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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> s6-fdholder-retrieve executes into <tt><a href="s6-ipcclient.html">s6-ipcclient</a> <em>path</em>
<a href="s6-fdholder-retrievec.html">s6-fdholder-retrievec</a> <em>id</em> <em>prog...</em></tt>.
It does nothing else: it is just a convenience program.
The <a href="s6-ipcclient.html">s6-ipcclient</a> program connects
to a Unix socket at <em>path</em>, and the
<a href="s6-fdholder-retrievec.html">s6-fdholder-retrievec</a> program
gets the file descriptor identified by <em>id</em> over the socket. </li>
<li> It should be used to connect to a
<a href="s6-fdholderd.html">s6-fdholderd</a> daemon, which will pass the
file descriptor to the client over the <em>path</em> socket. </li>
<li> It executes into <em>prog...</em> with the retrieved file
descriptor as <em>prog...</em>'s 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> 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> s6-fdholder-retrieve really executes into <tt>s6-ipcclient
s6-fdholder-retrievec
<a href="//skarnet.org/software/execline/fdclose.html">fdclose</a>
6 fdclose 7 <em>prog...</em></tt>, so that <em>prog...</em> does not
have a connection with the fd-holding daemon anymore. If you want to
keep the server connection open for <em>prog...</em>, use
<tt>s6-ipcclient s6-fdholder-retrievec</tt> manually. </li>
<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>. Be aware that <em>n</em>
cannot be 6 or 7, internally used by s6-fdholder-retrieve. </li>
</ul>
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