Markus Heiser | 5377d91 | 2016-06-30 15:18:56 +0200 | [diff] [blame] | 1 | .. -*- coding: utf-8; mode: rst -*- |
| 2 | |
| 3 | .. _func-mmap: |
| 4 | |
| 5 | *********** |
| 6 | V4L2 mmap() |
| 7 | *********** |
| 8 | |
| 9 | *man v4l2-mmap(2)* |
| 10 | |
| 11 | Map device memory into application address space |
| 12 | |
| 13 | |
| 14 | Synopsis |
| 15 | ======== |
| 16 | |
| 17 | .. code-block:: c |
| 18 | |
| 19 | #include <unistd.h> |
| 20 | #include <sys/mman.h> |
| 21 | |
| 22 | |
| 23 | .. c:function:: void *mmap( void *start, size_t length, int prot, int flags, int fd, off_t offset ) |
| 24 | |
| 25 | Arguments |
| 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 | :ref:`v4l2_buffer <v4l2-buffer>` ``length`` field for the |
| 39 | single-planar API, and the same value as returned by the driver in |
| 40 | the struct :ref:`v4l2_plane <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. Note the Linux ``video-buf`` kernel |
| 49 | module, which is used by the bttv, saa7134, saa7146, cx88 and vivi |
| 50 | driver supports only ``PROT_READ`` | ``PROT_WRITE``. When the |
| 51 | driver does not support the desired protection the |
| 52 | :c:func:`mmap()` function fails. |
| 53 | |
| 54 | Note device memory accesses (e. g. the memory on a graphics card |
| 55 | with video capturing hardware) may incur a performance penalty |
| 56 | compared to main memory accesses, or reads may be significantly |
| 57 | slower than writes or vice versa. Other I/O methods may be more |
| 58 | efficient in this case. |
| 59 | |
| 60 | ``flags`` |
| 61 | The ``flags`` parameter specifies the type of the mapped object, |
| 62 | mapping options and whether modifications made to the mapped copy of |
| 63 | the page are private to the process or are to be shared with other |
| 64 | references. |
| 65 | |
| 66 | ``MAP_FIXED`` requests that the driver selects no other address than |
| 67 | the one specified. If the specified address cannot be used, |
| 68 | :c:func:`mmap()` will fail. If ``MAP_FIXED`` is specified, |
| 69 | ``start`` must be a multiple of the pagesize. Use of this option is |
| 70 | discouraged. |
| 71 | |
| 72 | One of the ``MAP_SHARED`` or ``MAP_PRIVATE`` flags must be set. |
| 73 | ``MAP_SHARED`` allows applications to share the mapped memory with |
| 74 | other (e. g. child-) processes. Note the Linux ``video-buf`` module |
| 75 | which is used by the bttv, saa7134, saa7146, cx88 and vivi driver |
| 76 | supports only ``MAP_SHARED``. ``MAP_PRIVATE`` requests copy-on-write |
| 77 | semantics. V4L2 applications should not set the ``MAP_PRIVATE``, |
| 78 | ``MAP_DENYWRITE``, ``MAP_EXECUTABLE`` or ``MAP_ANON`` flag. |
| 79 | |
| 80 | ``fd`` |
| 81 | File descriptor returned by :ref:`open() <func-open>`. |
| 82 | |
| 83 | ``offset`` |
| 84 | Offset of the buffer in device memory. This must be the same value |
| 85 | as returned by the driver in the struct |
| 86 | :ref:`v4l2_buffer <v4l2-buffer>` ``m`` union ``offset`` field for |
| 87 | the single-planar API, and the same value as returned by the driver |
| 88 | in the struct :ref:`v4l2_plane <v4l2-plane>` ``m`` union |
| 89 | ``mem_offset`` field for the multi-planar API. |
| 90 | |
| 91 | |
| 92 | Description |
| 93 | =========== |
| 94 | |
| 95 | The :c:func:`mmap()` function asks to map ``length`` bytes starting at |
| 96 | ``offset`` in the memory of the device specified by ``fd`` into the |
| 97 | application address space, preferably at address ``start``. This latter |
| 98 | address is a hint only, and is usually specified as 0. |
| 99 | |
| 100 | Suitable length and offset parameters are queried with the |
| 101 | :ref:`VIDIOC_QUERYBUF <vidioc-querybuf>` ioctl. Buffers must be |
| 102 | allocated with the :ref:`VIDIOC_REQBUFS <vidioc-reqbufs>` ioctl |
| 103 | before they can be queried. |
| 104 | |
| 105 | To unmap buffers the :ref:`munmap() <func-munmap>` function is used. |
| 106 | |
| 107 | |
| 108 | Return Value |
| 109 | ============ |
| 110 | |
| 111 | On success :c:func:`mmap()` returns a pointer to the mapped buffer. On |
| 112 | error ``MAP_FAILED`` (-1) is returned, and the ``errno`` variable is set |
| 113 | appropriately. Possible error codes are: |
| 114 | |
| 115 | EBADF |
| 116 | ``fd`` is not a valid file descriptor. |
| 117 | |
| 118 | EACCES |
| 119 | ``fd`` is not open for reading and writing. |
| 120 | |
| 121 | EINVAL |
| 122 | The ``start`` or ``length`` or ``offset`` are not suitable. (E. g. |
| 123 | they are too large, or not aligned on a ``PAGESIZE`` boundary.) |
| 124 | |
| 125 | The ``flags`` or ``prot`` value is not supported. |
| 126 | |
| 127 | No buffers have been allocated with the |
| 128 | :ref:`VIDIOC_REQBUFS <vidioc-reqbufs>` ioctl. |
| 129 | |
| 130 | ENOMEM |
| 131 | Not enough physical or virtual memory was available to complete the |
| 132 | request. |
| 133 | |
| 134 | |
| 135 | .. ------------------------------------------------------------------------------ |
| 136 | .. This file was automatically converted from DocBook-XML with the dbxml |
| 137 | .. library (https://github.com/return42/sphkerneldoc). The origin XML comes |
| 138 | .. from the linux kernel, refer to: |
| 139 | .. |
| 140 | .. * https://github.com/torvalds/linux/tree/master/Documentation/DocBook |
| 141 | .. ------------------------------------------------------------------------------ |