问题
Does pthread_cond_signal unblock exactly one thread? If not, what will be the case it releases more than one thread? The specification says as follows:
The pthread_cond_signal() function shall unblock at least one of the threads that are blocked on the specified condition variable cond (if any threads are blocked on cond).
回答1:
The pthreads specification allows for "spurious wakeups" in an implementation. See, for example, the hypothetical implementation of pthread_cond_signal and pthread_cond_wait sketched in the specification that allows for just this condition.
The possibility of spurious wakeups is why one always associates some predicate with a condition, and checks that predicate upon wakeup.
来源：https://stackoverflow.com/questions/18391146/pthread-cond-signal-release-exactly-one-thread