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pthread_cond_wait(3c)
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
#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 condi‐
tion 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 "atomi‐
cally 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 sub‐
sequent 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 ac‐
quired 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_rel‐
timedwait_np() functions could occur. Since the return from
pthread_cond_wait(), pthread_cond_timedwait(), or pthread_cond_rel‐
timedwait_np() does not imply anything about the value of this predi‐
cate, the predicate should always be reevaluated.
The order in which blocked threads are awakened by pthread_cond_sig‐
nal() or pthread_cond_broadcast() is determined by the scheduling pol‐
icy. See pthreads(7).
The effect of using more than one mutex for concurrent
pthread_cond_wait(), pthread_cond_timedwait(), or pthread_cond_rel‐
timedwait_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 cancellability 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 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 ex‐
ceeds 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 mu‐
tex 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() func‐
tion 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_rel‐
timedwait_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 be‐
ginning 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 val‐
ues. 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(7) for descriptions of the following attributes:
tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i) ATTRIBUTE TYPEAT‐
TRIBUTE VALUE _ Interface StabilityCommitted _ MT-LevelMT-Safe _ Stan‐
dardSee standards(7).
SEE ALSO
pthread_cond_broadcast(3C), pthread_cond_signal(3C), pthread_mu‐
tex_lock(3C), pthread_mutexattr_getrobust(3C), time.h(3HEAD), attrib‐
utes(7), condition(7), pthreads(7), standards(7)
Oracle Solaris 11.4 13 Nov 2020 pthread_cond_wait(3C)