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All Rights Reserved. .\" .TH PTHREAD_COND_WAIT 3C "Nov 11, 2008" .SH NAME pthread_cond_wait, pthread_cond_timedwait, pthread_cond_reltimedwait_np \- wait on a condition .SH SYNOPSIS .LP .nf cc -mt [ \fIflag\fR... ] \fIfile\fR... -lpthread [ \fIlibrary\fR... ] #include \fBint\fR \fBpthread_cond_wait\fR(\fBpthread_cond_t *restrict\fR \fIcond\fR, \fBpthread_mutex_t *restrict\fR \fImutex\fR); .fi .LP .nf \fBint\fR \fBpthread_cond_timedwait\fR(\fBpthread_cond_t *restrict\fR \fIcond\fR, \fBpthread_mutex_t *restrict\fR \fImutex\fR, \fBconst struct timespec *restrict\fR \fIabstime\fR); .fi .LP .nf \fBint\fR \fBpthread_cond_reltimedwait_np\fR(\fBpthread_cond_t *\fR\fIcond\fR, \fBpthread_mutex_t *\fR\fImutex\fR, \fBconst struct timespec *\fR\fIreltime\fR); .fi .SH DESCRIPTION .sp .LP The \fBpthread_cond_wait()\fR, \fBpthread_cond_timedwait()\fR, and \fBpthread_cond_reltimedwait_np()\fR functions are used to block on a condition variable. They are called with \fImutex\fR locked by the calling thread or undefined behavior will result. .sp .LP These functions atomically release \fImutex\fR and cause the calling thread to block on the condition variable \fIcond\fR. 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 \fBpthread_cond_signal()\fR or \fBpthread_cond_broadcast()\fR in that thread behaves as if it were issued after the about-to-block thread has blocked. .sp .LP Upon successful return, the mutex has been locked and is owned by the calling thread. If mutex is a robust mutex where an owner terminated while holding the lock and the state is recoverable, the mutex is acquired even though the function returns an error value. .sp .LP 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 \fBpthread_cond_wait()\fR, \fBpthread_cond_timedwait()\fR, or \fBpthread_cond_reltimedwait_np()\fR functions could occur. Since the return from \fBpthread_cond_wait()\fR, \fBpthread_cond_timedwait()\fR, or \fBpthread_cond_reltimedwait_np()\fR does not imply anything about the value of this predicate, the predicate should always be reevaluated. .sp .LP The order in which blocked threads are awakened by \fBpthread_cond_signal()\fR or \fBpthread_cond_broadcast()\fR is determined by the scheduling policy. See \fBpthreads\fR(7). .sp .LP The effect of using more than one mutex for concurrent \fBpthread_cond_wait()\fR, \fBpthread_cond_timedwait()\fR, or \fBpthread_cond_reltimedwait_np()\fR operations on the same condition variable will result in undefined behavior. .sp .LP A condition wait (whether timed or not) is a cancellation point. When the cancelability enable state of a thread is set to \fBPTHREAD_CANCEL_DEFERRED\fR, 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. .sp .LP A thread that has been unblocked because it has been canceled while blocked in a call to \fBpthread_cond_wait()\fR or \fBpthread_cond_timedwait()\fR 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. .sp .LP The \fBpthread_cond_timedwait()\fR function is the same as \fBpthread_cond_wait()\fR except that an error is returned if the absolute time specified by \fIabstime\fR passes (that is, system time equals or exceeds \fIabstime\fR) before the condition \fIcond\fR is signaled or broadcast, or if the absolute time specified by \fIabstime\fR has already been passed at the time of the call. The \fIabstime\fR argument is of type \fBstruct timespec\fR, defined in \fBtime.h\fR(3HEAD). When such time-outs occur, \fBpthread_cond_timedwait()\fR will nonetheless release and reacquire the mutex referenced by \fImutex\fR. The function \fBpthread_cond_timedwait()\fR is also a cancellation point. .sp .LP The \fBpthread_cond_reltimedwait_np()\fR function is a non-standard extension provided by the Solaris version of POSIX threads as indicated by the ``\fB_np\fR'' (non-portable) suffix. The \fBpthread_cond_reltimedwait_np()\fR function is the same as \fBpthread_cond_timedwait()\fR except that the \fIreltime\fR argument specifies a non-negative time relative to the current system time rather than an absolute time. The \fIreltime\fR argument is of type \fBstruct timespec\fR, defined in \fBtime.h\fR(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 \fIcond\fR is signaled or broadcast. When such timeouts occur, \fBpthread_cond_reltimedwait_np()\fR releases and reacquires the mutex referenced by \fImutex\fR. The \fBpthread_cond_reltimedwait_np()\fR function is also a cancellation point. .sp .LP 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 \fB0\fR due to spurious wakeup. .SH RETURN VALUES .sp .LP Except in the case of \fBETIMEDOUT\fR, \fBEOWNERDEAD\fR, or \fBENOTRECOVERABLE\fR, 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 \fImutex\fR or the condition variable specified by \fIcond\fR. .sp .LP Upon successful completion, \fB0\fR is returned. Otherwise, an error value is returned to indicate the error. .SH ERRORS .sp .LP These functions will fail if: .sp .ne 2 .na \fB\fBEPERM\fR\fR .ad .RS 9n The mutex type is \fBPTHREAD_MUTEX_ERRORCHECK\fR or the mutex is a robust mutex, and the current thread does not own the mutex. .RE .sp .LP The \fBpthread_cond_timedwait()\fR function will fail if: .sp .ne 2 .na \fB\fBETIMEDOUT\fR\fR .ad .RS 13n The absolute time specified by \fIabstime\fR to \fBpthread_cond_timedwait()\fR has passed. .RE .sp .LP The \fBpthread_cond_reltimedwait_np()\fR function will fail if: .sp .ne 2 .na \fB\fBEINVAL\fR\fR .ad .RS 13n The value specified by \fIreltime\fR is invalid. .RE .sp .ne 2 .na \fB\fBETIMEDOUT\fR\fR .ad .RS 13n The relative time specified by \fIreltime\fR to \fBpthread_cond_reltimedwait_np()\fR has passed. .RE .sp .LP These functions may fail if: .sp .ne 2 .na \fB\fBEINVAL\fR\fR .ad .RS 10n The value specified by \fIcond\fR, \fImutex\fR, \fIabstime\fR, or \fIreltime\fR is invalid. .RE .sp .ne 2 .na \fB\fBEINVAL\fR\fR .ad .RS 10n Different mutexes were supplied for concurrent operations on the same condition variable. .RE .sp .LP If the mutex specified by \fImutex\fR is a robust mutex (initialized with the robustness attribute \fBPTHREAD_MUTEX_ROBUST\fR), the \fBpthread_cond_wait()\fR, \fBpthread_cond_timedwait()\fR, and \fBpthread_cond_reltimedwait_np()\fR functions will, under the specified conditions, return the following error values. For complete information, see the \fBpthread_mutex_lock\fR(3C) and \fBpthread_mutexattr_setrobust\fR(3C) manual pages. .sp .ne 2 .na \fB\fBEOWNERDEAD\fR\fR .ad .RS 19n 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. .RE .sp .ne 2 .na \fB\fBENOTRECOVERABLE\fR\fR .ad .RS 19n The mutex was protecting state that has now been left irrecoverable. The mutex has not been acquired. .RE .SH ATTRIBUTES .sp .LP See \fBattributes\fR(7) for descriptions of the following attributes: .sp .sp .TS box; c | c l | l . ATTRIBUTE TYPE ATTRIBUTE VALUE _ Interface Stability Standard _ MT-Level MT-Safe .TE .SH SEE ALSO .sp .LP .BR pthread_cond_broadcast (3C), .BR pthread_cond_signal (3C), .BR pthread_mutex_lock (3C), .BR pthread_mutexattr_getrobust (3C), .BR time.h (3HEAD), .BR attributes (7), .BR condition (7), .BR pthreads (7), .BR standards (7)