NAME

pthread_cond — condition variable interface

LIBRARY

POSIX Threads Library (libpthread, -lpthread)

SYNOPSIS

#include <pthread.h>

int
pthread_cond_init(pthread_cond_t * restrict cond, const pthread_condattr_t * restrict attr);

pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

int
pthread_cond_destroy(pthread_cond_t *cond);

int
pthread_cond_broadcast(pthread_cond_t *cond);

int
pthread_cond_signal(pthread_cond_t *cond);

int
pthread_cond_wait(pthread_cond_t * restrict cond, pthread_mutex_t * restrict mutex);

int
pthread_cond_timedwait(pthread_cond_t * restrict cond, pthread_mutex_t * restrict mutex, const struct timespec * restrict abstime);

DESCRIPTION

Condition variables are intended to be used to communicate changes in the state of data shared between threads. Condition variables are always associated with a mutex to provide synchronized access to the shared data. A single predicate should always be associated with a condition variable. The predicate should identify a state of the shared data that must be true before the thread proceeds.

The pthread_cond_init() function creates a new condition variable, with attributes specified with attr. If attr is NULL the default attributes are used. The pthread_cond_destroy() function frees the resources allocated by the condition variable cond.

The macro PTHREAD_COND_INITIALIZER can be used to initialize a condition variable when it can be statically allocated and the default attributes are appropriate. The effect is similar to calling pthread_cond_init() with attr specified as NULL, except that no error checking is done.

The difference between pthread_cond_broadcast() and pthread_cond_signal() is that the former unblocks all threads waiting for the condition variable, whereas the latter blocks only one waiting thread. If no threads are waiting on cond, neither function has any effect. If more than one thread is blocked on a condition variable, the used scheduling policy determines the order in which threads are unblocked. The same mutex used for waiting must be held while calling either function. Although neither function strictly enforces this requirement, undefined behavior may follow if the mutex is not held.

The pthread_cond_wait() function atomically blocks the current thread waiting on the condition variable specified by cond, and unlocks the mutex specified by mutex. The pthread_cond_timedwait() function behaves similarly, but unblocks also if the system time reaches the time specified in abstime, represented as struct timespec (see timespec(3)). With both functions the waiting thread unblocks after another thread calls pthread_cond_signal() or pthread_cond_broadcast() with the same condition variable and by holding the same mutex that was associated with cond by either one of the blocking functions. The current thread holds the lock on mutex upon return from either function.

Note that a call to pthread_cond_wait() or pthread_cond_timedwait() may wake up spontaneously, without a call to pthread_cond_signal() or pthread_cond_broadcast(). The caller should prepare for this by invoking either function within a predicate loop that tests whether the thread should proceed.

As noted, when calling either function that waits on a condition variable, a temporary binding is established between the condition variable cond and the mutex mutex. During this time, the effect of an attempt by any thread to wait on that condition variable using a different mutex is undefined. The same mutex must be held while broadcasting or signaling on cond. Additionally, the same mutex must be used for concurrent calls to pthread_cond_wait() and pthread_cond_timedwait(). Only when a condition variable is known to be quiescent may an application change the mutex associated with it. In this implementation, none of the functions enforce this requirement, but if the mutex is not held or independent mutexes are used the resulting behaviour is undefined.

RETURN VALUES

If successful, all functions return zero. Otherwise, an error number will be returned to indicate the error.

ERRORS

The pthread_cond_init() function may fail if:

[EINVAL]

The value specified by attr is invalid.

The pthread_cond_destroy() function may fail if:

[EBUSY]

The variable cond is locked by another thread.

[EINVAL]

The value specified by cond is invalid.

Both pthread_cond_broadcast() and pthread_cond_signal() may fail if:

[EINVAL]

The value specified by cond is invalid.

Both pthread_cond_wait() and pthread_cond_timedwait() may fail if:

[EINVAL]

The value specified by cond or the value specified by mutex is invalid.

[EPERM]

The value specified by mutex was not locked in the condition wait.

The pthread_cond_timedwait() function may additionally fail if:

[ETIMEDOUT]

The system time has reached or exceeded the time specified in abstime.

SEE ALSO

pthread(3), pthread_barrier(3), pthread_condattr(3), pthread_mutex(3), pthread_rwlock(3), pthread_spin(3)

STANDARDS

These functions conform to IEEE Std 1003.1-2001 (“POSIX.1”).