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<?xml version="1.0" encoding="iso-8859-1"?>
<!-- $Revision: 1.17 $ -->
<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).
</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>
<listitem>
<simpara>
<constant>LOCK_NB</constant> if you don't want
<function>flock</function> to block while locking.
(not supported on Windows)
</simpara>
</listitem>
</itemizedlist>
</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).
</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
$fp = fopen("/tmp/lock.txt", "w+");
if (flock($fp, LOCK_EX)) { // do an exclusive lock
fwrite($fp, "Write something here\n");
flock($fp, LOCK_UN); // release the lock
} else {
echo "Couldn't lock the file !";
}
fclose($fp);
?>
]]>
</programlisting>
</example>
</para>
</refsect1>
<refsect1 role="notes">
&reftitle.notes;
<note>
<para>
<function>flock</function> locks mandatory under Windows.
</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>
<warning>
<para>
<function>flock</function> will not work on NFS and many other networked
file systems. Check your operating system documentation for more
details.
</para>
<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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