 Offset of the buffer in device memory. This must be the same value
0092 as returned by the driver in the struct
0093 :c:type:`v4l2_buffer` ``m`` union ``offset`` field for
0094 the single-planar API, and the same value as returned by the driver
0095 in the struct :c:type:`v4l2_plane` ``m`` union
0096 ``mem_offset`` field for the multi-planar API.
0097
0098 Description
0099 ===========
0100
0101 The :c:func:`mmap()` function asks to map ``length`` bytes starting at
0102 ``offset`` in the memory of the device specified by ``fd`` into the
0103 application address space, preferably at address ``start``. This latter
0104 address is a hint only, and is usually specified as 0.
0105
0106 Suitable length and offset parameters are queried with the
0107 :ref:`VIDIOC_QUERYBUF` ioctl. Buffers must be
0108 allocated with the :ref:`VIDIOC_REQBUFS` ioctl
0109 before they can be queried.
0110
0111 To unmap buffers the :c:func:`munmap()` function is used.
0112
0113 Return Value
0114 ============
0115
0116 On success :c:func:`mmap()` returns a pointer to the mapped buffer. On
0117 error ``MAP_FAILED`` (-1) is returned, and the ``errno`` variable is set
0118 appropriately. Possible error codes are:
0119
0120 EBADF
0121 ``fd`` is not a valid file descriptor.
0122
0123 EACCES
0124 ``fd`` is not open for reading and writing.
0125
0126 EINVAL
0127 The ``start`` or ``length`` or ``offset`` are not suitable. (E. g.
0128 they are too large, or not aligned on a ``PAGESIZE`` boundary.)
0129
0130 The ``flags`` or ``prot`` value is not supported.
0131
0132 No buffers have been allocated with the
0133 :ref:`VIDIOC_REQBUFS` ioctl.
0134
0135 ENOMEM
0136 Not enough physical or virtual memory was available to complete the
0137 request.
