OSF DCE Application Development Guide--Core Components

Thread Concepts and Operations

Thread B updates the predicate named ready associated with the condition variable to

the state thread A is waiting for. It also executes a signal on the condition variable while

holding the mutex mutex_ready.

Figure 7-5. Thread A Wakes Up and Proceeds

Thread B

var

mutex_var

lock block

Thread A

access

Thread A wakes up, veriﬁes that the condition variable is in the correct state, and

proceeds to execute task Y (see Figure 7-3).

Note that, although the condition variable is used for explicit communications among

threads, the communications are anonymous. Thread B does not necessarily know that

thread A is waiting on the condition variable that thread B signals. And thread A does

not know that it was thread B that woke it up from its wait on the condition variable.

Use the pthread_cond_init() routine to create a condition variable. To create condition

variables as part of the program’s one-time initialization code, see Section 7.5.

Use the pthread_cond_wait() routine to cause a thread to wait until the condition is

signaled or broadcast. This routine speciﬁes a condition variable and a mutex that you

have locked. (If you have not locked the mutex, the results of pthread_cond_wait( ) are

unpredictable.) This routine unlocks the mutex and causes the calling thread to wait on

the condition variable until another thread calls one of the following routines:

• The pthread_cond_signal( ) routine to wake one thread that is waiting on the

condition variable

• The pthread_cond_broadcast() routine to wake all threads that are waiting on a

condition variable

If you want to limit the time that a thread waits for a condition to be signaled or

broadcast, use the pthread_cond_timedwait( ) routine. This routine speciﬁes the

condition variable, mutex, and absolute time at which the wait should expire if the

condition variable is not signaled or broadcast.

You can delete a condition variable and reclaim its storage by calling the

pthread_cond_destroy( ) routine. Use this routine only after the condition variable is no

longer needed by any thread. Condition variables are automatically deleted when the

program terminates.

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