A condition (short for “condition variable”) is a synchronization device that allows threads to suspend execution 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 isNULL
. 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 constantPTHREAD_COND_INITIALIZER
.This function always returns 0.
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.This function always returns 0.
pthread_cond_broadcast
restarts all the threads that are waiting on the condition variable cond. Nothing happens if no threads are waiting on cond.This function always returns 0.
pthread_cond_wait
atomically unlocks the mutex (as perpthread_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 topthread_cond_wait
. Before returning to the calling thread,pthread_cond_wait
re-acquires mutex (as perpthread_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.
This function always returns 0.
pthread_cond_timedwait
atomically unlocks mutex and waits on cond, aspthread_cond_wait
does, but it also bounds the duration of the wait. If cond has not been signaled before time abstime, the mutex mutex is re-acquired andpthread_cond_timedwait
returns the error codeETIMEDOUT
. The wait can also be interrupted by a signal; in that casepthread_cond_timedwait
returnsEINTR
.The abstime parameter specifies an absolute time, with the same origin as
time
andgettimeofday
: an abstime of 0 corresponds to 00:00:00 GMT, January 1, 1970.
pthread_cond_destroy
destroys the condition variable cond, freeing the resources it might hold. If any threads are waiting on the condition variable,pthread_cond_destroy
leaves cond untouched and returnsEBUSY
. Otherwise it returns 0, and cond must not be used again until it is reinitialized.In the LinuxThreads implementation, no resources are associated with condition variables, so
pthread_cond_destroy
actually does nothing.
pthread_cond_wait
and pthread_cond_timedwait
are
cancellation points. If a thread is canceled while suspended in one of
these functions, the thread immediately resumes execution, relocks the
mutex specified by mutex, and finally executes the cancellation.
Consequently, cleanup handlers are assured that mutex is locked
when they are called.
It is not safe to call the condition variable functions from a signal
handler. In particular, calling pthread_cond_signal
or
pthread_cond_broadcast
from a signal handler may deadlock the
calling thread.
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);
Condition attributes can be specified at condition creation time, by
passing a condition attribute object as second argument to
pthread_cond_init
. Passing NULL
is equivalent to passing
a condition attribute object with all attributes set to their default
values.
The LinuxThreads implementation supports no attributes for conditions. The functions on condition attributes are included only for compliance with the POSIX standard.
pthread_condattr_init
initializes the condition attribute object attr and fills it with default values for the attributes.pthread_condattr_destroy
destroys the condition attribute object attr.Both functions do nothing in the LinuxThreads implementation.
pthread_condattr_init
andpthread_condattr_destroy
always return 0.