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<title>skalibs: the selfpipe library interface</title>
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<h1> The <tt>selfpipe</tt> library interface </h1>
<p>
The selfpipe functions are declared in the
<tt>skalibs/selfpipe.h</tt> header and implemented in the <tt>libskarnet.a</tt>
or <tt>libskarnet.so</tt> library.
</p>
<h2> What does it do ? </h2>
<p>
Signal handlers suck.
</p>
<p>
They do. I don't care how experienced you are with C/Unix programming,
they do. You can be Ken Thompson, if you use signal handlers as a
regular part of your C programming model, you <em>are</em> going to
screw up, and write buggy code.
</p>
<p>
Unix is tricky enough with interruptions. Even when you have a single
thread, signals can make the execution flow very non-intuitive.
They mess up the logic of linear and structured code,
they introduce non-determinism; you always have to think "and what
if I get interrupted here and the flow goes into a handler...". This
is annoying.
</p>
<p>
Moreover, signal handler code is <em>very</em> limited in what it can
do. It can't use any non-reentrant function! If you call a non-reentrant
function, and by chance you were precisely in that non-reentrant function
code when you got interrupted by a signal... you lose. That means, no
malloc(). No bufferized IO. No globals. The list goes on and on. <br />
If you're going to catch signals, you'll want to handle them <em>outside</em>
the signal handler. You actually want to spend <em>the least possible
time</em> inside a signal handler - just enough to notify your main
execution flow that there's a signal to take care of.
</p>
<p>
And, of course, signal handlers don't mix with event loops, which is
a classic source of headaches for programmers and led to the birth of
abominations such as
<a href="http://www.opengroup.org/onlinepubs/009695399/functions/pselect.html">
pselect</a>. So much for the "everything is a file" concept that Unix was
built on.
</p>
<p>
A signal should be an event like any other.
There should be a unified interface - receiving a signal should make some
fd readable or something.
</p>
<p>
And that's exactly what the
<a href="http://cr.yp.to/docs/selfpipe.html">self-pipe trick</a>, invented
by <a href="../djblegacy.html">DJB</a>, does.
</p>
<p>
As long as you're in some kind of event loop, the self-pipe trick allows
you to forget about signal handlers... <em>forever</em>. It works this way:
</p>
<ol>
<li> Create a pipe <tt>p</tt>. Make both ends close-on-exec and nonblocking. </li>
<li> Write a tiny signal handler ("top half") for all the signals you want to
catch. This
signal handler should just write one byte into <tt>p[1]</tt>, and do nothing
more; ideally, the written byte identifies the signal. </li>
<li> In your event loop, add <tt>p[0]</tt> to the list of fds you're watching
for readability. </li>
</ol>
<p>
When you get a signal, a byte will be written to the self-pipe, and your
execution flow will resume. When you next go through the event loop,
<tt>p[0]</tt> will be readable; you'll then be able to read a byte from
it, identify the signal, and handle it - in your unrestricted main
environment (the "bottom half" of the handler).
</p>
<p>
The selfpipe library does it all for you - you don't even have to write
the top half yourself. You can forget their existence and recover
some peace of mind.
</p>
<p>
Note that in an asynchronous event loop, you need to protect your
system calls against EINTR by using <a href="safewrappers.html">safe
wrappers</a>.
</p>
<h2> How do I use it ? </h2>
<h3> Starting </h3>
<pre>
int fd = selfpipe_init() ;
</pre>
<p>
<tt>selfpipe_init()</tt> sets up a selfpipe. You must use that
function first. <br />
If <tt>fd</tt> is -1, then an error occurred. Else <tt>fd</tt> is a
non-blocking descriptor that can be used in your event loop. It will
be selected for readability when you've caught a signal.
</p>
<h3> Trapping/untrapping signals </h3>
<pre>
int r = selfpipe_trap(SIGTERM) ;
</pre>
<p>
<tt>selfpipe_trap()</tt> catches a signal and sends it to the selfpipe.
Uncaught signals won't trigger the selfpipe. <tt>r</tt> is 0 if
the operation succeeded, and -1 if it failed. If it succeeded, you
can forget about the trapped signal entirely. <br />
In our example, if <tt>r</tt> is 0, then a SIGTERM will instantly
trigger readability on <tt>fd</tt>.
</p>
<pre>
int r = selfpipe_untrap(SIGTERM) ;
</pre>
<p>
Conversely, <tt>selfpipe_untrap()</tt> uncatches a signal; the selfpipe
will not manage it anymore. <tt>r</tt> is 0 if the operation succeeded
and -1 if it failed.
</p>
<pre>
int r ;
sigset_t set ;
sigemptyset(&set) ;
sigaddset(&set, SIGTERM) ;
sigaddset(&set, SIGHUP) ;
r = selfpipe_trapset(&set) ;
</pre>
<p>
<tt>selfpipe_trap()</tt> and <tt>selfpipe_untrap()</tt> handle signals one
by one. Alternatively (and often preferrably), you can use
<tt>selfpipe_trapset()</tt> to directly handle signal sets. When you call
<tt>selfpipe_trapset()</tt>, signals that are present in <tt>set</tt> will
be caught by the selfpipe, and signals that are absent from <tt>set</tt>
will be uncaught. <tt>r</tt> is 0 if the operation succeeded and -1 if it
failed.
</p>
<h3> Handling events </h3>
<pre>
int c = selfpipe_read() ;
</pre>
<p>
Call <tt>selfpipe_read()</tt> when your <tt>fd</tt> is readable.
That's where you write your <em>real</em> signal handler: in the
body of your event loop, in a "normal" context. <br />
<tt>c</tt> is -1 if an error occurred - in which case chances are
it's a serious one and your system has become very unstable.
<tt>c</tt> is 0 if there are no more pending signals. If <tt>c</tt>
is positive, it is the number of the signal that was caught.
</p>
<h3> Finishing </h3>
<pre>
selfpipe_finish() ;
</pre>
<p>
Call <tt>selfpipe_finish()</tt> when you're done using the selfpipe.
Signal handlers will be restored to their previous value.
</p>
<h2> Any limitations ? </h2>
<p>
Some, as always.
</p>
<ul>
<li> The selfpipe library uses a global pipe;
so, it's not safe for multithreading. I'm not sure how multithreaded
programs handle signals; I personally don't like multithreading and
never use it, so I'm not knowledgeable about it. Anyway, if your
program is multithreaded, chances are you don't have an asynchronous
event loop, so the self-pipe trick has less benefits for you. </li>
<li> In rare cases, the self-pipe can theoretically be filled, if some
application sends more than PIPE_BUF signals before you have time to
<tt>selfpipe_read()</tt>. On most Unix systems, PIPE_BUF is 4096,
so it's a very acceptable margin. Unless your code is waiting where
it should not be, only malicious applications will fill the self-pipe
- and malicious applications could just send you a SIGKILL and be done
with you, so this is not a concern. Protect yourself from malicious
applications with clever use of uids. </li>
</ul>
<h2> Hey, Linux has <a href="http://www.kernel.org/doc/man-pages/online/pages/man2/signalfd.2.html">signalfd()</a> for this ! </h2>
<p>
Yes, the Linux team loves to gratuitously add new system calls to do
things that could already be done before without much effort. This
adds API complexity, which is not a sign of good engineering.
</p>
<p>
However, now that <tt>signalfd()</tt> exists, it is indeed marginally more
efficient than a pipe, and it saves one fd: so the selfpipe library
is implemented via <tt>signalfd()</tt> when this call is available.
</p>
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