pthread_cond_wait()

Arguments:

cond

A pointer to the pthread_cond_t object that you want the threads to block on.

mutex

The mutex that you want to unlock.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The pthread_cond_wait() function blocks the calling thread on the condition variable cond, and unlocks the associated mutex mutex. The calling thread must have locked mutex before waiting on the condition variable. On return from the function, the mutex is again locked and owned by the calling thread.

The calling thread is blocked until either another thread performs a signal or broadcast on the condition variable, a signal is delivered to the thread, or the thread is canceled (waiting on a condition variable is a cancellation point). In all cases, the thread reacquires the mutex before being unblocked.

If a thread that's blocked on a condition variable is canceled, the thread reacquires the mutex that's guarding the condition variable, so that the thread's cleanup handlers run in the same state as the critical code before and after the call to this function. If some other thread owns the lock, the canceled thread blocks until the mutex is available.

Returns:

EOK

Success, or the call was interrupted by a signal.

EFAULT

A fault occurred trying to access the buffers provided.

EINVAL

One or both of the following are true:

• One or more of cond or mutex is invalid.
• The mutex has died.

EPERM

The current thread doesn't own the mutex.

ESRCH

The condvar was destroyed while the thread was blocked on it.

Examples:

Use condition variables to synchronize producer and consumer threads:

#include <stdio.h>
#include <pthread.h>

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
int condition = 0;
int count = 0;

int consume( void )
{
   while( 1 )
   {
      pthread_mutex_lock( &mutex );
      while( condition == 0 )
         pthread_cond_wait( &cond, &mutex );
      printf( "Consumed %d\n", count );
      condition = 0;
      pthread_cond_signal( &cond );      
      pthread_mutex_unlock( &mutex );
   }

   return( 0 );
}

void*  produce( void * arg )
{
   while( 1 )
   {
      pthread_mutex_lock( &mutex );
      while( condition == 1 )
         pthread_cond_wait( &cond, &mutex );
      printf( "Produced %d\n", count++ );
      condition = 1;
      pthread_cond_signal( &cond );      
      pthread_mutex_unlock( &mutex );
   }
   return( 0 );
}

int main( void )
{
   pthread_create( NULL, NULL, &produce, NULL );
   return consume();
}

Classification:

POSIX 1003.1