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<a name="Implementation-warts"></a>
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Next: <a href="Programming-with-signal-handling-in-mind.html#Programming-with-signal-handling-in-mind" accesskey="n" rel="next">Programming with signal handling in mind</a>, Previous: <a href="The-deferral-mechanism.html#The-deferral-mechanism" accesskey="p" rel="prev">The deferral mechanism</a>, Up: <a href="Signal-handling.html#Signal-handling" accesskey="u" rel="up">Signal handling</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p>
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<a name="Implementation-warts-1"></a>
<h3 class="section">7.3 Implementation warts</h3>
<a name="Miscellaneous-issues"></a>
<h4 class="subsection">7.3.1 Miscellaneous issues</h4>
<p>Signal handlers automatically restore errno and fp state, but
arrange_return_to_lisp_function does not restore errno.
</p>
<a name="POSIX-_002d_002d-Letter-and-Spirit"></a>
<h4 class="subsection">7.3.2 POSIX – Letter and Spirit</h4>
<p>POSIX restricts signal handlers to a use only a narrow subset of POSIX
functions, and declares anything else to have undefined semantics.
</p>
<p>Apparently the real reason is that a signal handler is potentially
interrupting a POSIX call: so the signal safety requirement is really
a re-entrancy requirement. We can work around the letter of the
standard by arranging to handle the interrupt when the signal handler
returns (see: <code>arrange_return_to_lisp_function</code>.) This does,
however, in no way protect us from the real issue of re-entrancy: even
though we would no longer be in a signal handler, we might still be in
the middle of an interrupted POSIX call.
</p>
<p>For some signals this appears to be a non-issue: <code>SIGSEGV</code> and
other synchronous signals are raised by our code for our code, and so
we can be sure that we are not interrupting a POSIX call with any of
them.
</p>
<p>For asynchronous signals like <code>SIGALARM</code> and <code>SIGINT</code> this
is a real issue.
</p>
<p>The right thing to do in multithreaded builds would probably be to use
POSIX semaphores (which are signal safe) to inform a separate handler
thread about such asynchronous events. In single-threaded builds there
does not seem to be any other option aside from generally blocking
asynch signals and listening for them every once and a while at safe
points. Neither of these is implemented as of SBCL 1.0.4.
</p>
<p>Currently all our handlers invoke unsafe functions without hesitation.
</p>
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Next: <a href="Programming-with-signal-handling-in-mind.html#Programming-with-signal-handling-in-mind" accesskey="n" rel="next">Programming with signal handling in mind</a>, Previous: <a href="The-deferral-mechanism.html#The-deferral-mechanism" accesskey="p" rel="prev">The deferral mechanism</a>, Up: <a href="Signal-handling.html#Signal-handling" accesskey="u" rel="up">Signal handling</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p>
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