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<h1>flock</h1>
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<a href="../index-functions.html">Perl functions A-Z</a> |
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<ul>
<li><a name="flock-FILEHANDLE%2cOPERATION"></a><b>flock FILEHANDLE,OPERATION</b>
<p> </p>
<p>Calls flock(2), or an emulation of it, on FILEHANDLE. Returns true
for success, false on failure. Produces a fatal error if used on a
machine that doesn't implement flock(2), fcntl(2) locking, or lockf(3).
<code class="inline"><a class="l_k" href="../functions/flock.html">flock</a></code> is Perl's portable file locking interface, although it locks
only entire files, not records.</p>
<p>Two potentially non-obvious but traditional <code class="inline"><a class="l_k" href="../functions/flock.html">flock</a></code> semantics are
that it waits indefinitely until the lock is granted, and that its locks
<b>merely advisory</b>. Such discretionary locks are more flexible, but offer
fewer guarantees. This means that programs that do not also use <code class="inline"><a class="l_k" href="../functions/flock.html">flock</a></code>
may modify files locked with <code class="inline"><a class="l_k" href="../functions/flock.html">flock</a></code>. See <a href="../perlport.html">perlport</a>,
your port's specific documentation, or your system-specific local manpages
for details. It's best to assume traditional behavior if you're writing
portable programs. (But if you're not, you should as always feel perfectly
free to write for your own system's idiosyncrasies (sometimes called
"features"). Slavish adherence to portability concerns shouldn't get
in the way of your getting your job done.)</p>
<p>OPERATION is one of LOCK_SH, LOCK_EX, or LOCK_UN, possibly combined with
LOCK_NB. These constants are traditionally valued 1, 2, 8 and 4, but
you can use the symbolic names if you import them from the Fcntl module,
either individually, or as a group using the ':flock' tag. LOCK_SH
requests a shared lock, LOCK_EX requests an exclusive lock, and LOCK_UN
releases a previously requested lock. If LOCK_NB is bitwise-or'ed with
LOCK_SH or LOCK_EX then <code class="inline"><a class="l_k" href="../functions/flock.html">flock</a></code> will return immediately rather than blocking
waiting for the lock (check the return status to see if you got it).</p>
<p>To avoid the possibility of miscoordination, Perl now flushes FILEHANDLE
before locking or unlocking it.</p>
<p>Note that the emulation built with lockf(3) doesn't provide shared
locks, and it requires that FILEHANDLE be open with write intent. These
are the semantics that lockf(3) implements. Most if not all systems
implement lockf(3) in terms of fcntl(2) locking, though, so the
differing semantics shouldn't bite too many people.</p>
<p>Note that the fcntl(2) emulation of flock(3) requires that FILEHANDLE
be open with read intent to use LOCK_SH and requires that it be open
with write intent to use LOCK_EX.</p>
<p>Note also that some versions of <code class="inline"><a class="l_k" href="../functions/flock.html">flock</a></code> cannot lock things over the
network; you would need to use the more system-specific <code class="inline"><a class="l_k" href="../functions/fcntl.html">fcntl</a></code> for
that. If you like you can force Perl to ignore your system's flock(2)
function, and so provide its own fcntl(2)-based emulation, by passing
the switch <code class="inline">-<span class="w">Ud_flock</span></code>
to the <i>Configure</i> program when you configure
perl.</p>
<p>Here's a mailbox appender for BSD systems.</p>
<pre class="verbatim"><ol><li> <a class="l_k" href="../functions/use.html">use</a> <span class="w">Fcntl</span> <span class="q">qw(:flock SEEK_END)</span><span class="sc">;</span> <span class="c"># import LOCK_* and SEEK_END constants</span></li><li></li><li><a name="lock"></a> sub <span class="m">lock</span> <span class="s">{</span></li><li> <a class="l_k" href="../functions/my.html">my</a> <span class="s">(</span><span class="i">$fh</span><span class="s">)</span> = <span class="i">@_</span><span class="sc">;</span></li><li> <a class="l_k" href="../functions/flock.html">flock</a><span class="s">(</span><span class="i">$fh</span><span class="cm">,</span> <span class="w">LOCK_EX</span><span class="s">)</span> or <a class="l_k" href="../functions/die.html">die</a> <span class="q">"Cannot lock mailbox - $!\n"</span><span class="sc">;</span></li><li></li><li> <span class="c"># and, in case someone appended while we were waiting...</span></li><li> <a class="l_k" href="../functions/seek.html">seek</a><span class="s">(</span><span class="i">$fh</span><span class="cm">,</span> <span class="n">0</span><span class="cm">,</span> <span class="w">SEEK_END</span><span class="s">)</span> or <a class="l_k" href="../functions/die.html">die</a> <span class="q">"Cannot seek - $!\n"</span><span class="sc">;</span></li><li> <span class="s">}</span></li><li></li><li><a name="unlock"></a> sub <span class="m">unlock</span> <span class="s">{</span></li><li> <a class="l_k" href="../functions/my.html">my</a> <span class="s">(</span><span class="i">$fh</span><span class="s">)</span> = <span class="i">@_</span><span class="sc">;</span></li><li> <a class="l_k" href="../functions/flock.html">flock</a><span class="s">(</span><span class="i">$fh</span><span class="cm">,</span> <span class="w">LOCK_UN</span><span class="s">)</span> or <a class="l_k" href="../functions/die.html">die</a> <span class="q">"Cannot unlock mailbox - $!\n"</span><span class="sc">;</span></li><li> <span class="s">}</span></li><li></li><li> <a class="l_k" href="../functions/open.html">open</a><span class="s">(</span><a class="l_k" href="../functions/my.html">my</a> <span class="i">$mbox</span><span class="cm">,</span> <span class="q">">>"</span><span class="cm">,</span> <span class="q">"/usr/spool/mail/$ENV{'USER'}"</span><span class="s">)</span></li><li> or <a class="l_k" href="../functions/die.html">die</a> <span class="q">"Can't open mailbox: $!"</span><span class="sc">;</span></li><li></li><li> <a class="l_k" href="../functions/lock.html">lock</a><span class="s">(</span><span class="i">$mbox</span><span class="s">)</span><span class="sc">;</span></li><li> <a class="l_k" href="../functions/print.html">print</a> <span class="i">$mbox</span> <span class="i">$msg</span><span class="cm">,</span><span class="q">"\n\n"</span><span class="sc">;</span></li><li> <span class="i">unlock</span><span class="s">(</span><span class="i">$mbox</span><span class="s">)</span><span class="sc">;</span></li></ol></pre><p>On systems that support a real flock(), locks are inherited across fork()
calls, whereas those that must resort to the more capricious fcntl()
function lose the locks, making it harder to write servers.</p>
<p>See also <a href="../DB_File.html">DB_File</a> for other flock() examples.</p>
</li>
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