mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/chenhuacai/linux-loongson
synced 2025-10-31 12:39:23 +00:00
8e080c2e6c
The V4L and DVB API's are there for a long time. however, up to now, no efforts were done to merge them to kernel DocBook. This patch adds the current versions of the specs as an unique compendium. Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
190 lines
6.7 KiB
XML
190 lines
6.7 KiB
XML
<refentry id="func-read">
|
|
<refmeta>
|
|
<refentrytitle>V4L2 read()</refentrytitle>
|
|
&manvol;
|
|
</refmeta>
|
|
|
|
<refnamediv>
|
|
<refname>v4l2-read</refname>
|
|
<refpurpose>Read from a V4L2 device</refpurpose>
|
|
</refnamediv>
|
|
|
|
<refsynopsisdiv>
|
|
<funcsynopsis>
|
|
<funcsynopsisinfo>#include <unistd.h></funcsynopsisinfo>
|
|
<funcprototype>
|
|
<funcdef>ssize_t <function>read</function></funcdef>
|
|
<paramdef>int <parameter>fd</parameter></paramdef>
|
|
<paramdef>void *<parameter>buf</parameter></paramdef>
|
|
<paramdef>size_t <parameter>count</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>buf</parameter></term>
|
|
<listitem>
|
|
<para></para>
|
|
</listitem>
|
|
</varlistentry>
|
|
<varlistentry>
|
|
<term><parameter>count</parameter></term>
|
|
<listitem>
|
|
<para></para>
|
|
</listitem>
|
|
</varlistentry>
|
|
</variablelist>
|
|
</refsect1>
|
|
|
|
<refsect1>
|
|
<title>Description</title>
|
|
|
|
<para><function>read()</function> attempts to read up to
|
|
<parameter>count</parameter> bytes from file descriptor
|
|
<parameter>fd</parameter> into the buffer starting at
|
|
<parameter>buf</parameter>. The layout of the data in the buffer is
|
|
discussed in the respective device interface section, see ##. If <parameter>count</parameter> is zero,
|
|
<function>read()</function> returns zero and has no other results. If
|
|
<parameter>count</parameter> is greater than
|
|
<constant>SSIZE_MAX</constant>, the result is unspecified. Regardless
|
|
of the <parameter>count</parameter> value each
|
|
<function>read()</function> call will provide at most one frame (two
|
|
fields) worth of data.</para>
|
|
|
|
<para>By default <function>read()</function> blocks until data
|
|
becomes available. When the <constant>O_NONBLOCK</constant> flag was
|
|
given to the &func-open; function it
|
|
returns immediately with an &EAGAIN; when no data is available. The
|
|
&func-select; or &func-poll; functions
|
|
can always be used to suspend execution until data becomes available. All
|
|
drivers supporting the <function>read()</function> function must also
|
|
support <function>select()</function> and
|
|
<function>poll()</function>.</para>
|
|
|
|
<para>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.</para>
|
|
|
|
<para><function>read()</function> 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.</para>
|
|
|
|
<para>The behavior of
|
|
<function>read()</function> 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 <function>read()</function> call as
|
|
soon as it is complete.</para>
|
|
|
|
<para>A driver discarding the newest frames stops capturing until
|
|
the next <function>read()</function> call. The frame being received at
|
|
<function>read()</function> 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 <function>read()</function> is called and returning when
|
|
the DMA finished.</para>
|
|
|
|
<para>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.</para>
|
|
|
|
<para>Applications can get and set the number of buffers used
|
|
internally by the driver with the &VIDIOC-G-PARM; and &VIDIOC-S-PARM;
|
|
ioctls. They are optional, however. The discarding policy is not
|
|
reported and cannot be changed. For minimum requirements see <xref
|
|
linkend="devices" />.</para>
|
|
</refsect1>
|
|
|
|
<refsect1>
|
|
<title>Return Value</title>
|
|
|
|
<para>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 <function>read()</function> was interrupted by a
|
|
signal. On error, -1 is returned, and the <varname>errno</varname>
|
|
variable is set appropriately. In this case the next read will start
|
|
at the beginning of a new frame. Possible error codes are:</para>
|
|
|
|
<variablelist>
|
|
<varlistentry>
|
|
<term><errorcode>EAGAIN</errorcode></term>
|
|
<listitem>
|
|
<para>Non-blocking I/O has been selected using
|
|
O_NONBLOCK and no data was immediately available for reading.</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
<varlistentry>
|
|
<term><errorcode>EBADF</errorcode></term>
|
|
<listitem>
|
|
<para><parameter>fd</parameter> is not a valid file
|
|
descriptor or is not open for reading, or the process already has the
|
|
maximum number of files open.</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
<varlistentry>
|
|
<term><errorcode>EBUSY</errorcode></term>
|
|
<listitem>
|
|
<para>The driver does not support multiple read streams and the
|
|
device is already in use.</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
<varlistentry>
|
|
<term><errorcode>EFAULT</errorcode></term>
|
|
<listitem>
|
|
<para><parameter>buf</parameter> references an inaccessible
|
|
memory area.</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
<varlistentry>
|
|
<term><errorcode>EINTR</errorcode></term>
|
|
<listitem>
|
|
<para>The call was interrupted by a signal before any
|
|
data was read.</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
<varlistentry>
|
|
<term><errorcode>EIO</errorcode></term>
|
|
<listitem>
|
|
<para>I/O error. This indicates some hardware problem or a
|
|
failure to communicate with a remote device (USB camera etc.).</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
<varlistentry>
|
|
<term><errorcode>EINVAL</errorcode></term>
|
|
<listitem>
|
|
<para>The <function>read()</function> function is not
|
|
supported by this driver, not on this device, or generally not on this
|
|
type of device.</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
</variablelist>
|
|
</refsect1>
|
|
</refentry>
|
|
|
|
<!--
|
|
Local Variables:
|
|
mode: sgml
|
|
sgml-parent-document: "v4l2.sgml"
|
|
indent-tabs-mode: nil
|
|
End:
|
|
-->
|