Spurious wakeup

In the POSIX thread API, the function pthread_cond_wait is used to wait on a condition variable. A naive programmer might expect that when a thread returns from this function, the condition associated with the condition variable will be true. However, it is recommended that all threads check the condition after returning from pthread_cond_wait because there are several reasons the condition might not be true. One of these reasons is a spurious wakeup; that is, a thread might get woken up even though no thread signalled the condition.

According to David R. Butenhof's Programming with POSIX Threads ISBN 0-201-63392-2:

"This means that when you wait on a condition variable, the wait may (occasionally) return when no thread specifically broadcast or signalled that condition variable. Spurious wakeups may sound strange, but on some multiprocessor systems, making condition wakeup completely predictable might substantially slow all condition variable operations. The race conditions that cause spurious wakeups should be considered rare."

Other reasons for verifying the invariant

Practical reasons exist for checking the invariant after a return from a wait other than spurious wakeups. For example, a woken-up thread may not be scheduled immediately after the wake up, but be at the mercy of the system scheduler. A scheduler may preempt a process abruptly or schedule other threads. It may be the case that in the mean time, an external entity (another process, hardware) has invalidated the invariant assumption. Wrapping the wait with a loop avoids such cases.

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