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<title>execline: the fdreserve program</title>
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<p>
<a href="index.html">execline</a><br />
<a href="//skarnet.org/software/">Software</a><br />
<a href="//skarnet.org/">skarnet.org</a>
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<h1> The <tt>fdreserve</tt> program </h1>
<p>
<tt>fdreserve</tt> updates the environment with file descriptors that
are guaranteed safe to use, then executes a program.
</p>
<h2> Interface </h2>
<pre>
fdreserve <em>n</em> <em>prog...</em>
</pre>
<ul>
<li> <tt>fdreserve</tt> tries to reserve <em>n</em> file descriptors. </li>
<li> <tt>fdreserve</tt> sets the <tt>FD0</tt>, <tt>FD1</tt>, ...,
<tt>FD<em>n-1</em></tt> environment variables: each FD<em>i</em> contains a
valid file descriptor, that can be safely opened. </li>
<li> <tt>fdreserve</tt> then execs into <em>prog</em> with its arguments.
</ul>
<h2> Common use </h2>
<p>
<tt>fdreserve</tt> can be used when you do not want to hardcode file
descriptors in your scripts. For instance, to create a pipe, you could
use:
</p>
<pre>
#!/command/execlineb
fdreserve 2
multisubstitute
{
importas fdr FD0
importas fdw FD1
}
piperw $fdr $fdw
<em>prog...</em>
</pre>
<p>
Warning: <tt>fdreserve</tt> does not allocate descriptors, it merely returns
descriptors that are free at the time it is run. A program like
</p>
<pre>
#!/command/execlineb
fdreserve 3
multisubstitute
{
importas fdr FD0
importas fdw FD1
}
piperw $fdr $fdw
fdreserve 1
multisubstitute
{
importas oldfd FD2
importas newfd FD0
}
<em>prog...</em>
</pre>
<p>
may fail, because <em>oldfd</em> and <em>newfd</em> may be the same.
To avoid that, you should make sure that all descriptors returned by
<tt>fdreserve</tt> are actually allocated before calling <tt>fdreserve</tt>
again.
(Thanks to <a href="https://code.dogmap.org/">Paul Jarc</a> for having
spotted that case.)
</p>
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