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<?xml version="1.0" encoding="iso-8859-1"?>
<!-- $Revision: 1.13 $ -->
<!-- splitted from ./en/functions/sockets.xml, last change in rev 1.27 -->
<refentry id="function.socket-select">
<refnamediv>
<refname>socket_select</refname>
<refpurpose>
Runs the select() system call on the given arrays of sockets
with a specified timeout
</refpurpose>
</refnamediv>
<refsect1>
<title>Description</title>
<methodsynopsis>
<type>int</type><methodname>socket_select</methodname>
<methodparam><type>array</type><parameter role="reference">read</parameter></methodparam>
<methodparam><type>array</type><parameter role="reference">write</parameter></methodparam>
<methodparam><type>array</type><parameter role="reference">except</parameter></methodparam>
<methodparam><type>int</type><parameter>tv_sec</parameter></methodparam>
<methodparam choice="opt"><type>int</type><parameter>tv_usec</parameter></methodparam>
</methodsynopsis>
<para>
<function>socket_select</function> accepts arrays of sockets and
waits for them to change status. Those coming with BSD sockets background
will recognize that those socket resource arrays are in fact the
so-called file descriptor sets. Three independent arrays of socket
resources are watched.
</para>
<para>
The sockets listed in the <parameter>read</parameter> array will be watched to
see if characters become available for reading (more precisely, to see if
a read will not block - in particular, a socket resource is also ready on
end-of-file, in which case a <function>socket_read</function> will return
a zero length string).
</para>
<para>
The sockets listed in the <parameter>write</parameter> array will be
watched to see if a write will not block.
</para>
<para>
The sockets listed in the <parameter>except</parameter> array will be
watched for exceptions.
</para>
<warning>
<para>
On exit, the arrays are modified to indicate which socket resource
actually changed status.
</para>
</warning>
<para>
You do not need to pass every array to
<function>socket_select</function>. You can leave it out and use an
empty array or &null; instead. Also do not forget that those arrays are
passed <emphasis>by reference</emphasis> and will be modified after
<function>socket_select</function> returns.
</para>
<para>
<example>
<title><function>socket_select</function> example</title>
<programlisting role="php">
<![CDATA[
<?php
/* Prepare the read array */
$read = array($socket1, $socket2);
$num_changed_sockets = socket_select($read, $write = NULL, $except = NULL, 0);
if ($num_changed_sockets === false) {
/* Error handling */
} else if ($num_changed_sockets > 0) {
/* At least at one of the sockets something interesting happened */
}
?>
]]>
</programlisting>
</example>
</para>
<note>
<para>
Due a limitation in the current Zend Engine it is not possible to pass a
constant modifier like &null; directly as a parameter to a function
which expects this parameter to be passed by reference. Instead use a
temporary variable or an expression with the leftmost member being a
temporary variable:
<example>
<title>Using &null; with <function>socket_select</function></title>
<programlisting role="php">
<![CDATA[
<?php
socket_select($r, $w, $e = NULL, 0);
?>
]]>
</programlisting>
</example>
</para>
</note>
<para>
The <parameter>tv_sec</parameter> and <parameter>tv_usec</parameter>
together form the <emphasis>timeout</emphasis> parameter. The
<emphasis>timeout</emphasis> is an upper bound on the amount of time
elapsed before <function>socket_select</function> return.
<parameter>tv_sec</parameter> may be zero , causing
<function>socket_select</function> to return immediately. This is useful
for polling. If <parameter>tv_sec</parameter> is &null; (no timeout),
<function>socket_select</function> can block indefinitely.
</para>
<para>
On success <function>socket_select</function> returns the number of
socket resources contained in the modified arrays, which may be zero if
the timeout expires before anything interesting happens. On error &false;
is returned. The error code can be retrieved with
<function>socket_last_error</function>.
</para>
<note>
<para>
Be sure to use the <literal>===</literal> operator when checking for an
error. Since the <function>socket_select</function> may return 0 the
comparison with <literal>==</literal> would evaluate to &true;:
<example>
<title>Understanding <function>socket_select</function>'s result</title>
<programlisting role="php">
<![CDATA[
<?php
if (false === socket_select($r, $w, $e = NULL, 0)) {
echo "socket_select() failed, reason: " .
socket_strerror(socket_last_error()) . "\n";
}
?>
]]>
</programlisting>
</example>
</para>
</note>
<note>
<para>
Be aware that some socket implementations need to be handled very
carefully. A few basic rules:
<itemizedlist>
<listitem>
<simpara>
You should always try to use <function>socket_select</function>
without timeout. Your program should have nothing to do if there is
no data available. Code that depends on timeouts is not usually
portable and difficult to debug.
</simpara>
</listitem>
<listitem>
<simpara>
No socket resource must be added to any set if you do not intend to
check its result after the <function>socket_select</function> call,
and respond appropriately. After <function>socket_select</function>
returns, all socket resources in all arrays must be checked. Any
socket resource that is available for writing must be written to, and
any socket resource available for reading must be read from.
</simpara>
</listitem>
<listitem>
<simpara>
If you read/write to a socket returns in the arrays be aware that
they do not necessarily read/write the full amount of data you have
requested. Be prepared to even only be able to read/write a single
byte.
</simpara>
</listitem>
<listitem>
<simpara>
It's common to most socket implementations that the only exception
caught with the <parameter>except</parameter> array is out-of-bound
data received on a socket.
</simpara>
</listitem>
</itemizedlist>
</para>
</note>
<para>
See also
<function>socket_read</function>,
<function>socket_write</function>,
<function>socket_last_error</function> and
<function>socket_strerror</function>.
</para>
</refsect1>
</refentry>
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