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

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  <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>int</type><parameter>fp</parameter></methodparam>
      <methodparam><type>int</type><parameter>operation</parameter></methodparam>
      <methodparam choice="opt"><type>int</type><parameter>&amp;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>
    <simpara>
     <function>flock</function> operates on <parameter>fp</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 LOCK_SH (set to 1 prior to
         PHP 4.0.1).
       </simpara>
      </listitem>
      <listitem>
       <simpara>
         To acquire an exclusive lock (writer), set
         <parameter>operation</parameter> to LOCK_EX (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 LOCK_UN (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 LOCK_NB (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)
    </simpara>
    <simpara>
     <function>flock</function> returns &true; on success and &false; on
     error (e.g. when a lock could not be acquired).
    </simpara>
    <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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