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-<refentry id="func-mmap">
- <refmeta>
- <refentrytitle>V4L2 mmap()</refentrytitle>
- &manvol;
- </refmeta>
-
- <refnamediv>
- <refname>v4l2-mmap</refname>
- <refpurpose>Map device memory into application address space</refpurpose>
- </refnamediv>
-
- <refsynopsisdiv>
- <funcsynopsis>
- <funcsynopsisinfo>
-#include &lt;unistd.h&gt;
-#include &lt;sys/mman.h&gt;</funcsynopsisinfo>
- <funcprototype>
- <funcdef>void *<function>mmap</function></funcdef>
- <paramdef>void *<parameter>start</parameter></paramdef>
- <paramdef>size_t <parameter>length</parameter></paramdef>
- <paramdef>int <parameter>prot</parameter></paramdef>
- <paramdef>int <parameter>flags</parameter></paramdef>
- <paramdef>int <parameter>fd</parameter></paramdef>
- <paramdef>off_t <parameter>offset</parameter></paramdef>
- </funcprototype>
- </funcsynopsis>
- </refsynopsisdiv>
-
- <refsect1>
- <title>Arguments</title>
- <variablelist>
- <varlistentry>
- <term><parameter>start</parameter></term>
- <listitem>
- <para>Map the buffer to this address in the
-application's address space. When the <constant>MAP_FIXED</constant>
-flag is specified, <parameter>start</parameter> must be a multiple of the
-pagesize and mmap will fail when the specified address
-cannot be used. Use of this option is discouraged; applications should
-just specify a <constant>NULL</constant> pointer here.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><parameter>length</parameter></term>
- <listitem>
- <para>Length of the memory area to map. This must be the
-same value as returned by the driver in the &v4l2-buffer;
-<structfield>length</structfield> field for the
-single-planar API, and the same value as returned by the driver
-in the &v4l2-plane; <structfield>length</structfield> field for the
-multi-planar API.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><parameter>prot</parameter></term>
- <listitem>
- <para>The <parameter>prot</parameter> argument describes the
-desired memory protection. Regardless of the device type and the
-direction of data exchange it should be set to
-<constant>PROT_READ</constant> | <constant>PROT_WRITE</constant>,
-permitting read and write access to image buffers. Drivers should
-support at least this combination of flags. Note the Linux
-<filename>video-buf</filename> kernel module, which is used by the
-bttv, saa7134, saa7146, cx88 and vivi driver supports only
-<constant>PROT_READ</constant> | <constant>PROT_WRITE</constant>. When
-the driver does not support the desired protection the
-<function>mmap()</function> function fails.</para>
- <para>Note device memory accesses (&eg; the memory on a
-graphics card with video capturing hardware) may incur a performance
-penalty compared to main memory accesses, or reads may be
-significantly slower than writes or vice versa. Other I/O methods may
-be more efficient in this case.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><parameter>flags</parameter></term>
- <listitem>
- <para>The <parameter>flags</parameter> parameter
-specifies the type of the mapped object, mapping options and whether
-modifications made to the mapped copy of the page are private to the
-process or are to be shared with other references.</para>
- <para><constant>MAP_FIXED</constant> requests that the
-driver selects no other address than the one specified. If the
-specified address cannot be used, <function>mmap()</function> will fail. If
-<constant>MAP_FIXED</constant> is specified,
-<parameter>start</parameter> must be a multiple of the pagesize. Use
-of this option is discouraged.</para>
- <para>One of the <constant>MAP_SHARED</constant> or
-<constant>MAP_PRIVATE</constant> flags must be set.
-<constant>MAP_SHARED</constant> allows applications to share the
-mapped memory with other (&eg; child-) processes. Note the Linux
-<filename>video-buf</filename> module which is used by the bttv,
-saa7134, saa7146, cx88 and vivi driver supports only
-<constant>MAP_SHARED</constant>. <constant>MAP_PRIVATE</constant>
-requests copy-on-write semantics. V4L2 applications should not set the
-<constant>MAP_PRIVATE</constant>, <constant>MAP_DENYWRITE</constant>,
-<constant>MAP_EXECUTABLE</constant> or <constant>MAP_ANON</constant>
-flag.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><parameter>fd</parameter></term>
- <listitem>
- <para>&fd;</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><parameter>offset</parameter></term>
- <listitem>
- <para>Offset of the buffer in device memory. This must be the
-same value as returned by the driver in the &v4l2-buffer;
-<structfield>m</structfield> union <structfield>offset</structfield> field for
-the single-planar API, and the same value as returned by the driver
-in the &v4l2-plane; <structfield>m</structfield> union
-<structfield>mem_offset</structfield> field for the multi-planar API.</para>
- </listitem>
- </varlistentry>
- </variablelist>
- </refsect1>
-
- <refsect1>
- <title>Description</title>
-
- <para>The <function>mmap()</function> function asks to map
-<parameter>length</parameter> bytes starting at
-<parameter>offset</parameter> in the memory of the device specified by
-<parameter>fd</parameter> into the application address space,
-preferably at address <parameter>start</parameter>. This latter
-address is a hint only, and is usually specified as 0.</para>
-
- <para>Suitable length and offset parameters are queried with the
-&VIDIOC-QUERYBUF; ioctl. Buffers must be allocated with the
-&VIDIOC-REQBUFS; ioctl before they can be queried.</para>
-
- <para>To unmap buffers the &func-munmap; function is used.</para>
- </refsect1>
-
- <refsect1>
- <title>Return Value</title>
-
- <para>On success <function>mmap()</function> returns a pointer to
-the mapped buffer. On error <constant>MAP_FAILED</constant> (-1) is
-returned, and the <varname>errno</varname> variable is set
-appropriately. Possible error codes are:</para>
-
- <variablelist>
- <varlistentry>
- <term><errorcode>EBADF</errorcode></term>
- <listitem>
- <para><parameter>fd</parameter> is not a valid file
-descriptor.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><errorcode>EACCES</errorcode></term>
- <listitem>
- <para><parameter>fd</parameter> is
-not open for reading and writing.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><errorcode>EINVAL</errorcode></term>
- <listitem>
- <para>The <parameter>start</parameter> or
-<parameter>length</parameter> or <parameter>offset</parameter> are not
-suitable. (E.&nbsp;g. they are too large, or not aligned on a
-<constant>PAGESIZE</constant> boundary.)</para>
- <para>The <parameter>flags</parameter> or
-<parameter>prot</parameter> value is not supported.</para>
- <para>No buffers have been allocated with the
-&VIDIOC-REQBUFS; ioctl.</para>
- </listitem>
- </varlistentry>
- <varlistentry>
- <term><errorcode>ENOMEM</errorcode></term>
- <listitem>
- <para>Not enough physical or virtual memory was available to
-complete the request.</para>
- </listitem>
- </varlistentry>
- </variablelist>
- </refsect1>
-</refentry>