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Based on kernel version 3.13. Page generated on 2014-01-20 22:02 EST.

1	<refentry id="func-mmap">
2	  <refmeta>
3	    <refentrytitle>V4L2 mmap()</refentrytitle>
4	    &manvol;
5	  </refmeta>
6	
7	  <refnamediv>
8	    <refname>v4l2-mmap</refname>
9	    <refpurpose>Map device memory into application address space</refpurpose>
10	  </refnamediv>
11	
12	  <refsynopsisdiv>
13	    <funcsynopsis>
14	      <funcsynopsisinfo>
15	#include &lt;unistd.h&gt;
16	#include &lt;sys/mman.h&gt;</funcsynopsisinfo>
17	      <funcprototype>
18		<funcdef>void *<function>mmap</function></funcdef>
19		<paramdef>void *<parameter>start</parameter></paramdef>
20		<paramdef>size_t <parameter>length</parameter></paramdef>
21		<paramdef>int <parameter>prot</parameter></paramdef>
22		<paramdef>int <parameter>flags</parameter></paramdef>
23		<paramdef>int <parameter>fd</parameter></paramdef>
24		<paramdef>off_t <parameter>offset</parameter></paramdef>
25	      </funcprototype>
26	    </funcsynopsis>
27	  </refsynopsisdiv>
28	
29	  <refsect1>
30	    <title>Arguments</title>
31	    <variablelist>
32	      <varlistentry>
33		<term><parameter>start</parameter></term>
34		<listitem>
35		  <para>Map the buffer to this address in the
36	application's address space. When the <constant>MAP_FIXED</constant>
37	flag is specified, <parameter>start</parameter> must be a multiple of the
38	pagesize and mmap will fail when the specified address
39	cannot be used. Use of this option is discouraged; applications should
40	just specify a <constant>NULL</constant> pointer here.</para>
41		</listitem>
42	      </varlistentry>
43	      <varlistentry>
44		<term><parameter>length</parameter></term>
45		<listitem>
46		  <para>Length of the memory area to map. This must be the
47	same value as returned by the driver in the &v4l2-buffer;
48	<structfield>length</structfield> field for the
49	single-planar API, and the same value as returned by the driver
50	in the &v4l2-plane; <structfield>length</structfield> field for the
51	multi-planar API.</para>
52		</listitem>
53	      </varlistentry>
54	      <varlistentry>
55		<term><parameter>prot</parameter></term>
56		<listitem>
57		  <para>The <parameter>prot</parameter> argument describes the
58	desired memory protection. Regardless of the device type and the
59	direction of data exchange it should be set to
60	<constant>PROT_READ</constant> | <constant>PROT_WRITE</constant>,
61	permitting read and write access to image buffers. Drivers should
62	support at least this combination of flags. Note the Linux
63	<filename>video-buf</filename> kernel module, which is used by the
64	bttv, saa7134, saa7146, cx88 and vivi driver supports only
65	<constant>PROT_READ</constant> | <constant>PROT_WRITE</constant>. When
66	the driver does not support the desired protection the
67	<function>mmap()</function> function fails.</para>
68		  <para>Note device memory accesses (&eg; the memory on a
69	graphics card with video capturing hardware) may incur a performance
70	penalty compared to main memory accesses, or reads may be
71	significantly slower than writes or vice versa. Other I/O methods may
72	be more efficient in this case.</para>
73		</listitem>
74	      </varlistentry>
75	      <varlistentry>
76		<term><parameter>flags</parameter></term>
77		<listitem>
78		  <para>The <parameter>flags</parameter> parameter
79	specifies the type of the mapped object, mapping options and whether
80	modifications made to the mapped copy of the page are private to the
81	process or are to be shared with other references.</para>
82		  <para><constant>MAP_FIXED</constant> requests that the
83	driver selects no other address than the one specified. If the
84	specified address cannot be used, <function>mmap()</function> will fail. If
85	<constant>MAP_FIXED</constant> is specified,
86	<parameter>start</parameter> must be a multiple of the pagesize. Use
87	of this option is discouraged.</para>
88		  <para>One of the <constant>MAP_SHARED</constant> or
89	<constant>MAP_PRIVATE</constant> flags must be set.
90	<constant>MAP_SHARED</constant> allows applications to share the
91	mapped memory with other (&eg; child-) processes. Note the Linux
92	<filename>video-buf</filename> module which is used by the bttv,
93	saa7134, saa7146, cx88 and vivi driver supports only
94	<constant>MAP_SHARED</constant>. <constant>MAP_PRIVATE</constant>
95	requests copy-on-write semantics. V4L2 applications should not set the
96	<constant>MAP_PRIVATE</constant>, <constant>MAP_DENYWRITE</constant>,
97	<constant>MAP_EXECUTABLE</constant> or <constant>MAP_ANON</constant>
98	flag.</para>
99		</listitem>
100	      </varlistentry>
101	      <varlistentry>
102		<term><parameter>fd</parameter></term>
103		<listitem>
104		  <para>&fd;</para>
105		</listitem>
106	      </varlistentry>
107	      <varlistentry>
108		<term><parameter>offset</parameter></term>
109		<listitem>
110		  <para>Offset of the buffer in device memory. This must be the
111	same value as returned by the driver in the &v4l2-buffer;
112	<structfield>m</structfield> union <structfield>offset</structfield> field for
113	the single-planar API, and the same value as returned by the driver
114	in the &v4l2-plane; <structfield>m</structfield> union
115	<structfield>mem_offset</structfield> field for the multi-planar API.</para>
116		</listitem>
117	      </varlistentry>
118	    </variablelist>
119	  </refsect1>
120	
121	  <refsect1>
122	    <title>Description</title>
123	
124	    <para>The <function>mmap()</function> function asks to map
125	<parameter>length</parameter> bytes starting at
126	<parameter>offset</parameter> in the memory of the device specified by
127	<parameter>fd</parameter> into the application address space,
128	preferably at address <parameter>start</parameter>. This latter
129	address is a hint only, and is usually specified as 0.</para>
130	
131	    <para>Suitable length and offset parameters are queried with the
132	&VIDIOC-QUERYBUF; ioctl. Buffers must be allocated with the
133	&VIDIOC-REQBUFS; ioctl before they can be queried.</para>
134	
135	    <para>To unmap buffers the &func-munmap; function is used.</para>
136	  </refsect1>
137	
138	  <refsect1>
139	    <title>Return Value</title>
140	
141	    <para>On success <function>mmap()</function> returns a pointer to
142	the mapped buffer. On error <constant>MAP_FAILED</constant> (-1) is
143	returned, and the <varname>errno</varname> variable is set
144	appropriately. Possible error codes are:</para>
145	
146	    <variablelist>
147	      <varlistentry>
148		<term><errorcode>EBADF</errorcode></term>
149		<listitem>
150		  <para><parameter>fd</parameter> is not a valid file
151	descriptor.</para>
152		</listitem>
153	      </varlistentry>
154	      <varlistentry>
155		<term><errorcode>EACCES</errorcode></term>
156		<listitem>
157		  <para><parameter>fd</parameter> is
158	not open for reading and writing.</para>
159		</listitem>
160	      </varlistentry>
161	      <varlistentry>
162		<term><errorcode>EINVAL</errorcode></term>
163		<listitem>
164		  <para>The <parameter>start</parameter> or
165	<parameter>length</parameter> or <parameter>offset</parameter> are not
166	suitable. (E.&nbsp;g. they are too large, or not aligned on a
167	<constant>PAGESIZE</constant> boundary.)</para>
168		  <para>The <parameter>flags</parameter> or
169	<parameter>prot</parameter> value is not supported.</para>
170		  <para>No buffers have been allocated with the
171	&VIDIOC-REQBUFS; ioctl.</para>
172		</listitem>
173	      </varlistentry>
174	      <varlistentry>
175		<term><errorcode>ENOMEM</errorcode></term>
176		<listitem>
177		  <para>Not enough physical or virtual memory was available to
178	complete the request.</para>
179		</listitem>
180	      </varlistentry>
181	    </variablelist>
182	  </refsect1>
183	</refentry>
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