Operations on conditions
#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);
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.
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.
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.
All condition variable functions return 0 on success and a non-zero error code on error.
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.
Xavier Leroy <[email protected]>
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);