Based on kernel version 4.7.2. Page generated on 2016-08-22 22:45 EST.
1 <refentry id="func-read"> 2 <refmeta> 3 <refentrytitle>V4L2 read()</refentrytitle> 4 &manvol; 5 </refmeta> 6 7 <refnamediv> 8 <refname>v4l2-read</refname> 9 <refpurpose>Read from a V4L2 device</refpurpose> 10 </refnamediv> 11 12 <refsynopsisdiv> 13 <funcsynopsis> 14 <funcsynopsisinfo>#include <unistd.h></funcsynopsisinfo> 15 <funcprototype> 16 <funcdef>ssize_t <function>read</function></funcdef> 17 <paramdef>int <parameter>fd</parameter></paramdef> 18 <paramdef>void *<parameter>buf</parameter></paramdef> 19 <paramdef>size_t <parameter>count</parameter></paramdef> 20 </funcprototype> 21 </funcsynopsis> 22 </refsynopsisdiv> 23 24 <refsect1> 25 <title>Arguments</title> 26 27 <variablelist> 28 <varlistentry> 29 <term><parameter>fd</parameter></term> 30 <listitem> 31 <para>&fd;</para> 32 </listitem> 33 </varlistentry> 34 <varlistentry> 35 <term><parameter>buf</parameter></term> 36 <listitem> 37 <para></para> 38 </listitem> 39 </varlistentry> 40 <varlistentry> 41 <term><parameter>count</parameter></term> 42 <listitem> 43 <para></para> 44 </listitem> 45 </varlistentry> 46 </variablelist> 47 </refsect1> 48 49 <refsect1> 50 <title>Description</title> 51 52 <para><function>read()</function> attempts to read up to 53 <parameter>count</parameter> bytes from file descriptor 54 <parameter>fd</parameter> into the buffer starting at 55 <parameter>buf</parameter>. The layout of the data in the buffer is 56 discussed in the respective device interface section, see ##. If <parameter>count</parameter> is zero, 57 <function>read()</function> returns zero and has no other results. If 58 <parameter>count</parameter> is greater than 59 <constant>SSIZE_MAX</constant>, the result is unspecified. Regardless 60 of the <parameter>count</parameter> value each 61 <function>read()</function> call will provide at most one frame (two 62 fields) worth of data.</para> 63 64 <para>By default <function>read()</function> blocks until data 65 becomes available. When the <constant>O_NONBLOCK</constant> flag was 66 given to the &func-open; function it 67 returns immediately with an &EAGAIN; when no data is available. The 68 &func-select; or &func-poll; functions 69 can always be used to suspend execution until data becomes available. All 70 drivers supporting the <function>read()</function> function must also 71 support <function>select()</function> and 72 <function>poll()</function>.</para> 73 74 <para>Drivers can implement read functionality in different 75 ways, using a single or multiple buffers and discarding the oldest or 76 newest frames once the internal buffers are filled.</para> 77 78 <para><function>read()</function> never returns a "snapshot" of a 79 buffer being filled. Using a single buffer the driver will stop 80 capturing when the application starts reading the buffer until the 81 read is finished. Thus only the period of the vertical blanking 82 interval is available for reading, or the capture rate must fall below 83 the nominal frame rate of the video standard.</para> 84 85 <para>The behavior of 86 <function>read()</function> when called during the active picture 87 period or the vertical blanking separating the top and bottom field 88 depends on the discarding policy. A driver discarding the oldest 89 frames keeps capturing into an internal buffer, continuously 90 overwriting the previously, not read frame, and returns the frame 91 being received at the time of the <function>read()</function> call as 92 soon as it is complete.</para> 93 94 <para>A driver discarding the newest frames stops capturing until 95 the next <function>read()</function> call. The frame being received at 96 <function>read()</function> time is discarded, returning the following 97 frame instead. Again this implies a reduction of the capture rate to 98 one half or less of the nominal frame rate. An example of this model 99 is the video read mode of the bttv driver, initiating a DMA to user 100 memory when <function>read()</function> is called and returning when 101 the DMA finished.</para> 102 103 <para>In the multiple buffer model drivers maintain a ring of 104 internal buffers, automatically advancing to the next free buffer. 105 This allows continuous capturing when the application can empty the 106 buffers fast enough. Again, the behavior when the driver runs out of 107 free buffers depends on the discarding policy.</para> 108 109 <para>Applications can get and set the number of buffers used 110 internally by the driver with the &VIDIOC-G-PARM; and &VIDIOC-S-PARM; 111 ioctls. They are optional, however. The discarding policy is not 112 reported and cannot be changed. For minimum requirements see <xref 113 linkend="devices" />.</para> 114 </refsect1> 115 116 <refsect1> 117 <title>Return Value</title> 118 119 <para>On success, the number of bytes read is returned. It is not 120 an error if this number is smaller than the number of bytes requested, 121 or the amount of data required for one frame. This may happen for 122 example because <function>read()</function> was interrupted by a 123 signal. On error, -1 is returned, and the <varname>errno</varname> 124 variable is set appropriately. In this case the next read will start 125 at the beginning of a new frame. Possible error codes are:</para> 126 127 <variablelist> 128 <varlistentry> 129 <term><errorcode>EAGAIN</errorcode></term> 130 <listitem> 131 <para>Non-blocking I/O has been selected using 132 O_NONBLOCK and no data was immediately available for reading.</para> 133 </listitem> 134 </varlistentry> 135 <varlistentry> 136 <term><errorcode>EBADF</errorcode></term> 137 <listitem> 138 <para><parameter>fd</parameter> is not a valid file 139 descriptor or is not open for reading, or the process already has the 140 maximum number of files open.</para> 141 </listitem> 142 </varlistentry> 143 <varlistentry> 144 <term><errorcode>EBUSY</errorcode></term> 145 <listitem> 146 <para>The driver does not support multiple read streams and the 147 device is already in use.</para> 148 </listitem> 149 </varlistentry> 150 <varlistentry> 151 <term><errorcode>EFAULT</errorcode></term> 152 <listitem> 153 <para><parameter>buf</parameter> references an inaccessible 154 memory area.</para> 155 </listitem> 156 </varlistentry> 157 <varlistentry> 158 <term><errorcode>EINTR</errorcode></term> 159 <listitem> 160 <para>The call was interrupted by a signal before any 161 data was read.</para> 162 </listitem> 163 </varlistentry> 164 <varlistentry> 165 <term><errorcode>EIO</errorcode></term> 166 <listitem> 167 <para>I/O error. This indicates some hardware problem or a 168 failure to communicate with a remote device (USB camera etc.).</para> 169 </listitem> 170 </varlistentry> 171 <varlistentry> 172 <term><errorcode>EINVAL</errorcode></term> 173 <listitem> 174 <para>The <function>read()</function> function is not 175 supported by this driver, not on this device, or generally not on this 176 type of device.</para> 177 </listitem> 178 </varlistentry> 179 </variablelist> 180 </refsect1> 181 </refentry>
