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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 <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; |
| } |
| } |
| </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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