1.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later 2.. c:namespace:: V4L 3 4.. _func-mmap: 5 6*********** 7V4L2 mmap() 8*********** 9 10Name 11==== 12 13v4l2-mmap - Map device memory into application address space 14 15Synopsis 16======== 17 18.. code-block:: c 19 20 #include <unistd.h> 21 #include <sys/mman.h> 22 23.. c:function:: void *mmap( void *start, size_t length, int prot, int flags, int fd, off_t offset ) 24 25Arguments 26========= 27 28``start`` 29 Map the buffer to this address in the application's address space. 30 When the ``MAP_FIXED`` flag is specified, ``start`` must be a 31 multiple of the pagesize and mmap will fail when the specified 32 address cannot be used. Use of this option is discouraged; 33 applications should just specify a ``NULL`` pointer here. 34 35``length`` 36 Length of the memory area to map. This must be the same value as 37 returned by the driver in the struct 38 :c:type:`v4l2_buffer` ``length`` field for the 39 single-planar API, and the same value as returned by the driver in 40 the struct :c:type:`v4l2_plane` ``length`` field for 41 the multi-planar API. 42 43``prot`` 44 The ``prot`` argument describes the desired memory protection. 45 Regardless of the device type and the direction of data exchange it 46 should be set to ``PROT_READ`` | ``PROT_WRITE``, permitting read 47 and write access to image buffers. Drivers should support at least 48 this combination of flags. 49 50 .. note:: 51 52 #. The Linux ``videobuf`` kernel module, which is used by some 53 drivers supports only ``PROT_READ`` | ``PROT_WRITE``. When the 54 driver does not support the desired protection, the 55 :c:func:`mmap()` function fails. 56 57 #. Device memory accesses (e. g. the memory on a graphics card 58 with video capturing hardware) may incur a performance penalty 59 compared to main memory accesses, or reads may be significantly 60 slower than writes or vice versa. Other I/O methods may be more 61 efficient in such case. 62 63``flags`` 64 The ``flags`` parameter specifies the type of the mapped object, 65 mapping options and whether modifications made to the mapped copy of 66 the page are private to the process or are to be shared with other 67 references. 68 69 ``MAP_FIXED`` requests that the driver selects no other address than 70 the one specified. If the specified address cannot be used, 71 :c:func:`mmap()` will fail. If ``MAP_FIXED`` is specified, 72 ``start`` must be a multiple of the pagesize. Use of this option is 73 discouraged. 74 75 One of the ``MAP_SHARED`` or ``MAP_PRIVATE`` flags must be set. 76 ``MAP_SHARED`` allows applications to share the mapped memory with 77 other (e. g. child-) processes. 78 79 .. note:: 80 81 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. 86 87``fd`` 88 File descriptor returned by :c: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 :c:type:`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 :c:type:`v4l2_plane` ``m`` union 96 ``mem_offset`` field for the multi-planar API. 97 98Description 99=========== 100 101The :c:func:`mmap()` function asks to map ``length`` bytes starting at 102``offset`` in the memory of the device specified by ``fd`` into the 103application address space, preferably at address ``start``. This latter 104address is a hint only, and is usually specified as 0. 105 106Suitable length and offset parameters are queried with the 107:ref:`VIDIOC_QUERYBUF` ioctl. Buffers must be 108allocated with the :ref:`VIDIOC_REQBUFS` ioctl 109before they can be queried. 110 111To unmap buffers the :c:func:`munmap()` function is used. 112 113Return Value 114============ 115 116On success :c:func:`mmap()` returns a pointer to the mapped buffer. On 117error ``MAP_FAILED`` (-1) is returned, and the ``errno`` variable is set 118appropriately. Possible error codes are: 119 120EBADF 121 ``fd`` is not a valid file descriptor. 122 123EACCES 124 ``fd`` is not open for reading and writing. 125 126EINVAL 127 The ``start`` or ``length`` or ``offset`` are not suitable. (E. g. 128 they are too large, or not aligned on a ``PAGESIZE`` boundary.) 129 130 The ``flags`` or ``prot`` value is not supported. 131 132 No buffers have been allocated with the 133 :ref:`VIDIOC_REQBUFS` ioctl. 134 135ENOMEM 136 Not enough physical or virtual memory was available to complete the 137 request. 138