php-doc-en/reference/filesystem/functions/flock.xml

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<refentry xmlns="http://docbook.org/ns/docbook" xml:id="function.flock">
 <refnamediv>
  <refname>flock</refname>
  <refpurpose>Portable advisory file locking</refpurpose>
 </refnamediv>
 
 <refsect1 role="description">
  &reftitle.description;
  <methodsynopsis>
   <type>bool</type><methodname>flock</methodname>
   <methodparam><type>resource</type><parameter>handle</parameter></methodparam>
   <methodparam><type>int</type><parameter>operation</parameter></methodparam>
   <methodparam choice="opt"><type>int</type><parameter role="reference">wouldblock</parameter></methodparam>
  </methodsynopsis>
  <para>
   <function>flock</function> allows you to perform a simple reader/writer
   model which can be used on virtually every platform (including most Unix
   derivatives and even Windows).
  </para>
  <para>
   The lock is released also by <function>fclose</function> (which is also
   called automatically when script finished).
  </para>
  <para>
   PHP supports a portable way of locking complete files in an advisory way
   (which means all accessing programs have to use the same way of locking
   or it will not work). By default, this function will block until the
   requested lock is acquired; this may be controlled (on non-Windows
   platforms) with the <constant>LOCK_NB</constant> option documented below.
  </para>
 </refsect1>

 <refsect1 role="parameters">
  &reftitle.parameters;
  <para>
   <variablelist>
    <varlistentry>
     <term><parameter>handle</parameter></term>
     <listitem>
      <para>
       An open file pointer.
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>operation</parameter></term>
     <listitem>
      <para>
       <parameter>operation</parameter> is one of the following:
       <itemizedlist>
        <listitem>
         <simpara>
          <constant>LOCK_SH</constant> to acquire a shared lock (reader).
         </simpara>
        </listitem>
        <listitem>
         <simpara>
          <constant>LOCK_EX</constant> to acquire an exclusive lock (writer).
         </simpara>
        </listitem>
        <listitem>
         <simpara>
          <constant>LOCK_UN</constant> to release a lock (shared or exclusive).
         </simpara>
        </listitem>
       </itemizedlist>
      </para>
      <para>
       It is also possible to add <constant>LOCK_NB</constant> as a bitmask to one 
       of the above operations if you don't want <function>flock</function> to 
       block while locking. (not supported on Windows)
      </para>
     </listitem>
    </varlistentry>
    <varlistentry>
     <term><parameter>wouldblock</parameter></term>
     <listitem>
      <para>
       The optional third argument is set to &true; if the lock would block
       (EWOULDBLOCK errno condition). (not supported on Windows)
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
  </para>
 </refsect1>

 <refsect1 role="returnvalues">
  &reftitle.returnvalues;
  <para>
   &return.success;
  </para>
 </refsect1>

 <refsect1 role="changelog">
  &reftitle.changelog;
  <para>
   <informaltable>
    <tgroup cols="2">
     <thead>
      <row>
       <entry>&Version;</entry>
       <entry>&Description;</entry>
      </row>
     </thead>
     <tbody>
      <row>
       <entry>4.0.1</entry>
       <entry>
        The <literal>LOCK_XXX</literal> constants were added. Prior to that
        you must use 1 for <constant>LOCK_SH</constant>, 2 for
        <constant>LOCK_EX</constant>, 3 for <constant>LOCK_UN</constant> and
        4 for <constant>LOCK_NB</constant>
       </entry>
      </row>
     </tbody>
    </tgroup>
   </informaltable>
  </para>
 </refsect1>

 <refsect1 role="examples">
  &reftitle.examples;
  <para>
   <example>
    <title><function>flock</function> example</title>
    <programlisting role="php">
<![CDATA[
<?php

$fp = fopen("/tmp/lock.txt", "r+");

if (flock($fp, LOCK_EX)) { // do an exclusive lock
    ftruncate($fp, 0); // truncate file
    fwrite($fp, "Write something here\n");
    flock($fp, LOCK_UN); // release the lock
} else {
    echo "Couldn't get the lock!";
}

fclose($fp);

?>
]]>
    </programlisting>
   </example>
  </para>
  <para>
   <example>
    <title><function>flock</function> using the <constant>LOCK_NB</constant> option</title>
    <programlisting role="php">
<![CDATA[
<?php
$fp = fopen('/tmp/lock.txt', 'r+');

/* Activate the LOCK_NB option on an LOCK_EX operation */
if(!flock($fp, LOCK_EX | LOCK_NB)) {
    echo 'Unable to obtain lock';
    exit(-1);
}

/* ... */

fclose($fp);
?>
]]>
    </programlisting>
   </example>
  </para>
 </refsect1>

 <refsect1 role="notes">
  &reftitle.notes;
  <note>
   <para>
    <function>flock</function> locks mandatory under Windows aswell as some 
    POSIX based operating systems.
   </para>
  </note>
  <note>
   <para>
    Because <function>flock</function> requires a file pointer, you may have
    to use a special lock file to protect access to a file that you intend
    to truncate by opening it in write mode (with a "w" or "w+" argument to
    <function>fopen</function>).
   </para>
  </note>
  <note>
   <para>
    POSIX.1 compatible systems will release the scripts <function>flock</function>'s 
    on a file, if the scripts uses <function>fclose</function> on any file handle 
    on that file.
   </para>
  </note>
  <warning>
   <para>
    Assigning other value to <parameter>handle</parameter> argument in the
    consecutive code would release the lock.
   </para>
  </warning>
  <warning>
   <para>
    On some operating systems <function>flock</function> is implemented at
    the process level. When using a multithreaded server API like ISAPI you
    may not be able to rely on <function>flock</function> to protect files
    against other PHP scripts running in parallel threads of the same server
    instance!
   </para>
   <para>
    <function>flock</function> is not supported on antiquated filesystems like
    <literal>FAT</literal> and its derivates and will therefore always
    return &false; under this environments (this is especially true for
    Windows 98 users).
   </para>
  </warning>
 </refsect1>

</refentry>

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