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138 lines
4.5 KiB
ReStructuredText
138 lines
4.5 KiB
ReStructuredText
.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later
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.. c:namespace:: V4L
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.. _func-mmap:
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***********
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V4L2 mmap()
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***********
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Name
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====
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v4l2-mmap - Map device memory into application address space
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Synopsis
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========
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.. code-block:: c
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#include <unistd.h>
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#include <sys/mman.h>
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.. c:function:: void *mmap( void *start, size_t length, int prot, int flags, int fd, off_t offset )
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Arguments
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=========
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``start``
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Map the buffer to this address in the application's address space.
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When the ``MAP_FIXED`` flag is specified, ``start`` must be a
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multiple of the pagesize and mmap will fail when the specified
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address cannot be used. Use of this option is discouraged;
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applications should just specify a ``NULL`` pointer here.
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``length``
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Length of the memory area to map. This must be the same value as
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returned by the driver in the struct
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:c:type:`v4l2_buffer` ``length`` field for the
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single-planar API, and the same value as returned by the driver in
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the struct :c:type:`v4l2_plane` ``length`` field for
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the multi-planar API.
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``prot``
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The ``prot`` argument describes the desired memory protection.
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Regardless of the device type and the direction of data exchange it
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should be set to ``PROT_READ`` | ``PROT_WRITE``, permitting read
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and write access to image buffers. Drivers should support at least
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this combination of flags.
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.. note::
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#. The Linux ``videobuf`` kernel module, which is used by some
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drivers supports only ``PROT_READ`` | ``PROT_WRITE``. When the
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driver does not support the desired protection, the
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:c:func:`mmap()` function fails.
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#. Device memory accesses (e. g. the memory on a graphics card
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with video capturing hardware) may incur a performance penalty
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compared to main memory accesses, or reads may be significantly
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slower than writes or vice versa. Other I/O methods may be more
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efficient in such case.
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``flags``
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The ``flags`` parameter specifies the type of the mapped object,
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mapping options and whether modifications made to the mapped copy of
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the page are private to the process or are to be shared with other
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references.
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``MAP_FIXED`` requests that the driver selects no other address than
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the one specified. If the specified address cannot be used,
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:c:func:`mmap()` will fail. If ``MAP_FIXED`` is specified,
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``start`` must be a multiple of the pagesize. Use of this option is
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discouraged.
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One of the ``MAP_SHARED`` or ``MAP_PRIVATE`` flags must be set.
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``MAP_SHARED`` allows applications to share the mapped memory with
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other (e. g. child-) processes.
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.. note::
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The Linux ``videobuf`` module which is used by some
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drivers supports only ``MAP_SHARED``. ``MAP_PRIVATE`` requests
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copy-on-write semantics. V4L2 applications should not set the
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``MAP_PRIVATE``, ``MAP_DENYWRITE``, ``MAP_EXECUTABLE`` or ``MAP_ANON``
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flags.
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``fd``
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File descriptor returned by :c:func:`open()`.
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``offset``
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Offset of the buffer in device memory. This must be the same value
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as returned by the driver in the struct
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:c:type:`v4l2_buffer` ``m`` union ``offset`` field for
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the single-planar API, and the same value as returned by the driver
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in the struct :c:type:`v4l2_plane` ``m`` union
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``mem_offset`` field for the multi-planar API.
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Description
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===========
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The :c:func:`mmap()` function asks to map ``length`` bytes starting at
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``offset`` in the memory of the device specified by ``fd`` into the
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application address space, preferably at address ``start``. This latter
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address is a hint only, and is usually specified as 0.
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Suitable length and offset parameters are queried with the
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:ref:`VIDIOC_QUERYBUF` ioctl. Buffers must be
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allocated with the :ref:`VIDIOC_REQBUFS` ioctl
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before they can be queried.
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To unmap buffers the :c:func:`munmap()` function is used.
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Return Value
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============
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On success :c:func:`mmap()` returns a pointer to the mapped buffer. On
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error ``MAP_FAILED`` (-1) is returned, and the ``errno`` variable is set
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appropriately. Possible error codes are:
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EBADF
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``fd`` is not a valid file descriptor.
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EACCES
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``fd`` is not open for reading and writing.
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EINVAL
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The ``start`` or ``length`` or ``offset`` are not suitable. (E. g.
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they are too large, or not aligned on a ``PAGESIZE`` boundary.)
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The ``flags`` or ``prot`` value is not supported.
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No buffers have been allocated with the
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:ref:`VIDIOC_REQBUFS` ioctl.
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ENOMEM
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Not enough physical or virtual memory was available to complete the
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request.
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