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<h4 class="subsection">24.8.2 Problems with <code>pause</code></h4>

<p>The simplicity of <code>pause</code> can conceal serious timing errors that
can make a program hang mysteriously.

   <p>It is safe to use <code>pause</code> if the real work of your program is done
by the signal handlers themselves, and the &ldquo;main program&rdquo; does nothing
but call <code>pause</code>.  Each time a signal is delivered, the handler
will do the next batch of work that is to be done, and then return, so
that the main loop of the program can call <code>pause</code> again.

   <p>You can't safely use <code>pause</code> to wait until one more signal arrives,
and then resume real work.  Even if you arrange for the signal handler
to cooperate by setting a flag, you still can't use <code>pause</code>
reliably.  Here is an example of this problem:

<pre class="smallexample">     /* <code>usr_interrupt</code><span class="roman"> is set by the signal handler.</span>  */
     if (!usr_interrupt)
       pause ();
     
     /* <span class="roman">Do work once the signal arrives.</span>  */
     ...
</pre>
   <p class="noindent">This has a bug: the signal could arrive after the variable
<code>usr_interrupt</code> is checked, but before the call to <code>pause</code>. 
If no further signals arrive, the process would never wake up again.

   <p>You can put an upper limit on the excess waiting by using <code>sleep</code>
in a loop, instead of using <code>pause</code>.  (See <a href="Sleeping.html#Sleeping">Sleeping</a>, for more
about <code>sleep</code>.)  Here is what this looks like:

<pre class="smallexample">     /* <code>usr_interrupt</code><span class="roman"> is set by the signal handler.</span>
     while (!usr_interrupt)
       sleep (1);
     
     /* <span class="roman">Do work once the signal arrives.</span>  */
     ...
</pre>
   <p>For some purposes, that is good enough.  But with a little more
complexity, you can wait reliably until a particular signal handler is
run, using <code>sigsuspend</code>.

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