PTHREAD_COND(3) 					     Library Functions Manual						   PTHREAD_COND(3)

NAME

       pthread_cond_init,  pthread_cond_destroy,  pthread_cond_signal,	pthread_cond_broadcast, pthread_cond_wait, pthread_cond_timedwait - opera-
       tions on conditions

SYNOPSIS

       #include <pthread.h>

       pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

       int pthread_cond_init(pthread_cond_t *cond, pthread_condattr_t *cond_attr);

       int pthread_cond_signal(pthread_cond_t *cond);

       int pthread_cond_broadcast(pthread_cond_t *cond);

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

       int pthread_cond_timedwait(pthread_cond_t *cond, pthread_mutex_t *mutex, const struct timespec *abstime);

       int pthread_cond_destroy(pthread_cond_t *cond);

DESCRIPTION

       A condition (short for ``condition variable'') is a synchronization device that allows threads to suspend execution and relinquish the pro-
       cessors	until some predicate on shared data is satisfied. The basic operations on conditions are: signal the condition (when the predicate
       becomes true), and wait for the condition, suspending the thread execution until another thread signals the condition.

       A condition variable must always be associated with a mutex, to avoid the race condition where a thread prepares to  wait  on  a  condition
       variable and another thread signals the condition just before the first thread actually waits on it.

       pthread_cond_init  initializes the condition variable cond, using the condition attributes specified in cond_attr, or default attributes if
       cond_attr is NULL. The LinuxThreads implementation supports no attributes  for  conditions,  hence  the	cond_attr  parameter  is  actually
       ignored.

       Variables of type pthread_cond_t can also be initialized statically, using the constant PTHREAD_COND_INITIALIZER.

       pthread_cond_signal restarts one of the threads that are waiting on the condition variable cond. If no threads are waiting on cond, nothing
       happens. If several threads are waiting on cond, exactly one is restarted, but it is not specified which.

       pthread_cond_broadcast restarts all the threads that are waiting on the condition variable cond. Nothing happens if no threads are  waiting
       on cond.

       pthread_cond_wait  atomically unlocks the mutex (as per pthread_unlock_mutex) and waits for the condition variable cond to be signaled. The
       thread execution is suspended and does not consume any CPU time until the condition variable is signaled. The mutex must be locked  by  the
       calling	thread	on  entrance  to  pthread_cond_wait.  Before  returning to the calling thread, pthread_cond_wait re-acquires mutex (as per
       pthread_lock_mutex).

       Unlocking the mutex and suspending on the condition variable is done atomically. Thus, if all threads always acquire the mutex before  sig-
       naling  the  condition,	this guarantees that the condition cannot be signaled (and thus ignored) between the time a thread locks the mutex
       and the time it waits on the condition variable.

       pthread_cond_timedwait atomically unlocks mutex and waits on cond, as pthread_cond_wait does, but it also bounds the duration of the  wait.
       If  cond  has  not  been signaled within the amount of time specified by abstime, the mutex mutex is re-acquired and pthread_cond_timedwait
       returns the error ETIMEDOUT.  The abstime parameter specifies an absolute time, with the same origin as	time(2)  and  gettimeofday(2):	an
       abstime of 0 corresponds to 00:00:00 GMT, January 1, 1970.

       pthread_cond_destroy  destroys a condition variable, freeing the resources it might hold. No threads must be waiting on the condition vari-
       able on entrance to pthread_cond_destroy. In the LinuxThreads implementation, no resources are associated with  condition  variables,  thus
       pthread_cond_destroy actually does nothing except checking that the condition has no waiting threads.

CANCELLATION

       pthread_cond_wait  and  pthread_cond_timedwait are cancellation points. If a thread is cancelled while suspended in one of these functions,
       the thread immediately resumes execution, then locks again the mutex argument to pthread_cond_wait and pthread_cond_timedwait, and  finally
       executes the cancellation.  Consequently, cleanup handlers are assured that mutex is locked when they are called.

ASYNC-SIGNAL SAFETY
       The condition functions are not async-signal safe, and should not be called from a signal handler. In particular, calling pthread_cond_sig-
       nal or pthread_cond_broadcast from a signal handler may deadlock the calling thread.

RETURN VALUE

       All condition variable functions return 0 on success and a non-zero error code on error.

ERRORS

       pthread_cond_init, pthread_cond_signal, pthread_cond_broadcast, and pthread_cond_wait never return an error code.

       The pthread_cond_timedwait function returns the following error codes on error:

	      ETIMEDOUT
		     the condition variable was not signaled until the timeout specified by abstime

	      EINTR  pthread_cond_timedwait was interrupted by a signal

       The pthread_cond_destroy function returns the following error code on error:

	      EBUSY  some threads are currently waiting on cond.

AUTHOR

       Xavier Leroy <Xavier.Leroy@inria.fr>

SEE ALSO

       pthread_condattr_init(3), pthread_mutex_lock(3), pthread_mutex_unlock(3), gettimeofday(2), nanosleep(2).

EXAMPLE

       Consider two shared variables x and y, protected by the mutex mut, and a condition variable cond that is to be signaled whenever x  becomes
       greater than y.

	      int x,y;
	      pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER;
	      pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

       Waiting until x is greater than y is performed as follows:

	      pthread_mutex_lock(&mut);
	      while (x <= y) {
		      pthread_cond_wait(&cond, &mut);
	      }
	      /* operate on x and y */
	      pthread_mutex_unlock(&mut);

       Modifications on x and y that may cause x to become greater than y should signal the condition if needed:

	      pthread_mutex_lock(&mut);
	      /* modify x and y */
	      if (x > y) pthread_cond_broadcast(&cond);
	      pthread_mutex_unlock(&mut);

       If it can be proved that at most one waiting thread needs to be waken up (for instance, if there are only two threads communicating through
       x and  y),  pthread_cond_signal	can  be  used  as  a  slightly	more  efficient  alternative  to  pthread_cond_broadcast.  In  doubt,  use
       pthread_cond_broadcast.

       To wait for x to becomes greater than y with a timeout of 5 seconds, do:

	      struct timeval now;
	      struct timespec timeout;
	      int retcode;

	      pthread_mutex_lock(&mut);
	      gettimeofday(&now);
	      timeout.tv_sec = now.tv_sec + 5;
	      timeout.tv_nsec = now.tv_usec * 1000;
	      retcode = 0;
	      while (x <= y && retcode != ETIMEDOUT) {
		      retcode = pthread_cond_timedwait(&cond, &mut, &timeout);
	      }
	      if (retcode == ETIMEDOUT) {
		      /* timeout occurred */
	      } else {
		      /* operate on x and y */
	      }
	      pthread_mutex_unlock(&mut);

								   LinuxThreads 						   PTHREAD_COND(3)