pthread_cond_init(3) - Linux manual page (original) (raw)
pthreadcondinit(3) Library Functions Manual pthreadcondinit(3)
NAME top
pthread_cond_init, pthread_cond_signal, pthread_cond_broadcast,
pthread_cond_wait, pthread_cond_timedwait, pthread_cond_destroy -
operations on conditionsSYNOPSIS top
**#include <pthread.h>**
**pthread_cond_t** _cond_ **= PTHREAD_COND_INITIALIZER;**
**int pthread_cond_init(pthread_cond_t ***_cond_**,**
**pthread_condattr_t ***_condattr_**);**
**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 top
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 _condattr_, or default
attributes if _condattr_ is **NULL**. The LinuxThreads implementation
supports no attributes for conditions, hence the _condattr_
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_mutex_lock**).
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)](../man2/time.2.html) and [gettimeofday(2)](../man2/gettimeofday.2.html): 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 top
**pthread_cond_wait** and **pthread_cond_timedwait** are cancelation
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 cancelation.
Consequently, cleanup handlers are assured that _mutex_ is locked
when they are called.ASYNC-SIGNAL SAFETY top
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 top
All condition variable functions return 0 on success and a non-
zero error code on error.ERRORS top
**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_.
The **pthread_cond_destroy** function returns the following error code
on error:
**EBUSY** Some threads are currently waiting on _cond_.SEE ALSO top
[pthread_condattr_init(3)](../man3/pthread%5Fcondattr%5Finit.3.html), [pthread_mutex_lock(3)](../man3/pthread%5Fmutex%5Flock.3.html),
[pthread_mutex_unlock(3)](../man3/pthread%5Fmutex%5Funlock.3.html), [gettimeofday(2)](../man2/gettimeofday.2.html), [nanosleep(2)](../man2/nanosleep.2.html).EXAMPLE top
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 become 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);**COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2025-01-04 pthreadcondinit(3)
Pages that refer to this page:futex(2), PR_SET_TIMERSLACK(2const), pthread_condattr_init(3), pthreads(7)