<?xml version="1.0" encoding="ANSI_X3.4-1968" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968" /><title>V4L2 read()</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><link rel="home" href="index.html" title="LINUX MEDIA INFRASTRUCTURE API" /><link rel="up" href="apa.html" title="Appendix A. Function Reference" /><link rel="prev" href="re65.html" title="V4L2 poll()" /><link rel="next" href="re67.html" title="V4L2 select()" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">V4L2 read()</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="re65.html">Prev</a> </td><th width="60%" align="center">Appendix A. Function Reference</th><td width="20%" align="right"> <a accesskey="n" href="re67.html">Next</a></td></tr></table><hr /></div><div class="refentry" title="V4L2 read()"><a id="func-read"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>v4l2-read — Read from a V4L2 device</p></div><div class="refsynopsisdiv" title="Synopsis"><h2>Synopsis</h2><div class="funcsynopsis"><pre class="funcsynopsisinfo">#include <unistd.h></pre><table border="0" summary="Function synopsis" cellspacing="0" cellpadding="0" class="funcprototype-table"><tr><td><code class="funcdef">ssize_t <b class="fsfunc">read</b>(</code></td><td>int <var class="pdparam">fd</var>, </td></tr><tr><td> </td><td>void *<var class="pdparam">buf</var>, </td></tr><tr><td> </td><td>size_t <var class="pdparam">count</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div></div></div><div class="refsect1" title="Arguments"><a id="id2931220"></a><h2>Arguments</h2><div class="variablelist"><dl><dt><span class="term"><em class="parameter"><code>fd</code></em></span></dt><dd><p>File descriptor returned by <a class="link" href="re64.html" title="V4L2 open()"><code class="function">open()</code></a>.</p></dd><dt><span class="term"><em class="parameter"><code>buf</code></em></span></dt><dd><p></p></dd><dt><span class="term"><em class="parameter"><code>count</code></em></span></dt><dd><p></p></dd></dl></div></div><div class="refsect1" title="Description"><a id="id2931286"></a><h2>Description</h2><p><code class="function">read()</code> attempts to read up to <em class="parameter"><code>count</code></em> bytes from file descriptor <em class="parameter"><code>fd</code></em> into the buffer starting at <em class="parameter"><code>buf</code></em>. The layout of the data in the buffer is discussed in the respective device interface section, see ##. If <em class="parameter"><code>count</code></em> is zero, <code class="function">read()</code> returns zero and has no other results. If <em class="parameter"><code>count</code></em> is greater than <code class="constant">SSIZE_MAX</code>, the result is unspecified. Regardless of the <em class="parameter"><code>count</code></em> value each <code class="function">read()</code> call will provide at most one frame (two fields) worth of data.</p><p>By default <code class="function">read()</code> blocks until data becomes available. When the <code class="constant">O_NONBLOCK</code> flag was given to the <a class="link" href="re64.html" title="V4L2 open()"><code class="function">open()</code></a> function it returns immediately with an <span class="errorcode">EAGAIN</span> error code when no data is available. The <a class="link" href="re67.html" title="V4L2 select()"><code class="function">select()</code></a> or <a class="link" href="re65.html" title="V4L2 poll()"><code class="function">poll()</code></a> functions can always be used to suspend execution until data becomes available. All drivers supporting the <code class="function">read()</code> function must also support <code class="function">select()</code> and <code class="function">poll()</code>.</p><p>Drivers can implement read functionality in different ways, using a single or multiple buffers and discarding the oldest or newest frames once the internal buffers are filled.</p><p><code class="function">read()</code> never returns a "snapshot" of a buffer being filled. Using a single buffer the driver will stop capturing when the application starts reading the buffer until the read is finished. Thus only the period of the vertical blanking interval is available for reading, or the capture rate must fall below the nominal frame rate of the video standard.</p><p>The behavior of <code class="function">read()</code> when called during the active picture period or the vertical blanking separating the top and bottom field depends on the discarding policy. A driver discarding the oldest frames keeps capturing into an internal buffer, continuously overwriting the previously, not read frame, and returns the frame being received at the time of the <code class="function">read()</code> call as soon as it is complete.</p><p>A driver discarding the newest frames stops capturing until the next <code class="function">read()</code> call. The frame being received at <code class="function">read()</code> time is discarded, returning the following frame instead. Again this implies a reduction of the capture rate to one half or less of the nominal frame rate. An example of this model is the video read mode of the bttv driver, initiating a DMA to user memory when <code class="function">read()</code> is called and returning when the DMA finished.</p><p>In the multiple buffer model drivers maintain a ring of internal buffers, automatically advancing to the next free buffer. This allows continuous capturing when the application can empty the buffers fast enough. Again, the behavior when the driver runs out of free buffers depends on the discarding policy.</p><p>Applications can get and set the number of buffers used internally by the driver with the <a class="link" href="re47.html" title="ioctl VIDIOC_G_PARM, VIDIOC_S_PARM"><code class="constant">VIDIOC_G_PARM</code></a> and <a class="link" href="re47.html" title="ioctl VIDIOC_G_PARM, VIDIOC_S_PARM"><code class="constant">VIDIOC_S_PARM</code></a> ioctls. They are optional, however. The discarding policy is not reported and cannot be changed. For minimum requirements see <a class="xref" href="ch04.html" title="Chapter 4. Interfaces">Chapter 4, <i>Interfaces</i></a>.</p></div><div class="refsect1" title="Return Value"><a id="id2932434"></a><h2>Return Value</h2><p>On success, the number of bytes read is returned. It is not an error if this number is smaller than the number of bytes requested, or the amount of data required for one frame. This may happen for example because <code class="function">read()</code> was interrupted by a signal. On error, -1 is returned, and the <code class="varname">errno</code> variable is set appropriately. In this case the next read will start at the beginning of a new frame. Possible error codes are:</p><div class="variablelist"><dl><dt><span class="term"><span class="errorcode">EAGAIN</span></span></dt><dd><p>Non-blocking I/O has been selected using O_NONBLOCK and no data was immediately available for reading.</p></dd><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 descriptor or is not open for reading, or the process already has the maximum number of files open.</p></dd><dt><span class="term"><span class="errorcode">EBUSY</span></span></dt><dd><p>The driver does not support multiple read streams and the device is already in use.</p></dd><dt><span class="term"><span class="errorcode">EFAULT</span></span></dt><dd><p><em class="parameter"><code>buf</code></em> references an inaccessible memory area.</p></dd><dt><span class="term"><span class="errorcode">EINTR</span></span></dt><dd><p>The call was interrupted by a signal before any data was read.</p></dd><dt><span class="term"><span class="errorcode">EIO</span></span></dt><dd><p>I/O error. This indicates some hardware problem or a failure to communicate with a remote device (USB camera etc.).</p></dd><dt><span class="term"><span class="errorcode">EINVAL</span></span></dt><dd><p>The <code class="function">read()</code> function is not supported by this driver, not on this device, or generally not on this type of device.</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="re65.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="apa.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="re67.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">V4L2 poll() </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> V4L2 select()</td></tr></table></div></body></html>
