pthread_cond_wait

Causes a thread to wait for a condition variable to be signaled or broadcasted.

Syntax

pthread_cond_wait( cond, mutex );

Argument Data Type Access

cond opaque pthread_cond_t modify mutex opaque pthread_mutex_t modify

C Binding

#include 


int
pthread_cond_wait (
         pthread_cond_t   *cond,
         pthread_mutex_t   *mutex);

Arguments

cond: Condition variable that a thread waits on.
mutex: Mutex associated with the condition variable specified in cond.

Description

This routine causes a thread to wait for a condition variable to be signaled or broadcasted. Each condition corresponds to one or more Boolean relations (predicates) based on shared data. The calling thread waits for the data to reach a particular state for the predicate to become true. However, the return from this routine does not imply anything about the value of the predicate and it should be reevaluated upon return. Condition variables are discussed in Chapter 2 and Chapter 3.

Call this routine after you have locked the mutex specified in mutex. The results of this routine are unpredictable if this routine is called without first locking the mutex.

This routine atomically releases the mutex and causes the calling thread to wait on the condition. The atomicity is important, because it means the thread cannot miss a wakeup while the mutex is unlocked. When the wait is satisfied as a result of some thread calling pthread_cond_signal or pthread_cond_broadcast, the mutex is reacquired before returning to the caller.

A thread that changes the state of storage protected by the mutex in such a way that a predicate associated with a condition variable might now be true, must call either pthread_cond_signal or pthread_ cond_broadcast for that condition variable. If neither call is made, any thread waiting on the condition variable continues to wait.

This routine might (with low probability) return when the condition variable has not been signaled or broadcasted. When this occurs, the mutex is reacquired before the routine returns. To handle this type of situation, enclose this routine in a loop that checks the predicate. The loop provides documentation of your intent and protects against these spurious wakeups, while also allowing correct behavior even if another thread consumes the desired state before the awakened thread runs.

It is illegal for threads to wait on the same condition variable by specifying different mutexes.

Return Values

If an error condition occurs, this routine returns an integer value indicating the type of error. Possible return values are as follows:

Return Description

0 Successful completion. [EINVAL] The value specified by cond or mutex is invalid, or: Different mutexes are supplied for concurrent pthread_cond_wait or pthread_cond_timedwait operations on the same condition variable, or The mutex was not owned by the current thread at the time of the call. [ENOMEM] DECthreads cannot acquire memory needed to block using a statically initialized condition variable.

Associated Routines

   pthread_cond_broadcast
   pthread_cond_destroy
   pthread_cond_init
   pthread_cond_signal
   pthread_cond_timedwait