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
36application's address space. When the <constant>MAP_FIXED</constant>
37flag is specified, <parameter>start</parameter> must be a multiple of the
38pagesize and mmap will fail when the specified address
39cannot be used. Use of this option is discouraged; applications should
40just 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
47same value as returned by the driver in the &v4l2-buffer;
48<structfield>length</structfield> field for the
49single-planar API, and the same value as returned by the driver
50in the &v4l2-plane; <structfield>length</structfield> field for the
51multi-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
58desired memory protection. Regardless of the device type and the
59direction of data exchange it should be set to
60<constant>PROT_READ</constant> | <constant>PROT_WRITE</constant>,
61permitting read and write access to image buffers. Drivers should
62support at least this combination of flags. Note the Linux
63<filename>video-buf</filename> kernel module, which is used by the
64bttv, saa7134, saa7146, cx88 and vivi driver supports only
65<constant>PROT_READ</constant> | <constant>PROT_WRITE</constant>. When
66the 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
69graphics card with video capturing hardware) may incur a performance
70penalty compared to main memory accesses, or reads may be
71significantly slower than writes or vice versa. Other I/O methods may
72be 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
79specifies the type of the mapped object, mapping options and whether
80modifications made to the mapped copy of the page are private to the
81process or are to be shared with other references.</para>
82	  <para><constant>MAP_FIXED</constant> requests that the
83driver selects no other address than the one specified. If the
84specified 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
87of 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
91mapped memory with other (&eg; child-) processes. Note the Linux
92<filename>video-buf</filename> module which is used by the bttv,
93saa7134, saa7146, cx88 and vivi driver supports only
94<constant>MAP_SHARED</constant>. <constant>MAP_PRIVATE</constant>
95requests 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>
98flag.</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
111same value as returned by the driver in the &v4l2-buffer;
112<structfield>m</structfield> union <structfield>offset</structfield> field for
113the single-planar API, and the same value as returned by the driver
114in 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,
128preferably at address <parameter>start</parameter>. This latter
129address 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
142the mapped buffer. On error <constant>MAP_FAILED</constant> (-1) is
143returned, and the <varname>errno</varname> variable is set
144appropriately. 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
151descriptor.</para>
152	</listitem>
153      </varlistentry>
154      <varlistentry>
155	<term><errorcode>EACCES</errorcode></term>
156	<listitem>
157	  <para><parameter>fd</parameter> is
158not 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
166suitable. (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
178complete the request.</para>
179	</listitem>
180      </varlistentry>
181    </variablelist>
182  </refsect1>
183</refentry>
184