php-doc-en/reference/sockets/functions/socket-select.xml

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<refentry xml:id="function.socket-select" xmlns="http://docbook.org/ns/docbook">
 <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 class="union"><type>int</type><type>false</type></type><methodname>socket_select</methodname>
   <methodparam><type class="union"><type>array</type><type>null</type></type><parameter role="reference">read</parameter></methodparam>
   <methodparam><type class="union"><type>array</type><type>null</type></type><parameter role="reference">write</parameter></methodparam>
   <methodparam><type class="union"><type>array</type><type>null</type></type><parameter role="reference">except</parameter></methodparam>
   <methodparam><type class="union"><type>int</type><type>null</type></type><parameter>seconds</parameter></methodparam>
   <methodparam choice="opt"><type>int</type><parameter>microseconds</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 arrays are in fact the so-called file
   descriptor sets. Three independent arrays of sockets 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
       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>seconds</parameter></term>
     <listitem>
      <para>
       The <parameter>seconds</parameter> and <parameter>microseconds</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>seconds</parameter> may be zero , causing
       <function>socket_select</function> to return immediately. This is useful
       for polling. If <parameter>seconds</parameter> is &null; (no timeout),
       <function>socket_select</function> can block indefinitely.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>microseconds</parameter></term>
     <listitem>
      <para>
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
  <warning>
   <para>
    On exit, the arrays are modified to indicate which socket
    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
   sockets 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 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 sockets in all arrays must be checked. Any
       socket that is available for writing must be written to, and
       any socket 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>
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