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(libc.info)Non-atomic Example

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Problems with Non-Atomic Access
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   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>
     
     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.