pthread_cond_wait(3T)

NAME

pthread_cond_wait(), pthread_cond_timedwait() — wait or timed wait on a condition variable

SYNOPSIS

#include <pthread.h>

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
);

Parameters

cond: Pointer to the condition variable to be waited on.
mutex: Pointer to the mutex associated with the condition variable cond.
abstime: Absolute time at which the wait expires, even if the condition has not been signaled or broadcast.

DESCRIPTION

The pthread_cond_wait() function is used to wait for the occurrence of a condition associated with the condition variable cond.

The pthread_cond_timedwait() function is used to wait a limited amount of time for the occurrence of a condition associated with the condition variable cond. The abstime parameter specifies the time at which this function should time out. If the absolute time specified by abstime passes and the indicated condition has not been signaled, the function returns an error to the caller.

Note: abstime is the time at which the wait expires, not the length of time the thread will wait.

The condition variabled denoted by cond must have been dynamically initialized by a call to pthread_cond_init() or statically initialized with the macro PTHREAD_COND_INITIALIZER.

Both functions should be called with mutex locked by the calling thread. If mutex is not locked by the calling thread, undefined behavior will result. These functions atomically release mutex and cause the calling thread to block on the condition variable cond. If another thread is able to acquire the mutex after the about-to-block thread has released it but before it has actually blocked, a subsequent call to pthread_cond_signal() or pthread_cond_broadcast() by the other thread will behave as if it were issued after the about-to-block thread has blocked.

When the condition is signaled or the timed wait expires, the caller is unblocked and will reacquire mutex before returning. Whether these functions succeed or fail, mutex will always be reacquired before returning to the caller.

Using different mutexes for concurrent calls to these functions on the same condition variable results in undefined behavior.

When using condition variables, there is a predicate associated with the condition wait. If this predicate is false, the thread should perform a condition wait. Spurious wakeups may occur when waiting on a condition variable. A spurious wakeup occurs when a thread returns from a condition wait when it should really continue waiting. A normal signal being delivered to a thread may cause a spurious wakeup during a condition wait. Since the return values from pthread_cond_wait() and pthread_cond_timedwait() do not imply anything about the value of the predicate, the predicate should be re-evaluated.

A condition wait is a cancellation point. When the calling thread has deferred cancellation enabled, cancellation requested will be acted upon. If a cancellation request is acted upon while a thread is blocked in one of these functions, mutex is reacquired before calling the cancellation cleanup handlers. The cancellation cleanup handlers should release mutex so that application deadlock does not occur. If the condition signal and the cancellation request both occur, the canceled thread will not consume the condition signal (i.e., a different thread will be unblocked due to the condition signal).

If a signal is delivered to a thread waiting for a condition variable, upon return from the signal handler, the thread may return zero due to a spurious wakeup or continue waiting for the condition.

RETURN VALUE

pthread_cond_wait() and pthread_cond_timedwait() return the following values:

0: Successful completion.
<>0: Failure. An error number is returned to indicate the error. (The errno variable is not set.)

ERRORS

The following error value is returned by pthread_cond_timedwait() if the corresponding condition is detected.

ETIMEDOUT: abstime has passed and a condition signal has not been received.

One of the following error values is returned by pthread_cond_wait() and pthread_cond_timedwait() if the corresponding condition is detected.

EFAULT

A cond, mutex, or abstime parameter points to an illegal address.

EINVAL

The value specified by cond, mutex, or abstime is invalid.

mutex is not owned by the calling thread. This error is not returned for a PTHREAD_MUTEX_NO_OWNER_NP, PTHREAD_MUTEX_NORMAL, or PTHREAD_MUTEX_DEFAULT mutex on HP-UX.

EINVAL

Different mutexes are being used for cond. This error is not detected on HP-UX.

WARNINGS

It is important to note that when pthread_cond_wait() or pthread_cond_timedwait() return without error, the associated predicate may still be false. When pthread_cond_timedwait() returns with the timeout error, the associated predicate may be true. It is recommended that a condition wait be enclosed in the equivalent of a "while loop," which checks the predicate.

Undefined behavior results if these functions are called with a PTHREAD_MUTEX_RECURSIVE mutex.

EXAMPLES

pthread_cond_wait() is recommended to be used in a loop testing the predicate associated with it. This will take care of any spurious wakeups that may occur.

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER; 
pthread_cond_t cond = PTHREAD_COND_INITIALIZER; 
  
(void)pthread_mutex_lock(&mutex); 
while (predicate == FALSE) { 
    (void)pthread_cond_wait(&cond, &mutex); 
} 
  
(void)pthread_mutex_unlock(&mutex); 

pthread_cond_timedwait() is also recommended to be used in a loop. This function can return success even if the predicate is not true. It should be called in a loop while checking the predicate. If the function times out, the predicate may still have become true. The predicate should be checked before processing the timeout case. The example given below does not do any other error checking.

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER; 
pthread_cond_t cond = PTHREAD_COND_INITIALIZER; 
struct timespec abstime; 
  
(void)pthread_mutex_lock(&mutex); 
abstime = absolute time to timeout. 
  
while (predicate == FALSE) { 
    ret = pthread_cond_timedwait(&cond, &mutex, &abstime); 
    if (ret == ETIMEDOUT) { 
        if (predicate == FALSE) { 
            /* Code for time-out condition */ 
        } else { 
            /* success condition */ 
            break; 
        } 
    } 
} 
(void)pthread_mutex_unlock(&mutex); 
  
Code for success condition. 

AUTHOR

pthread_cond_wait() and pthread_cond_timedwait() were derived from the IEEE POSIX P1003.1c standard.

STANDARDS CONFORMANCE

pthread_cond_wait(): POSIX 1003.1c.
pthread_cond_timedwait(): POSIX 1003.1c.