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<title>s6: the s6-fdholder-retrieve program</title>
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<p>
<a href="index.html">s6</a><br />
<a href="//skarnet.org/software/">Software</a><br />
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<h1> The <tt>s6-fdholder-retrieve</tt> program </h1>
<p>
<tt>s6-fdholder-retrieve</tt> connects to a
<a href="s6-fdholderd.html">fd-holding daemon</a> listening on a
Unix domain socket, and retrieves a file descriptor from that
daemon, then executes a program with that file descriptor as the
program's standard input.
</p>
<h2> Interface </h2>
<pre>
s6-fdholder-retrieve [ -D ] [ -t <em>timeout</em> ] <em>path</em> <em>id</em> <em>prog...</em>
</pre>
<ul>
<li> <tt>s6-fdholder-retrieve</tt> connects to a
<a href="s6-fdholderd.html">s6-fdholderd</a> server process listening on
<em>path</em>. </li>
<li> It attempts to retrieve a copy of the file descriptor that has been
stored into that daemon under identifier <em>id</em>.
<li> It then executes into <em>prog...</em>, with the retrieved
file descriptor as standard input. </li>
</ul>
<h2> Options </h2>
<ul>
<li> <tt>-D</tt> : delete the file descriptor from the server's
storage after retrieval. This option requires writing rights over the
given identifier as well as reading rights: check the server's
<a href="s6-fdholderd.html#configuration">configuration</a>. </li>
<li> <tt>-t <em>timeout</em></tt> : if the operation cannot be
processed in <em>timeout</em> milliseconds, then fail with an error message.
Communications with the server should be near-instant, so this option is
only here to protect users against programming errors (connecting to the
wrong socket, for instance). </li>
</ul>
<h2> Exit codes </h2>
<ul>
<li> On success, the program doesn't exit; instead, it execs into <em>prog...</em>. </li>
<li> 1: the server denied the operation. The meaning of the error messages
is explained <a href="s6-fdholder-errorcodes.html">here</a>. </li>
<li> 100: wrong usage. </li>
<li> 111: system call failed - that includes attempting to connect to a
nonexistent socket, or one where no <a href="s6-fdholderd.html">s6-fdholderd</a>
daemon is listening. </li>
</ul>
<h2> Usage example </h2>
<pre>
s6-fdholder-retrieve /service/fdholderd/s MYSOCKET s6-ipcserverd cat
</pre>
<p>
will retrieve a file descriptor stored under the MYSOCKET identifier in
the s6-fdholderd daemon listening on the <tt>/service/fdholderd/s</tt>
socket, and execute into <tt>s6-ipcserverd cat</tt> with that file
descriptor as stdin. In this case, if MYSOCKET referred to a Unix domain
socket, <a href="s6-ipcserverd.html">s6-ipcserverd</a> will then accept
client connections on it and spawn a <tt>cat</tt> program for every
connection.
</p>
<h2> Notes </h2>
<ul>
<li> To execute <em>prog</em> with the newly retrieved file descriptor
as number <em>n</em> while preserving stdin, use the following construct:
<tt>fdmove <em>n</em> 0 s6-fdholder-retrieve <em>path</em> <em>id</em>
<a href="//skarnet.org/software/execline/fdswap.html">fdswap</a>
0 <em>n</em> <em>prog...</em></tt>. </li>
</ul>
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