1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
2
3.. _dmx-mmap:
4
5*****************
6Digital TV mmap()
7*****************
8
9Name
10====
11
12dmx-mmap - Map device memory into application address space
13
14.. warning:: this API is still experimental
15
16Synopsis
17========
18
19.. code-block:: c
20
21    #include <unistd.h>
22    #include <sys/mman.h>
23
24
25.. c:function:: void *mmap( void *start, size_t length, int prot, int flags, int fd, off_t offset )
26    :name: dmx-mmap
27
28Arguments
29=========
30
31``start``
32    Map the buffer to this address in the application's address space.
33    When the ``MAP_FIXED`` flag is specified, ``start`` must be a
34    multiple of the pagesize and mmap will fail when the specified
35    address cannot be used. Use of this option is discouraged;
36    applications should just specify a ``NULL`` pointer here.
37
38``length``
39    Length of the memory area to map. This must be a multiple of the
40    DVB packet length (188, on most drivers).
41
42``prot``
43    The ``prot`` argument describes the desired memory protection.
44    Regardless of the device type and the direction of data exchange it
45    should be set to ``PROT_READ`` | ``PROT_WRITE``, permitting read
46    and write access to image buffers. Drivers should support at least
47    this combination of flags.
48
49``flags``
50    The ``flags`` parameter specifies the type of the mapped object,
51    mapping options and whether modifications made to the mapped copy of
52    the page are private to the process or are to be shared with other
53    references.
54
55    ``MAP_FIXED`` requests that the driver selects no other address than
56    the one specified. If the specified address cannot be used,
57    :ref:`mmap() <dmx-mmap>` will fail. If ``MAP_FIXED`` is specified,
58    ``start`` must be a multiple of the pagesize. Use of this option is
59    discouraged.
60
61    One of the ``MAP_SHARED`` or ``MAP_PRIVATE`` flags must be set.
62    ``MAP_SHARED`` allows applications to share the mapped memory with
63    other (e. g. child-) processes.
64
65    .. note::
66
67       The Linux Digital TV applications should not set the
68       ``MAP_PRIVATE``, ``MAP_DENYWRITE``, ``MAP_EXECUTABLE`` or ``MAP_ANON``
69       flags.
70
71``fd``
72    File descriptor returned by :ref:`open() <dmx_fopen>`.
73
74``offset``
75    Offset of the buffer in device memory, as returned by
76    :ref:`DMX_QUERYBUF` ioctl.
77
78
79Description
80===========
81
82The :ref:`mmap() <dmx-mmap>` function asks to map ``length`` bytes starting at
83``offset`` in the memory of the device specified by ``fd`` into the
84application address space, preferably at address ``start``. This latter
85address is a hint only, and is usually specified as 0.
86
87Suitable length and offset parameters are queried with the
88:ref:`DMX_QUERYBUF` ioctl. Buffers must be allocated with the
89:ref:`DMX_REQBUFS` ioctl before they can be queried.
90
91To unmap buffers the :ref:`munmap() <dmx-munmap>` function is used.
92
93
94Return Value
95============
96
97On success :ref:`mmap() <dmx-mmap>` returns a pointer to the mapped buffer. On
98error ``MAP_FAILED`` (-1) is returned, and the ``errno`` variable is set
99appropriately. Possible error codes are:
100
101EBADF
102    ``fd`` is not a valid file descriptor.
103
104EACCES
105    ``fd`` is not open for reading and writing.
106
107EINVAL
108    The ``start`` or ``length`` or ``offset`` are not suitable. (E. g.
109    they are too large, or not aligned on a ``PAGESIZE`` boundary.)
110
111    The ``flags`` or ``prot`` value is not supported.
112
113    No buffers have been allocated with the
114    :ref:`DMX_REQBUFS` ioctl.
115
116ENOMEM
117    Not enough physical or virtual memory was available to complete the
118    request.
119