Condition variables are intended to be used to communicate changes in the state of data shared between threads. Condition variables are always associated with a mutex to provide synchronized access to the shared data. A single predicate should always be associated with a condition variable. The predicate should identify a state of the shared data that must be true before the thread proceeds.
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 blocks 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.