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DESCRIPTION

The pthread_cond_init() function creates a new condition variable, with attributes specified with attr. If attr is NULL the default attributes are used. The pthread_cond_destroy() function frees the resources allocated by the condition variable cond.

The macro PTHREAD_COND_INITIALIZER can be used to initialize a condition variable when it can be statically allocated and the default attributes are appropriate. The effect is similar to calling pthread_cond_init() with attr specified as NULL, except that no error checking is done.

The difference between pthread_cond_broadcast() and pthread_cond_signal() is that the former unblocks all threads waiting for the condition variable, whereas the latter unblocks only one waiting thread. If no threads are waiting on cond, neither function has any effect. If more than one thread is blocked on a condition variable, the used scheduling policy determines the order in which threads are unblocked. The same mutex used for waiting must be held while calling either function. Although neither function strictly enforces this requirement, undefined behavior may follow if the mutex is not held.

The pthread_cond_wait() function atomically blocks the current thread waiting on the condition variable specified by cond, and unlocks the mutex specified by mutex. The pthread_cond_timedwait() function behaves similarly, but unblocks also if the system time reaches the time specified in abstime, represented as struct timespec (see timespec(3)). With both functions the waiting thread unblocks after another thread calls pthread_cond_signal() or pthread_cond_broadcast() with the same condition variable and by holding the same mutex that was associated with cond by either one of the blocking functions. The current thread holds the lock on mutex upon return from either function.

Note that a call to pthread_cond_wait() or pthread_cond_timedwait() may wake up spontaneously, without a call to pthread_cond_signal() or pthread_cond_broadcast(). The caller should prepare for this by invoking either function within a predicate loop that tests whether the thread should proceed.

As noted, when calling either function that waits on a condition variable, a temporary binding is established between the condition variable cond and the mutex mutex. During this time, the effect of an attempt by any thread to wait on that condition variable using a different mutex is undefined. The same mutex must be held while broadcasting or signaling on cond. Additionally, the same mutex must be used for concurrent calls to pthread_cond_wait() and pthread_cond_timedwait(). Only when a condition variable is known to be quiescent may an application change the mutex associated with it. In this implementation, none of the functions enforce this requirement, but if the mutex is not held or independent mutexes are used the resulting behaviour is undefined.