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<h5 class="subsubsection">24.4.7.1 Problems with Non-Atomic Access</h5>

<p>Here is an example which shows what can happen if a signal handler runs
in the middle of modifying a variable.  (Interrupting the reading of a
variable can also lead to paradoxical results, but here we only show
writing.)

<pre class="smallexample">     #include &lt;signal.h&gt;
     #include &lt;stdio.h&gt;
     
     volatile struct two_words { int a, b; } memory;
     
     void
     handler(int signum)
     {
        printf ("%d,%d\n", memory.a, memory.b);
        alarm (1);
     }
     
     int
     main (void)
     {
        static struct two_words zeros = { 0, 0 }, ones = { 1, 1 };
        signal (SIGALRM, handler);
        memory = zeros;
        alarm (1);
        while (1)
          {
            memory = zeros;
            memory = ones;
          }
     }
</pre>
   <p>This program fills <code>memory</code> with zeros, ones, zeros, ones,
alternating forever; meanwhile, once per second, the alarm signal handler
prints the current contents.  (Calling <code>printf</code> in the handler is
safe in this program because it is certainly not being called outside
the handler when the signal happens.)

   <p>Clearly, this program can print a pair of zeros or a pair of ones.  But
that's not all it can do!  On most machines, it takes several
instructions to store a new value in <code>memory</code>, and the value is
stored one word at a time.  If the signal is delivered in between these
instructions, the handler might find that <code>memory.a</code> is zero and
<code>memory.b</code> is one (or vice versa).

   <p>On some machines it may be possible to store a new value in
<code>memory</code> with just one instruction that cannot be interrupted.  On
these machines, the handler will always print two zeros or two ones.

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