socket-select.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<refentry xmlns="http://docbook.org/ns/docbook" xml: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 role="description">
  &reftitle.description;
  <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><initializer>0</initializer></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>
 </refsect1>

 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>read</parameter></term>
     <listitem>
      <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>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>write</parameter></term>
     <listitem>
      <para>
       The sockets listed in the <parameter>write</parameter> array will be
       watched to see if a write will not block.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>except</parameter></term>
     <listitem>
      <para>
       The sockets listed in the <parameter>except</parameter> array will be
       watched for exceptions.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>tv_sec</parameter></term>
     <listitem>
      <para>
       The <parameter>tv_sec</parameter> and <parameter>tv_usec</parameter>
       together form the <literal>timeout</literal> parameter. The
       <literal>timeout</literal> 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>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>tv_usec</parameter></term>
     <listitem>
      <para>
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </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>
  <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
$e = NULL;
socket_select($r, $w, $e, 0);
?>
]]>
     </programlisting>
    </example>
   </para>
  </note>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <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
$e = NULL;
if (false === socket_select($r, $w, $e, 0)) {
    echo "socket_select() failed, reason: " .
        socket_strerror(socket_last_error()) . "\n";
}
?>
]]>
     </programlisting>
    </example>
   </para>
  </note>
 </refsect1>

 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example>
    <title><function>socket_select</function> example</title>
    <programlisting role="php">
<![CDATA[
<?php
/* Prepare the read array */
$read   = array($socket1, $socket2);
$write  = NULL;
$except = NULL;
$num_changed_sockets = socket_select($read, $write, $except, 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>
 </refsect1>

 <refsect1 role="notes">
  &reftitle.notes;
  <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>
 </refsect1>

 <refsect1 role="seealso">
  &reftitle.seealso;
  <para>
   <simplelist>
    <member><function>socket_read</function></member>
    <member><function>socket_write</function></member>
    <member><function>socket_last_error</function></member>
    <member><function>socket_strerror</function></member>
   </simplelist>
  </para>
 </refsect1>

</refentry>

<!-- Keep this comment at the end of the file
Local variables:
mode: sgml
sgml-omittag:t
sgml-shorttag:t
sgml-minimize-attributes:nil
sgml-always-quote-attributes:t
sgml-indent-step:1
sgml-indent-data:t
indent-tabs-mode:nil
sgml-parent-document:nil
sgml-default-dtd-file:"~/.phpdoc/manual.ced"
sgml-exposed-tags:nil
sgml-local-catalogs:nil
sgml-local-ecat-files:nil
End:
vim600: syn=xml fen fdm=syntax fdl=2 si
vim: et tw=78 syn=sgml
vi: ts=1 sw=1
-->