In our working producer and consumer solutions with a single buffer slot using condition variables, we made the producer check that count was 0 in a while loop and the consumer similarly check that it was 1 in a while loop. Why is a while loop necessary here instead of an if statement?

Question

In our working producer and consumer solutions with a single buffer slot using condition variables, we made the producer check that  count  was 0 in a while loop and the consumer similarly check that it was 1 in a while loop. Why is a while loop necessary here instead of an if statement?

Accepted Answer

If a producer wakes a sleeping consumer thread, but before it runs another consumer thread empties the buffer, then when the woken consumer thread eventually does run it could try to consume from an empty buffer, likely crashing your program. Since there is no guarantee of when an woken thread will run, it is essential to recheck the condition before continuing, hence the while loop instead of an if statement.

Answer

The while loop is essential for the woken thread to reacquire the lock  m . With only an if statement, the functions  get  and  put  can be called to alter the buffer without mutual exclusion and therefore creating race conditions.

Answer

The while loop is not necessary on most modern machines but is required for it to be backwards compatible with older systems built with Mesa semantics where a thread is not guaranteed to run immediately after its creation/awakening.

Answer

The while loop is essential because  Pthread_cond_wait  only makes the thread sleep for a fixed amount of time. If the time delay is not sufficient for the other thread(s) to have finished, then we have created a race condition. Therefore, we need to make sure that the thread rechecks the condition before continuing.

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In our working producer and consumer solutions with a single buffer slot using c...

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