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<?xml version="1.0" encoding="iso-8859-1"?>
<!-- $Revision: 1.11 $ -->
<!-- splitted from ./en/functions/filesystem.xml, last change in rev 1.2 -->
<refentry id="function.flock">
<refnamediv>
<refname>flock</refname>
<refpurpose>Portable advisory file locking</refpurpose>
</refnamediv>
<refsect1>
<title>Description</title>
<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>
<simpara>
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).
</simpara>
<note>
<para>
<function>flock</function> is mandatory under Windows.
</para>
</note>
<simpara>
<function>flock</function> operates on <parameter>handle</parameter>
which must be an open file
pointer. <parameter>operation</parameter> is one of the following
values:
</simpara>
<para>
<itemizedlist>
<listitem>
<simpara>
To acquire a shared lock (reader), set
<parameter>operation</parameter> to <constant>LOCK_SH</constant> (set to 1 prior to
PHP 4.0.1).
</simpara>
</listitem>
<listitem>
<simpara>
To acquire an exclusive lock (writer), set
<parameter>operation</parameter> to <constant>LOCK_EX</constant> (set to 2 prior to
PHP 4.0.1).
</simpara>
</listitem>
<listitem>
<simpara>
To release a lock (shared or exclusive), set
<parameter>operation</parameter> to <constant>LOCK_UN</constant> (set to 3 prior to
PHP 4.0.1).
</simpara>
</listitem>
<listitem>
<simpara>
If you don't want <function>flock</function> to block while
locking, add <constant>LOCK_NB</constant> (4 prior to PHP 4.0.1) to
<parameter>operation</parameter>.
</simpara>
</listitem>
</itemizedlist>
</para>
<simpara>
<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). The optional third
argument is set to &true; if the lock would block (EWOULDBLOCK
errno condition). The lock is released also by <function>fclose</function>
(which is also called automatically when script finished).
</simpara>
<simpara>
&return.success;
</simpara>
<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>
<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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