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Diffstat (limited to 'linuxthreads/man/pthread_cond_init.man')
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diff --git a/linuxthreads/man/pthread_cond_init.man b/linuxthreads/man/pthread_cond_init.man new file mode 100644 index 0000000000..b803f08361 --- /dev/null +++ b/linuxthreads/man/pthread_cond_init.man @@ -0,0 +1,235 @@ +.TH PTHREAD_COND 3 LinuxThreads + +.XREF pthread_cond_signal +.XREF pthread_cond_broadcast +.XREF pthread_cond_wait +.XREF pthread_cond_timedwait +.XREF pthread_cond_destroy + +.SH NAME +pthread_cond_init, pthread_cond_destroy, pthread_cond_signal, pthread_cond_broadcast, pthread_cond_wait, pthread_cond_timedwait \- operations on conditions + +.SH 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); + +.SH 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. + +.SH 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. + +.SH "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. + +.SH "RETURN VALUE" + +All condition variable functions return 0 on success and a non-zero +error code on error. + +.SH 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: +.RS +.TP +!ETIMEDOUT! +the condition variable was not signaled until the timeout specified by +|abstime| + +.TP +!EINTR! +!pthread_cond_timedwait! was interrupted by a signal +.RE + +The !pthread_cond_destroy! function returns the following error code +on error: +.RS +.TP +!EBUSY! +some threads are currently waiting on |cond|. +.RE + +.SH AUTHOR +Xavier Leroy <Xavier.Leroy@inria.fr> + +.SH "SEE ALSO" +!pthread_condattr_init!(3), +!pthread_mutex_lock!(3), +!pthread_mutex_unlock!(3), +!gettimeofday!(2), +!nanosleep!(2). + +.SH 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|. + +.RS +.ft 3 +.nf +.sp +int x,y; +pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER; +pthread_cond_t cond = PTHREAD_COND_INITIALIZER; +.ft +.LP +.RE +.fi + +Waiting until |x| is greater than |y| is performed as follows: + +.RS +.ft 3 +.nf +.sp +pthread_mutex_lock(&mut); +while (x <= y) { + pthread_cond_wait(&cond, &mut); +} +/* operate on x and y */ +pthread_mutex_unlock(&mut); +.ft +.LP +.RE +.fi + +Modifications on |x| and |y| that may cause |x| to become greater than +|y| should signal the condition if needed: + +.RS +.ft 3 +.nf +.sp +pthread_mutex_lock(&mut); +/* modify x and y */ +if (x > y) pthread_mutex_broadcast(&cond); +pthread_mutex_unlock(&mut); +.ft +.LP +.RE +.fi + +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: + +.RS +.ft 3 +.nf +.sp +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); +.ft +.LP +.RE +.fi + |