169 lines
6.1 KiB
XML
169 lines
6.1 KiB
XML
<refentry id="vidioc-qbuf">
|
|
<refmeta>
|
|
<refentrytitle>ioctl VIDIOC_QBUF, VIDIOC_DQBUF</refentrytitle>
|
|
&manvol;
|
|
</refmeta>
|
|
|
|
<refnamediv>
|
|
<refname>VIDIOC_QBUF</refname>
|
|
<refname>VIDIOC_DQBUF</refname>
|
|
<refpurpose>Exchange a buffer with the driver</refpurpose>
|
|
</refnamediv>
|
|
|
|
<refsynopsisdiv>
|
|
<funcsynopsis>
|
|
<funcprototype>
|
|
<funcdef>int <function>ioctl</function></funcdef>
|
|
<paramdef>int <parameter>fd</parameter></paramdef>
|
|
<paramdef>int <parameter>request</parameter></paramdef>
|
|
<paramdef>struct v4l2_buffer *<parameter>argp</parameter></paramdef>
|
|
</funcprototype>
|
|
</funcsynopsis>
|
|
</refsynopsisdiv>
|
|
|
|
<refsect1>
|
|
<title>Arguments</title>
|
|
|
|
<variablelist>
|
|
<varlistentry>
|
|
<term><parameter>fd</parameter></term>
|
|
<listitem>
|
|
<para>&fd;</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
<varlistentry>
|
|
<term><parameter>request</parameter></term>
|
|
<listitem>
|
|
<para>VIDIOC_QBUF, VIDIOC_DQBUF</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
<varlistentry>
|
|
<term><parameter>argp</parameter></term>
|
|
<listitem>
|
|
<para></para>
|
|
</listitem>
|
|
</varlistentry>
|
|
</variablelist>
|
|
</refsect1>
|
|
|
|
<refsect1>
|
|
<title>Description</title>
|
|
|
|
<para>Applications call the <constant>VIDIOC_QBUF</constant> 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.</para>
|
|
|
|
<para>To enqueue a <link linkend="mmap">memory mapped</link>
|
|
buffer applications set the <structfield>type</structfield> field of a
|
|
&v4l2-buffer; to the same buffer type as previously &v4l2-format;
|
|
<structfield>type</structfield> and &v4l2-requestbuffers;
|
|
<structfield>type</structfield>, the <structfield>memory</structfield>
|
|
field to <constant>V4L2_MEMORY_MMAP</constant> and the
|
|
<structfield>index</structfield> field. Valid index numbers range from
|
|
zero to the number of buffers allocated with &VIDIOC-REQBUFS;
|
|
(&v4l2-requestbuffers; <structfield>count</structfield>) minus one. The
|
|
contents of the struct <structname>v4l2_buffer</structname> returned
|
|
by a &VIDIOC-QUERYBUF; ioctl will do as well. When the buffer is
|
|
intended for output (<structfield>type</structfield> is
|
|
<constant>V4L2_BUF_TYPE_VIDEO_OUTPUT</constant> or
|
|
<constant>V4L2_BUF_TYPE_VBI_OUTPUT</constant>) applications must also
|
|
initialize the <structfield>bytesused</structfield>,
|
|
<structfield>field</structfield> and
|
|
<structfield>timestamp</structfield> fields. See <xref
|
|
linkend="buffer" /> for details. When
|
|
<constant>VIDIOC_QBUF</constant> is called with a pointer to this
|
|
structure the driver sets the
|
|
<constant>V4L2_BUF_FLAG_MAPPED</constant> and
|
|
<constant>V4L2_BUF_FLAG_QUEUED</constant> flags and clears the
|
|
<constant>V4L2_BUF_FLAG_DONE</constant> flag in the
|
|
<structfield>flags</structfield> field, or it returns an
|
|
&EINVAL;.</para>
|
|
|
|
<para>To enqueue a <link linkend="userp">user pointer</link>
|
|
buffer applications set the <structfield>type</structfield> field of a
|
|
&v4l2-buffer; to the same buffer type as previously &v4l2-format;
|
|
<structfield>type</structfield> and &v4l2-requestbuffers;
|
|
<structfield>type</structfield>, the <structfield>memory</structfield>
|
|
field to <constant>V4L2_MEMORY_USERPTR</constant> and the
|
|
<structfield>m.userptr</structfield> field to the address of the
|
|
buffer and <structfield>length</structfield> to its size. When the
|
|
buffer is intended for output additional fields must be set as above.
|
|
When <constant>VIDIOC_QBUF</constant> is called with a pointer to this
|
|
structure the driver sets the <constant>V4L2_BUF_FLAG_QUEUED</constant>
|
|
flag and clears the <constant>V4L2_BUF_FLAG_MAPPED</constant> and
|
|
<constant>V4L2_BUF_FLAG_DONE</constant> flags in the
|
|
<structfield>flags</structfield> 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 &VIDIOC-STREAMOFF; or &VIDIOC-REQBUFS; ioctl are
|
|
called, or until the device is closed.</para>
|
|
|
|
<para>Applications call the <constant>VIDIOC_DQBUF</constant>
|
|
ioctl to dequeue a filled (capturing) or displayed (output) buffer
|
|
from the driver's outgoing queue. They just set the
|
|
<structfield>type</structfield> and <structfield>memory</structfield>
|
|
fields of a &v4l2-buffer; as above, when <constant>VIDIOC_DQBUF</constant>
|
|
is called with a pointer to this structure the driver fills the
|
|
remaining fields or returns an error code.</para>
|
|
|
|
<para>By default <constant>VIDIOC_DQBUF</constant> blocks when no
|
|
buffer is in the outgoing queue. When the
|
|
<constant>O_NONBLOCK</constant> flag was given to the &func-open;
|
|
function, <constant>VIDIOC_DQBUF</constant> returns immediately
|
|
with an &EAGAIN; when no buffer is available.</para>
|
|
|
|
<para>The <structname>v4l2_buffer</structname> structure is
|
|
specified in <xref linkend="buffer" />.</para>
|
|
</refsect1>
|
|
|
|
<refsect1>
|
|
&return-value;
|
|
|
|
<variablelist>
|
|
<varlistentry>
|
|
<term><errorcode>EAGAIN</errorcode></term>
|
|
<listitem>
|
|
<para>Non-blocking I/O has been selected using
|
|
<constant>O_NONBLOCK</constant> and no buffer was in the outgoing
|
|
queue.</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
<varlistentry>
|
|
<term><errorcode>EINVAL</errorcode></term>
|
|
<listitem>
|
|
<para>The buffer <structfield>type</structfield> is not
|
|
supported, or the <structfield>index</structfield> is out of bounds,
|
|
or no buffers have been allocated yet, or the
|
|
<structfield>userptr</structfield> or
|
|
<structfield>length</structfield> are invalid.</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
<varlistentry>
|
|
<term><errorcode>ENOMEM</errorcode></term>
|
|
<listitem>
|
|
<para>Not enough physical or virtual memory was available to
|
|
enqueue a user pointer buffer.</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
<varlistentry>
|
|
<term><errorcode>EIO</errorcode></term>
|
|
<listitem>
|
|
<para><constant>VIDIOC_DQBUF</constant> 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.</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
</variablelist>
|
|
</refsect1>
|
|
</refentry>
|
|
|
|
<!--
|
|
Local Variables:
|
|
mode: sgml
|
|
sgml-parent-document: "v4l2.sgml"
|
|
indent-tabs-mode: nil
|
|
End:
|
|
-->
|