pthread_cond_signal

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 -
       operations 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
       processors 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
       signaling 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
       variable 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_signal 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)