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