pthread_cond_wait(3C) 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);
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 behaviour 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_reltimed-
wait_np() functions may 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 re-evaluated.
The order in which blocked threads are awakened by pthread_cond_signal() or pthread_cond_broadcast() is determined by the scheduling pol-
icy. 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_CAN-
CEL_DEFERRED, a side effect of acting upon a cancellation request while in a condition wait is that the mutex is re-acquired 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 broadcasted, or if the absolute
time specified by abstime has already been passed at the time of the call. When such time-outs occur, pthread_cond_timedwait() will none-
theless 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 pthreads 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. 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 broadcasted. 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.
Except in the case of ETIMEDOUT, all 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 spec-
ified by cond.
Upon successful completion, 0 is returned. Otherwise, an error value is returned to indicate the error.
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 con-
dition variable.
EPERM The mutex was not owned by the current thread at the time of the call.
When a thread makes a call to pthread_cond_wait(), pthread_cond_timedwait(), or pthread_cond_reltimedwait_np(), if the symbol
_POSIX_THREAD_PRIO_INHERIT is defined and the mutex is initialized with the protocol attribute having the value PTHREAD_PRIO_INHERIT and
the robustness attribute having the value PTHREAD_MUTEX_ROBUST_NP (see pthread_mutexattr_getrobust_np(3C)), the pthread_cond_wait(),
pthread_cond_timedwait(), and pthread_cond_reltimedwait_np() functions will fail if:
EOWNERDEAD The last owner of this mutex died while holding the mutex. This mutex is now owned by the caller. The caller must
now attempt to make the state protected by the mutex consistent. If it is able to clean up the state, then it
should call pthread_mutex_consistent_np() for the mutex and unlock the mutex. Subsequent calls to
pthread_cond_wait(), pthread_cond_timedwait(), and pthread_cond_reltimedwait_np() will behave normally, as before.
If the caller is not able to clean up the state, pthread_mutex_consistent_np() should not be called for the mutex,
but it should be unlocked. Subsequent calls to pthread_cond_wait(), pthread_cond_timedwait(), and pthread_cond_rel-
timedwait_np() will fail to acquire the mutex with the error value ENOTRECOVERABLE. If the owner who acquired the
lock with EOWNERDEAD dies, the next owner will acquire the lock with EOWNERDEAD.
ENOTRECOVERABLE The mutex trying to be acquired is protecting the state that has been left irrecoverable by the mutex's last owner,
who died while holding the lock. The mutex has not been acquired. This condition can occur when the lock was previ-
ously acquired with EOWNERDEAD, and the owner was not able to clean up the state and unlocked the mutex without
making the mutex consistent.
See attributes(5) for descriptions of the following attributes:
+-----------------------------+-----------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+-----------------------------+-----------------------------+
|Interface Stability |Standard |
+-----------------------------+-----------------------------+
|MT-Level |MT-Safe |
+-----------------------------+-----------------------------+
pthread_cond_signal(3C), pthread_cond_broadcast(3C), attributes(5), condition(5), pthreads(5), standards(5)
23 Mar 2005 pthread_cond_wait(3C)