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 <unistd.h> 16#include <sys/mman.h></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.</para> 49 </listitem> 50 </varlistentry> 51 <varlistentry> 52 <term><parameter>prot</parameter></term> 53 <listitem> 54 <para>The <parameter>prot</parameter> argument describes the 55desired memory protection. Regardless of the device type and the 56direction of data exchange it should be set to 57<constant>PROT_READ</constant> | <constant>PROT_WRITE</constant>, 58permitting read and write access to image buffers. Drivers should 59support at least this combination of flags. Note the Linux 60<filename>video-buf</filename> kernel module, which is used by the 61bttv, saa7134, saa7146, cx88 and vivi driver supports only 62<constant>PROT_READ</constant> | <constant>PROT_WRITE</constant>. When 63the driver does not support the desired protection the 64<function>mmap()</function> function fails.</para> 65 <para>Note device memory accesses (⪚ the memory on a 66graphics card with video capturing hardware) may incur a performance 67penalty compared to main memory accesses, or reads may be 68significantly slower than writes or vice versa. Other I/O methods may 69be more efficient in this case.</para> 70 </listitem> 71 </varlistentry> 72 <varlistentry> 73 <term><parameter>flags</parameter></term> 74 <listitem> 75 <para>The <parameter>flags</parameter> parameter 76specifies the type of the mapped object, mapping options and whether 77modifications made to the mapped copy of the page are private to the 78process or are to be shared with other references.</para> 79 <para><constant>MAP_FIXED</constant> requests that the 80driver selects no other address than the one specified. If the 81specified address cannot be used, <function>mmap()</function> will fail. If 82<constant>MAP_FIXED</constant> is specified, 83<parameter>start</parameter> must be a multiple of the pagesize. Use 84of this option is discouraged.</para> 85 <para>One of the <constant>MAP_SHARED</constant> or 86<constant>MAP_PRIVATE</constant> flags must be set. 87<constant>MAP_SHARED</constant> allows applications to share the 88mapped memory with other (⪚ child-) processes. Note the Linux 89<filename>video-buf</filename> module which is used by the bttv, 90saa7134, saa7146, cx88 and vivi driver supports only 91<constant>MAP_SHARED</constant>. <constant>MAP_PRIVATE</constant> 92requests copy-on-write semantics. V4L2 applications should not set the 93<constant>MAP_PRIVATE</constant>, <constant>MAP_DENYWRITE</constant>, 94<constant>MAP_EXECUTABLE</constant> or <constant>MAP_ANON</constant> 95flag.</para> 96 </listitem> 97 </varlistentry> 98 <varlistentry> 99 <term><parameter>fd</parameter></term> 100 <listitem> 101 <para>&fd;</para> 102 </listitem> 103 </varlistentry> 104 <varlistentry> 105 <term><parameter>offset</parameter></term> 106 <listitem> 107 <para>Offset of the buffer in device memory. This must be the 108same value as returned by the driver in the &v4l2-buffer; 109<structfield>m</structfield> union <structfield>offset</structfield> field.</para> 110 </listitem> 111 </varlistentry> 112 </variablelist> 113 </refsect1> 114 115 <refsect1> 116 <title>Description</title> 117 118 <para>The <function>mmap()</function> function asks to map 119<parameter>length</parameter> bytes starting at 120<parameter>offset</parameter> in the memory of the device specified by 121<parameter>fd</parameter> into the application address space, 122preferably at address <parameter>start</parameter>. This latter 123address is a hint only, and is usually specified as 0.</para> 124 125 <para>Suitable length and offset parameters are queried with the 126&VIDIOC-QUERYBUF; ioctl. Buffers must be allocated with the 127&VIDIOC-REQBUFS; ioctl before they can be queried.</para> 128 129 <para>To unmap buffers the &func-munmap; function is used.</para> 130 </refsect1> 131 132 <refsect1> 133 <title>Return Value</title> 134 135 <para>On success <function>mmap()</function> returns a pointer to 136the mapped buffer. On error <constant>MAP_FAILED</constant> (-1) is 137returned, and the <varname>errno</varname> variable is set 138appropriately. Possible error codes are:</para> 139 140 <variablelist> 141 <varlistentry> 142 <term><errorcode>EBADF</errorcode></term> 143 <listitem> 144 <para><parameter>fd</parameter> is not a valid file 145descriptor.</para> 146 </listitem> 147 </varlistentry> 148 <varlistentry> 149 <term><errorcode>EACCES</errorcode></term> 150 <listitem> 151 <para><parameter>fd</parameter> is 152not open for reading and writing.</para> 153 </listitem> 154 </varlistentry> 155 <varlistentry> 156 <term><errorcode>EINVAL</errorcode></term> 157 <listitem> 158 <para>The <parameter>start</parameter> or 159<parameter>length</parameter> or <parameter>offset</parameter> are not 160suitable. (E. g. they are too large, or not aligned on a 161<constant>PAGESIZE</constant> boundary.)</para> 162 <para>The <parameter>flags</parameter> or 163<parameter>prot</parameter> value is not supported.</para> 164 <para>No buffers have been allocated with the 165&VIDIOC-REQBUFS; ioctl.</para> 166 </listitem> 167 </varlistentry> 168 <varlistentry> 169 <term><errorcode>ENOMEM</errorcode></term> 170 <listitem> 171 <para>Not enough physical or virtual memory was available to 172complete the request.</para> 173 </listitem> 174 </varlistentry> 175 </variablelist> 176 </refsect1> 177</refentry> 178 179<!-- 180Local Variables: 181mode: sgml 182sgml-parent-document: "v4l2.sgml" 183indent-tabs-mode: nil 184End: 185--> 186