Non-atomic Example - The GNU C Library

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##### 24.4.7.1 Problems with Non-Atomic Access

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.)

         #include <signal.h>
         #include <stdio.h>

         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;
              }
         }

This program fills `memory` with zeros, ones, zeros, ones, alternating forever; meanwhile, once per second, the alarm signal handler prints the current contents. (Calling `printf` in the handler is safe in this program because it is certainly not being called outside the handler when the signal happens.)

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 `memory`, and the value is stored one word at a time. If the signal is delivered in between these instructions, the handler might find that `memory.a` is zero and `memory.b` is one (or vice versa).

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