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<h1>fork</h1>
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Perl 5 version 10.1 documentation
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<h1>fork</h1>
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<a href="../index-functions.html">Perl functions A-Z</a> |
<a href="../index-functions-by-cat.html">Perl functions by category</a> |
<a href="../perlfunc.html">The 'perlfunc' manpage</a>
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<ul>
<li><a name="fork"></a><b>fork</b>
<p> </p>
<p>Does a fork(2) system call to create a new process running the
same program at the same point. It returns the child pid to the
parent process, <code class="inline"><span class="n">0</span></code>
to the child process, or <code class="inline"><a class="l_k" href="../functions/undef.html">undef</a></code> if the fork is
unsuccessful. File descriptors (and sometimes locks on those descriptors)
are shared, while everything else is copied. On most systems supporting
fork(), great care has gone into making it extremely efficient (for
example, using copy-on-write technology on data pages), making it the
dominant paradigm for multitasking over the last few decades.</p>
<p>Beginning with v5.6.0, Perl will attempt to flush all files opened for
output before forking the child process, but this may not be supported
on some platforms (see <a href="../perlport.html">perlport</a>). To be safe, you may need to set
<code class="inline"><span class="i">$|</span></code>
($AUTOFLUSH in English) or call the <code class="inline"><span class="i">autoflush</span><span class="s">(</span><span class="s">)</span></code>
method of
<code class="inline"><span class="w">IO::Handle</span></code>
on any open handles in order to avoid duplicate output.</p>
<p>If you <code class="inline"><a class="l_k" href="../functions/fork.html">fork</a></code> without ever waiting on your children, you will
accumulate zombies. On some systems, you can avoid this by setting
<code class="inline"><span class="i">$SIG</span>{<span class="w">CHLD</span>}</code>
to <code class="inline"><span class="q">"IGNORE"</span></code>
. See also <a href="../perlipc.html">perlipc</a> for more examples of
forking and reaping moribund children.</p>
<p>Note that if your forked child inherits system file descriptors like
STDIN and STDOUT that are actually connected by a pipe or socket, even
if you exit, then the remote server (such as, say, a CGI script or a
backgrounded job launched from a remote shell) won't think you're done.
You should reopen those to <i>/dev/null</i> if it's any issue.</p>
</li>
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