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| <h5 class="subsubsection">24.4.7.3 Atomic Usage Patterns</h5> |
| |
| <p>Certain patterns of access avoid any problem even if an access is |
| interrupted. For example, a flag which is set by the handler, and |
| tested and cleared by the main program from time to time, is always safe |
| even if access actually requires two instructions. To show that this is |
| so, we must consider each access that could be interrupted, and show |
| that there is no problem if it is interrupted. |
| |
| <p>An interrupt in the middle of testing the flag is safe because either it's |
| recognized to be nonzero, in which case the precise value doesn't |
| matter, or it will be seen to be nonzero the next time it's tested. |
| |
| <p>An interrupt in the middle of clearing the flag is no problem because |
| either the value ends up zero, which is what happens if a signal comes |
| in just before the flag is cleared, or the value ends up nonzero, and |
| subsequent events occur as if the signal had come in just after the flag |
| was cleared. As long as the code handles both of these cases properly, |
| it can also handle a signal in the middle of clearing the flag. (This |
| is an example of the sort of reasoning you need to do to figure out |
| whether non-atomic usage is safe.) |
| |
| <p>Sometimes you can insure uninterrupted access to one object by |
| protecting its use with another object, perhaps one whose type |
| guarantees atomicity. See <a href="Merged-Signals.html#Merged-Signals">Merged Signals</a>, for an example. |
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