<?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>ioctl VIDIOC_QBUF, VIDIOC_DQBUF</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="re53.html" title="ioctl VIDIOC_OVERLAY" /><link rel="next" href="re55.html" title="ioctl VIDIOC_QUERYBUF" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center">ioctl VIDIOC_QBUF, VIDIOC_DQBUF</th></tr><tr><td width="20%" align="left"><a accesskey="p" href="re53.html">Prev</a> </td><th width="60%" align="center">Appendix A. Function Reference</th><td width="20%" align="right"> <a accesskey="n" href="re55.html">Next</a></td></tr></table><hr /></div><div class="refentry" title="ioctl VIDIOC_QBUF, VIDIOC_DQBUF"><a id="vidioc-qbuf"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>VIDIOC_QBUF, VIDIOC_DQBUF — Exchange a buffer with the driver</p></div><div class="refsynopsisdiv" title="Synopsis"><h2>Synopsis</h2><div class="funcsynopsis"><table border="0" summary="Function synopsis" cellspacing="0" cellpadding="0" class="funcprototype-table"><tr><td><code class="funcdef">int <b class="fsfunc">ioctl</b>(</code></td><td>int <var class="pdparam">fd</var>, </td></tr><tr><td> </td><td>int <var class="pdparam">request</var>, </td></tr><tr><td> </td><td>struct v4l2_buffer *<var class="pdparam">argp</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div></div></div><div class="refsect1" title="Arguments"><a id="id2729099"></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>request</code></em></span></dt><dd><p>VIDIOC_QBUF, VIDIOC_DQBUF</p></dd><dt><span class="term"><em class="parameter"><code>argp</code></em></span></dt><dd><p></p></dd></dl></div></div><div class="refsect1" title="Description"><a id="id2729168"></a><h2>Description</h2><p>Applications call the <code class="constant">VIDIOC_QBUF</code> ioctl to enqueue an empty (capturing) or filled (output) buffer in the driver's incoming queue. The semantics depend on the selected I/O method.</p><p>To enqueue a <a class="link" href="ch03s02.html" title="Streaming I/O (Memory Mapping)">memory mapped</a> buffer applications set the <em class="structfield"><code>type</code></em> field of a struct <a class="link" href="ch03s05.html#v4l2-buffer" title="Table 3.1. struct v4l2_buffer">v4l2_buffer</a> to the same buffer type as previously struct <a class="link" href="re41.html#v4l2-format" title="Table A.46. struct v4l2_format">v4l2_format</a> <em class="structfield"><code>type</code></em> and struct <a class="link" href="re59.html#v4l2-requestbuffers" title="Table A.72. struct v4l2_requestbuffers">v4l2_requestbuffers</a> <em class="structfield"><code>type</code></em>, the <em class="structfield"><code>memory</code></em> field to <code class="constant">V4L2_MEMORY_MMAP</code> and the <em class="structfield"><code>index</code></em> field. Valid index numbers range from zero to the number of buffers allocated with <a class="link" href="re59.html" title="ioctl VIDIOC_REQBUFS"><code class="constant">VIDIOC_REQBUFS</code></a> (struct <a class="link" href="re59.html#v4l2-requestbuffers" title="Table A.72. struct v4l2_requestbuffers">v4l2_requestbuffers</a> <em class="structfield"><code>count</code></em>) minus one. The contents of the struct <span class="structname">v4l2_buffer</span> returned by a <a class="link" href="re55.html" title="ioctl VIDIOC_QUERYBUF"><code class="constant">VIDIOC_QUERYBUF</code></a> ioctl will do as well. When the buffer is intended for output (<em class="structfield"><code>type</code></em> is <code class="constant">V4L2_BUF_TYPE_VIDEO_OUTPUT</code> or <code class="constant">V4L2_BUF_TYPE_VBI_OUTPUT</code>) applications must also initialize the <em class="structfield"><code>bytesused</code></em>, <em class="structfield"><code>field</code></em> and <em class="structfield"><code>timestamp</code></em> fields. See <a class="xref" href="ch03s05.html" title="Buffers">the section called “Buffers”</a> for details. When <code class="constant">VIDIOC_QBUF</code> is called with a pointer to this structure the driver sets the <code class="constant">V4L2_BUF_FLAG_MAPPED</code> and <code class="constant">V4L2_BUF_FLAG_QUEUED</code> flags and clears the <code class="constant">V4L2_BUF_FLAG_DONE</code> flag in the <em class="structfield"><code>flags</code></em> field, or it returns an <span class="errorcode">EINVAL</span> error code.</p><p>To enqueue a <a class="link" href="ch03s03.html" title="Streaming I/O (User Pointers)">user pointer</a> buffer applications set the <em class="structfield"><code>type</code></em> field of a struct <a class="link" href="ch03s05.html#v4l2-buffer" title="Table 3.1. struct v4l2_buffer">v4l2_buffer</a> to the same buffer type as previously struct <a class="link" href="re41.html#v4l2-format" title="Table A.46. struct v4l2_format">v4l2_format</a> <em class="structfield"><code>type</code></em> and struct <a class="link" href="re59.html#v4l2-requestbuffers" title="Table A.72. struct v4l2_requestbuffers">v4l2_requestbuffers</a> <em class="structfield"><code>type</code></em>, the <em class="structfield"><code>memory</code></em> field to <code class="constant">V4L2_MEMORY_USERPTR</code> and the <em class="structfield"><code>m.userptr</code></em> field to the address of the buffer and <em class="structfield"><code>length</code></em> to its size. When the buffer is intended for output additional fields must be set as above. When <code class="constant">VIDIOC_QBUF</code> is called with a pointer to this structure the driver sets the <code class="constant">V4L2_BUF_FLAG_QUEUED</code> flag and clears the <code class="constant">V4L2_BUF_FLAG_MAPPED</code> and <code class="constant">V4L2_BUF_FLAG_DONE</code> flags in the <em class="structfield"><code>flags</code></em> field, or it returns an error code. This ioctl locks the memory pages of the buffer in physical memory, they cannot be swapped out to disk. Buffers remain locked until dequeued, until the <a class="link" href="re61.html" title="ioctl VIDIOC_STREAMON, VIDIOC_STREAMOFF"><code class="constant">VIDIOC_STREAMOFF</code></a> or <a class="link" href="re59.html" title="ioctl VIDIOC_REQBUFS"><code class="constant">VIDIOC_REQBUFS</code></a> ioctl are called, or until the device is closed.</p><p>Applications call the <code class="constant">VIDIOC_DQBUF</code> ioctl to dequeue a filled (capturing) or displayed (output) buffer from the driver's outgoing queue. They just set the <em class="structfield"><code>type</code></em> and <em class="structfield"><code>memory</code></em> fields of a struct <a class="link" href="ch03s05.html#v4l2-buffer" title="Table 3.1. struct v4l2_buffer">v4l2_buffer</a> as above, when <code class="constant">VIDIOC_DQBUF</code> is called with a pointer to this structure the driver fills the remaining fields or returns an error code.</p><p>By default <code class="constant">VIDIOC_DQBUF</code> blocks when no buffer is in the outgoing queue. 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, <code class="constant">VIDIOC_DQBUF</code> returns immediately with an <span class="errorcode">EAGAIN</span> error code when no buffer is available.</p><p>The <span class="structname">v4l2_buffer</span> structure is specified in <a class="xref" href="ch03s05.html" title="Buffers">the section called “Buffers”</a>.</p></div><div class="refsect1" title="Return Value"><a id="id2729507"></a><h2>Return Value</h2><p>On success <span class="returnvalue">0</span> is returned, on error <span class="returnvalue">-1</span> and the <code class="varname">errno</code> variable is set appropriately:</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 <code class="constant">O_NONBLOCK</code> and no buffer was in the outgoing queue.</p></dd><dt><span class="term"><span class="errorcode">EINVAL</span></span></dt><dd><p>The buffer <em class="structfield"><code>type</code></em> is not supported, or the <em class="structfield"><code>index</code></em> is out of bounds, or no buffers have been allocated yet, or the <em class="structfield"><code>userptr</code></em> or <em class="structfield"><code>length</code></em> are invalid.</p></dd><dt><span class="term"><span class="errorcode">ENOMEM</span></span></dt><dd><p>Not enough physical or virtual memory was available to enqueue a user pointer buffer.</p></dd><dt><span class="term"><span class="errorcode">EIO</span></span></dt><dd><p><code class="constant">VIDIOC_DQBUF</code> failed due to an internal error. Can also indicate temporary problems like signal loss. Note the driver might dequeue an (empty) buffer despite returning an error, or even stop capturing.</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="re53.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="re55.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">ioctl VIDIOC_OVERLAY </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> ioctl VIDIOC_QUERYBUF</td></tr></table></div></body></html>
