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Editorial Team
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Asked: 2026-05-17T16:37:54+00:00

There’s an article about semaphores on OS X . The author tests the semaphore

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There’s an article about semaphores on OS X. The author tests the semaphore by incrementing and decrementing a static variable in two threads. With semaphore guarding the variable access, the variable ends up being zero. Without the guard the variable ends up having some bogus value. I tried the code and it works. What I don’t understand is how may the concurrent access from the two threads make a difference in the final variable value. After all it seems to me like a bunch of +1s and –1s that should be comutative, right? I feel I am missing something glaring obvious, what is it? 🙂

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1 Answer

  1. Editorial Team

    The problem is that ++ / -- are not atomic. They are essentially three operations:

    1. Load
    2. Inc/dec
    3. Store

    So if two threads load value simultaneously and store it also simultaneously. Difference will be 1 instead of 2.

    Here is sample

    Thread A Thread B
Load 5   Load 5
Inc 6    Inc 6
Store 6  Store 6
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