pthread_cond_wait()

Wait on condition variable

Synopsis:

#include <pthread.h>

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

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.

---

Don't use a recursive mutex with condition variables.

---

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.

---

Make sure that the thread's cleanup handlers unlock the mutex.

---

Returns:

EOK

Success, or the call was interrupted by a signal.

EAGAIN

Insufficient system resources are available to wait on the condition.

EFAULT

A fault occurred trying to access the buffers provided.

EINVAL

One or more of the following is true:

• One or more of cond or mutex is invalid.
• Concurrent waits on cond used different mutexes.

EPERM

The current thread doesn't own the mutex.

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 THR

See also:

pthread_cond_broadcast(), pthread_cond_init(), pthread_cond_signal(), pthread_cond_timedwait(), SyncCondvarWait()

Processes and Threads chapter of Getting Started with QNX Neutrino