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  <div class="section" id="wsgirestrictsignal">
<h1>WSGIRestrictSignal<a class="headerlink" href="#wsgirestrictsignal" title="Permalink to this headline">¶</a></h1>
<table class="docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field-odd field"><th class="field-name">Description:</th><td class="field-body">Enable restrictions on use of signal().</td>
</tr>
<tr class="field-even field"><th class="field-name">Syntax:</th><td class="field-body"><tt class="docutils literal"><span class="pre">WSGIRestrictSignal</span> <span class="pre">On|Off</span></tt></td>
</tr>
<tr class="field-odd field"><th class="field-name">Default:</th><td class="field-body"><tt class="docutils literal"><span class="pre">WSGIRestrictSignal</span> <span class="pre">On</span></tt></td>
</tr>
<tr class="field-even field"><th class="field-name">Context:</th><td class="field-body">server config</td>
</tr>
</tbody>
</table>
<p>A well behaved Python WSGI application should not in general register any
signal handlers of its own using <tt class="docutils literal"><span class="pre">signal.signal()</span></tt>. The reason for this
is that the web server which is hosting a WSGI application will more than
likely register signal handlers of its own. If a WSGI application were to
override such signal handlers it could interfere with the operation of the
web server, preventing actions such as server shutdown and restart.</p>
<p>In the interests of promoting portability of WSGI applications, mod_wsgi
restricts use of <tt class="docutils literal"><span class="pre">signal.signal()</span></tt> and will ensure that any attempts
to register signal handlers are ignored. A warning notice will be output
to the Apache error log indicating that this action has been taken.</p>
<p>If for some reason there is a need for a WSGI application to register some
special signal handler this behaviour can be turned off, however an
application should avoid the signals <tt class="docutils literal"><span class="pre">SIGTERM</span></tt>, <tt class="docutils literal"><span class="pre">SIGINT</span></tt>,
<tt class="docutils literal"><span class="pre">SIGHUP</span></tt>, <tt class="docutils literal"><span class="pre">SIGWINCH</span></tt> and <tt class="docutils literal"><span class="pre">SIGUSR1</span></tt> as these are all used by
Apache.</p>
<p>Apache will ensure that the signal <tt class="docutils literal"><span class="pre">SIGPIPE</span></tt> is set to <tt class="docutils literal"><span class="pre">SIG_IGN</span></tt>.
If a WSGI application needs to override this, it must ensure that it is
reset to <tt class="docutils literal"><span class="pre">SIG_IGN</span></tt> before any Apache code is run. In a multi threaded
MPM this would be practically impossible to ensure so it is preferable that
the handler for <tt class="docutils literal"><span class="pre">SIG_PIPE</span></tt> also not be changed.</p>
<p>Apache does not use <tt class="docutils literal"><span class="pre">SIGALRM</span></tt>, but it is generally preferable that
other techniques be used to achieve the same affect.</p>
<p>Do note that if enabling the ability to register signal handlers, such a
registration can only reliably be done from within code which is
implemented as a side effect of importing a script file identified by the
WSGIImportScript directive. This is because signal handlers can only be
registered from the main Python interpreter thread, and request handlers
when using embedded mode and a multithreaded Apache MPM would generally
execute from secondary threads. Similarly, when using daemon mode, request
handlers would executed from secondary threads. Only code run as a side
effect of WSGIImportScript is guaranteed to be executed in main Python
interpreter thread.</p>
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