PTHREAD_COND_WAIT(3C) Standard C 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. 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.


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


These functions will fail if:

EPERM
The mutex type is PTHREAD_MUTEX_ERRORCHECK or the mutex is a
robust mutex, and the current thread does not own the mutex.


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.


These functions may fail if:

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


EINVAL
Different mutexes were supplied for concurrent operations on
the same condition variable.


If the mutex specified by mutex is a robust mutex (initialized with the
robustness attribute PTHREAD_MUTEX_ROBUST), 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(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 |
+--------------------+-----------------+

SEE ALSO


pthread_cond_signal(3C), pthread_cond_broadcast(3C),
pthread_mutex_lock(3C), pthread_mutexattr_getrobust(3C), time.h(3HEAD),
attributes(5), condition(5), pthreads(5), standards(5)


November 11, 2008 PTHREAD_COND_WAIT(3C)