1<html><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968"><title>V4L2 mmap()</title><meta name="generator" content="DocBook XSL Stylesheets V1.78.1"><link rel="home" href="index.html" title="LINUX MEDIA INFRASTRUCTURE API"><link rel="up" href="user-func.html" title="Appendix A. Function Reference"><link rel="prev" href="vidioc-subscribe-event.html" title="ioctl VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT"><link rel="next" href="func-munmap.html" title="V4L2 munmap()"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">V4L2 mmap()</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="vidioc-subscribe-event.html">Prev</a> </td><th width="60%" align="center">Appendix A. Function Reference</th><td width="20%" align="right"> <a accesskey="n" href="func-munmap.html">Next</a></td></tr></table><hr></div><div class="refentry"><a name="func-mmap"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>v4l2-mmap — Map device memory into application address space</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><div class="funcsynopsis"><pre class="funcsynopsisinfo"> 2#include <unistd.h> 3#include <sys/mman.h></pre><table border="0" class="funcprototype-table" summary="Function synopsis" style="cellspacing: 0; cellpadding: 0;"><tr><td><code class="funcdef">void *<b class="fsfunc">mmap</b>(</code></td><td>void *<var class="pdparam">start</var>, </td></tr><tr><td> </td><td>size_t <var class="pdparam">length</var>, </td></tr><tr><td> </td><td>int <var class="pdparam">prot</var>, </td></tr><tr><td> </td><td>int <var class="pdparam">flags</var>, </td></tr><tr><td> </td><td>int <var class="pdparam">fd</var>, </td></tr><tr><td> </td><td>off_t <var class="pdparam">offset</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div></div></div><div class="refsect1"><a name="idp1116945684"></a><h2>Arguments</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="parameter"><code>start</code></em></span></dt><dd><p>Map the buffer to this address in the 4application's address space. When the <code class="constant">MAP_FIXED</code> 5flag is specified, <em class="parameter"><code>start</code></em> must be a multiple of the 6pagesize and mmap will fail when the specified address 7cannot be used. Use of this option is discouraged; applications should 8just specify a <code class="constant">NULL</code> pointer here.</p></dd><dt><span class="term"><em class="parameter"><code>length</code></em></span></dt><dd><p>Length of the memory area to map. This must be the 9same value as returned by the driver in the struct <a class="link" href="buffer.html#v4l2-buffer" title="Table 3.1. struct v4l2_buffer">v4l2_buffer</a> 10<em class="structfield"><code>length</code></em> field for the 11single-planar API, and the same value as returned by the driver 12in the struct <a class="link" href="buffer.html#v4l2-plane" title="Table 3.2. struct v4l2_plane">v4l2_plane</a> <em class="structfield"><code>length</code></em> field for the 13multi-planar API.</p></dd><dt><span class="term"><em class="parameter"><code>prot</code></em></span></dt><dd><p>The <em class="parameter"><code>prot</code></em> argument describes the 14desired memory protection. Regardless of the device type and the 15direction of data exchange it should be set to 16<code class="constant">PROT_READ</code> | <code class="constant">PROT_WRITE</code>, 17permitting read and write access to image buffers. Drivers should 18support at least this combination of flags. Note the Linux 19<code class="filename">video-buf</code> kernel module, which is used by the 20bttv, saa7134, saa7146, cx88 and vivi driver supports only 21<code class="constant">PROT_READ</code> | <code class="constant">PROT_WRITE</code>. When 22the driver does not support the desired protection the 23<code class="function">mmap()</code> function fails.</p><p>Note device memory accesses (e. g. the memory on a 24graphics card with video capturing hardware) may incur a performance 25penalty compared to main memory accesses, or reads may be 26significantly slower than writes or vice versa. Other I/O methods may 27be more efficient in this case.</p></dd><dt><span class="term"><em class="parameter"><code>flags</code></em></span></dt><dd><p>The <em class="parameter"><code>flags</code></em> parameter 28specifies the type of the mapped object, mapping options and whether 29modifications made to the mapped copy of the page are private to the 30process or are to be shared with other references.</p><p><code class="constant">MAP_FIXED</code> requests that the 31driver selects no other address than the one specified. If the 32specified address cannot be used, <code class="function">mmap()</code> will fail. If 33<code class="constant">MAP_FIXED</code> is specified, 34<em class="parameter"><code>start</code></em> must be a multiple of the pagesize. Use 35of this option is discouraged.</p><p>One of the <code class="constant">MAP_SHARED</code> or 36<code class="constant">MAP_PRIVATE</code> flags must be set. 37<code class="constant">MAP_SHARED</code> allows applications to share the 38mapped memory with other (e. g. child-) processes. Note the Linux 39<code class="filename">video-buf</code> module which is used by the bttv, 40saa7134, saa7146, cx88 and vivi driver supports only 41<code class="constant">MAP_SHARED</code>. <code class="constant">MAP_PRIVATE</code> 42requests copy-on-write semantics. V4L2 applications should not set the 43<code class="constant">MAP_PRIVATE</code>, <code class="constant">MAP_DENYWRITE</code>, 44<code class="constant">MAP_EXECUTABLE</code> or <code class="constant">MAP_ANON</code> 45flag.</p></dd><dt><span class="term"><em class="parameter"><code>fd</code></em></span></dt><dd><p>File descriptor returned by <a class="link" href="func-open.html" title="V4L2 open()"><code class="function">open()</code></a>.</p></dd><dt><span class="term"><em class="parameter"><code>offset</code></em></span></dt><dd><p>Offset of the buffer in device memory. This must be the 46same value as returned by the driver in the struct <a class="link" href="buffer.html#v4l2-buffer" title="Table 3.1. struct v4l2_buffer">v4l2_buffer</a> 47<em class="structfield"><code>m</code></em> union <em class="structfield"><code>offset</code></em> field for 48the single-planar API, and the same value as returned by the driver 49in the struct <a class="link" href="buffer.html#v4l2-plane" title="Table 3.2. struct v4l2_plane">v4l2_plane</a> <em class="structfield"><code>m</code></em> union 50<em class="structfield"><code>mem_offset</code></em> field for the multi-planar API.</p></dd></dl></div></div><div class="refsect1"><a name="idp1116964676"></a><h2>Description</h2><p>The <code class="function">mmap()</code> function asks to map 51<em class="parameter"><code>length</code></em> bytes starting at 52<em class="parameter"><code>offset</code></em> in the memory of the device specified by 53<em class="parameter"><code>fd</code></em> into the application address space, 54preferably at address <em class="parameter"><code>start</code></em>. This latter 55address is a hint only, and is usually specified as 0.</p><p>Suitable length and offset parameters are queried with the 56<a class="link" href="vidioc-querybuf.html" title="ioctl VIDIOC_QUERYBUF"><code class="constant">VIDIOC_QUERYBUF</code></a> ioctl. Buffers must be allocated with the 57<a class="link" href="vidioc-reqbufs.html" title="ioctl VIDIOC_REQBUFS"><code class="constant">VIDIOC_REQBUFS</code></a> ioctl before they can be queried.</p><p>To unmap buffers the <a class="link" href="func-munmap.html" title="V4L2 munmap()"><code class="function">munmap()</code></a> function is used.</p></div><div class="refsect1"><a name="idp1116969220"></a><h2>Return Value</h2><p>On success <code class="function">mmap()</code> returns a pointer to 58the mapped buffer. On error <code class="constant">MAP_FAILED</code> (-1) is 59returned, and the <code class="varname">errno</code> variable is set 60appropriately. Possible error codes are:</p><div class="variablelist"><dl class="variablelist"><dt><span class="term"><span class="errorcode">EBADF</span></span></dt><dd><p><em class="parameter"><code>fd</code></em> is not a valid file 61descriptor.</p></dd><dt><span class="term"><span class="errorcode">EACCES</span></span></dt><dd><p><em class="parameter"><code>fd</code></em> is 62not open for reading and writing.</p></dd><dt><span class="term"><span class="errorcode">EINVAL</span></span></dt><dd><p>The <em class="parameter"><code>start</code></em> or 63<em class="parameter"><code>length</code></em> or <em class="parameter"><code>offset</code></em> are not 64suitable. (E. g. they are too large, or not aligned on a 65<code class="constant">PAGESIZE</code> boundary.)</p><p>The <em class="parameter"><code>flags</code></em> or 66<em class="parameter"><code>prot</code></em> value is not supported.</p><p>No buffers have been allocated with the 67<a class="link" href="vidioc-reqbufs.html" title="ioctl VIDIOC_REQBUFS"><code class="constant">VIDIOC_REQBUFS</code></a> ioctl.</p></dd><dt><span class="term"><span class="errorcode">ENOMEM</span></span></dt><dd><p>Not enough physical or virtual memory was available to 68complete the request.</p></dd></dl></div></div></div><div class="navfooter"><hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="vidioc-subscribe-event.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="user-func.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="func-munmap.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">ioctl VIDIOC_SUBSCRIBE_EVENT, VIDIOC_UNSUBSCRIBE_EVENT </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> V4L2 munmap()</td></tr></table></div></body></html> 69