man pages section 3: Basic Library Functions

pthread_cond_wait(3C)

Name

pthread_cond_wait, pthread_cond_timedwait, pthread_cond_reltimedwait_np– wait on a condition

Synopsis

cc –mt [ flag... ] file... –lpthread [ library... ]
#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);

int pthread_cond_reltimedwait_np(pthread_cond_t *cond,
     pthread_mutex_t *mutex, const struct timespec *reltime);

Description

The pthread_cond_wait(), pthread_cond_timedwait(), and pthread_cond_reltimedwait_np() functions are used to block on a condition variable. They are called with mutex locked by the calling thread or undefined behavior will result.

These functions atomically release mutex and cause the calling thread to block on the condition variable cond. Atomically here means ``atomically with respect to access by another thread to the mutex and then the condition variable.” That is, if another thread is able to acquire the mutex after the about-to-block thread has released it, then a subsequent call to pthread_cond_signal() or pthread_cond_broadcast() in that thread behaves as if it were issued after the about-to-block thread has blocked.

Upon successful return, the mutex has been locked and is owned by the calling thread.

When using condition variables there is always a boolean predicate, an invariant, associated with each condition wait that must be true before the thread should proceed. Spurious wakeups from the pthread_cond_wait(), pthread_cond_timedwait(), or pthread_cond_reltimedwait_np() functions could occur. Since the return from pthread_cond_wait(), pthread_cond_timedwait(), or pthread_cond_reltimedwait_np() does not imply anything about the value of this predicate, the predicate should always be reevaluated.

The order in which blocked threads are awakened by pthread_cond_signal() or pthread_cond_broadcast() is determined by the scheduling policy. See pthreads(5).

The effect of using more than one mutex for concurrent pthread_cond_wait(), pthread_cond_timedwait(), or pthread_cond_reltimedwait_np() operations on the same condition variable will result in undefined behavior.

A condition wait (whether timed or not) is a cancellation point. When the cancelability enable state of a thread is set to PTHREAD_CANCEL_DEFERRED, a side effect of acting upon a cancellation request while in a condition wait is that the mutex is reacquired before calling the first cancellation cleanup handler.

A thread that has been unblocked because it has been canceled while blocked in a call to pthread_cond_wait() or pthread_cond_timedwait() does not consume any condition signal that may be directed concurrently at the condition variable if there are other threads blocked on the condition variable.

The pthread_cond_timedwait() function is the same as pthread_cond_wait() except that an error is returned if the absolute time specified by abstime passes (that is, system time equals or exceeds abstime) before the condition cond is signaled or broadcast, or if the absolute time specified by abstime has already been passed at the time of the call. The abstime argument is of type struct timespec, defined in time.h(3HEAD). When such time-outs occur, pthread_cond_timedwait() will nonetheless release and reacquire the mutex referenced by mutex. The function pthread_cond_timedwait() is also a cancellation point.

The pthread_cond_reltimedwait_np() function is a non-standard extension provided by the Solaris version of POSIX threads as indicated by the ``_np'' (non-portable) suffix. The pthread_cond_reltimedwait_np() function is the same as pthread_cond_timedwait() except that the reltime argument specifies a non-negative time relative to the current system time rather than an absolute time. The reltime argument is of type struct timespec, defined in time.h(3HEAD). An error value is returned if the relative time passes (that is, system time equals or exceeds the starting system time plus the relative time) before the condition cond is signaled or broadcast. When such timeouts occur, pthread_cond_reltimedwait_np() releases and reacquires the mutex referenced by mutex. The pthread_cond_reltimedwait_np() function is also a cancellation point.

If a signal is delivered to a thread waiting for a condition variable, upon return from the signal handler the thread resumes waiting for the condition variable as if it was not interrupted, or it returns 0 due to spurious wakeup.

Return Values

Except in the case of ETIMEDOUT, EOWNERDEAD, or ENOTRECOVERABLE, all of these error checks act as if they were performed immediately at the beginning of processing for the function and cause an error return, in effect, prior to modifying the state of the mutex specified by mutex or the condition variable specified by cond.

Upon successful completion, 0 is returned. Otherwise, an error value is returned to indicate the error.

Errors

The pthread_cond_timedwait() function will fail if:

ETIMEDOUT: The absolute time specified by abstime to pthread_cond_timedwait() has passed.

The pthread_cond_reltimedwait_np() function will fail if:

EINVAL: The value specified by reltime is invalid.
ETIMEDOUT: The relative time specified by reltime to pthread_cond_reltimedwait_np() has passed.

The pthread_cond_wait() and pthread_cond_timedwait() functions may fail if:

EINVAL: The value specified by cond, mutex, or abstime is invalid.
EINVAL: Different mutexes were supplied for concurrent pthread_cond_wait() or pthread_cond_timedwait(), operations on the same condition variable.
EPERM: The mutex was not owned by the current thread at the time of the call.

If the mutex specified by mutex is a robust mutex (initialized with the robustness attribute PTHREAD_MUTEX_ROBUST_NP), the pthread_cond_wait(), pthread_cond_timedwait(), and pthread_cond_reltimedwait_np() functions will, under the specified conditions, return the following error values. For complete information, see the pthread_mutex_lock(3C) and pthread_mutexattr_setrobust_np(3C) manual pages.

EOWNERDEAD: The last owner of this mutex died while holding the mutex, leaving the state it was protecting possibly inconsistent. The mutex is now owned by the caller.
ENOTRECOVERABLE: The mutex was protecting state that has now been left irrecoverable. The mutex has not been acquired.

Attributes

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE	ATTRIBUTE VALUE
Interface Stability	Standard
MT-Level	MT-Safe