PTHREAD_COND_TIMEDWAIT(3)  POSIX Programmer's Manual PTHREAD_COND_TIMEDWAIT(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       pthread_cond_timedwait, pthread_cond_wait -- wait on a condition

SYNOPSIS
       #include <pthread.h>

       int pthread_cond_timedwait(pthread_cond_t *restrict cond,
           pthread_mutex_t *restrict mutex,
           const struct timespec *restrict abstime);
       int pthread_cond_wait(pthread_cond_t *restrict cond,
           pthread_mutex_t *restrict mutex);

DESCRIPTION
       The pthread__cond__timedwait() and pthread__cond__wait() functions shall
       block on a condition variable. The application shall ensure that these
       functions are called with mutex locked by the calling thread;
       otherwise, an error (for PTHREAD_MUTEX_ERRORCHECK and robust mutexes)
       or undefined behavior (for other mutexes) results.

       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__broadcast() or pthread__cond__signal()
       in that thread shall behave as if it were issued after the about-to-
       block thread has blocked.

       Upon successful return, the mutex shall have been locked and shall be
       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 shall be acquired even though the function returns an error code.

       When using condition variables there is always a Boolean predicate
       involving shared variables associated with each condition wait that is
       true if the thread should proceed. Spurious wakeups from the
       pthread__cond__timedwait() or pthread__cond__wait() functions may occur.
       Since the return from pthread__cond__timedwait() or pthread__cond__wait()
       does not imply anything about the value of this predicate, the
       predicate should be re-evaluated upon such return.

       When a thread waits on a condition variable, having specified a
       particular mutex to either the pthread__cond__timedwait() or the
       pthread__cond__wait() operation, a dynamic binding is formed between that
       mutex and condition variable that remains in effect as long as at least
       one thread is blocked on the condition variable. During this time, the
       effect of an attempt by any thread to wait on that condition variable
       using a different mutex is undefined. Once all waiting threads have
       been unblocked (as by the pthread__cond__broadcast() operation), the next
       wait operation on that condition variable shall form a new dynamic
       binding with the mutex specified by that wait operation. Even though
       the dynamic binding between condition variable and mutex may be removed
       or replaced between the time a thread is unblocked from a wait on the
       condition variable and the time that it returns to the caller or begins
       cancellation cleanup, the unblocked thread shall always re-acquire the
       mutex specified in the condition wait operation call from which it is
       returning.

       A condition wait (whether timed or not) is a cancellation point. When
       the cancelability type 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 (in effect) re-acquired before calling the
       first cancellation cleanup handler. The effect is as if the thread were
       unblocked, allowed to execute up to the point of returning from the
       call to pthread__cond__timedwait() or pthread__cond__wait(), but at that
       point notices the cancellation request and instead of returning to the
       caller of pthread__cond__timedwait() or pthread__cond__wait(), starts the
       thread cancellation activities, which includes calling cancellation
       cleanup handlers.

       A thread that has been unblocked because it has been canceled while
       blocked in a call to pthread__cond__timedwait() or pthread__cond__wait()
       shall 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 shall be equivalent to
       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 timeouts occur,
       pthread__cond__timedwait() shall nonetheless release and re-acquire the
       mutex referenced by mutex, and may consume a condition signal directed
       concurrently at the condition variable.

       The condition variable shall have a clock attribute which specifies the
       clock that shall be used to measure the time specified by the abstime
       argument. The pthread__cond__timedwait() 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 shall return
       zero due to spurious wakeup.

       The behavior is undefined if the value specified by the cond or mutex
       argument to these functions does not refer to an initialized condition
       variable or an initialized mutex object, respectively.

RETURN VALUE
       Except for [ETIMEDOUT], [ENOTRECOVERABLE], and [EOWNERDEAD], all these
       error checks shall act as if they were performed immediately at the
       beginning of processing for the function and shall 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, a value of zero shall be returned;
       otherwise, an error number shall be returned to indicate the error.

ERRORS
       These functions shall fail if:

       ENOTRECOVERABLE
              The state protected by the mutex is not recoverable.

       EOWNERDEAD
              The mutex is a robust mutex and the process containing the
              previous owning thread terminated while holding the mutex lock.
              The mutex lock shall be acquired by the calling thread and it is
              up to the new owner to make the state consistent.

       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 shall fail if:

       ETIMEDOUT
              The time specified by abstime to pthread__cond__timedwait() has
              passed.

       EINVAL The abstime argument specified a nanosecond value less than zero
              or greater than or equal to 1000 million.

       These functions may fail if:

       EOWNERDEAD
              The mutex is a robust mutex and the previous owning thread
              terminated while holding the mutex lock. The mutex lock shall be
              acquired by the calling thread and it is up to the new owner to
              make the state consistent.

       These functions shall not return an error code of [EINTR].

       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       Applications that have assumed that non-zero return values are errors
       will need updating for use with robust mutexes, since a valid return
       for a thread acquiring a mutex which is protecting a currently
       inconsistent state is [EOWNERDEAD].  Applications that do not check the
       error returns, due to ruling out the possibility of such errors
       arising, should not use robust mutexes. If an application is supposed
       to work with normal and robust mutexes, it should check all return
       values for error conditions and if necessary take appropriate action.

RATIONALE
       If an implementation detects that the value specified by the cond
       argument to pthread__cond__timedwait() or pthread__cond__wait() does not
       refer to an initialized condition variable, or detects that the value
       specified by the mutex argument to pthread__cond__timedwait() or
       pthread__cond__wait() does not refer to an initialized mutex object, it
       is recommended that the function should fail and report an [EINVAL]
       error.

   Condition Wait Semantics
       It is important to note that when pthread__cond__wait() and
       pthread__cond__timedwait() return without error, the associated predicate
       may still be false.  Similarly, when pthread__cond__timedwait() returns
       with the timeout error, the associated predicate may be true due to an
       unavoidable race between the expiration of the timeout and the
       predicate state change.

       The application needs to recheck the predicate on any return because it
       cannot be sure there is another thread waiting on the thread to handle
       the signal, and if there is not then the signal is lost. The burden is
       on the application to check the predicate.

       Some implementations, particularly on a multi-processor, may sometimes
       cause multiple threads to wake up when the condition variable is
       signaled simultaneously on different processors.

       In general, whenever a condition wait returns, the thread has to re-
       evaluate the predicate associated with the condition wait to determine
       whether it can safely proceed, should wait again, or should declare a
       timeout. A return from the wait does not imply that the associated
       predicate is either true or false.

       It is thus recommended that a condition wait be enclosed in the
       equivalent of a ``while loop'' that checks the predicate.

   Timed Wait Semantics
       An absolute time measure was chosen for specifying the timeout
       parameter for two reasons. First, a relative time measure can be easily
       implemented on top of a function that specifies absolute time, but
       there is a race condition associated with specifying an absolute
       timeout on top of a function that specifies relative timeouts. For
       example, assume that clock__gettime() returns the current time and
       cond__relative__timed__wait() uses relative timeouts:


           clock_gettime(CLOCK_REALTIME, &now)
           reltime = sleep_til_this_absolute_time -now;
           cond_relative_timed_wait(c, m, &reltime);

       If the thread is preempted between the first statement and the last
       statement, the thread blocks for too long. Blocking, however, is
       irrelevant if an absolute timeout is used. An absolute timeout also
       need not be recomputed if it is used multiple times in a loop, such as
       that enclosing a condition wait.

       For cases when the system clock is advanced discontinuously by an
       operator, it is expected that implementations process any timed wait
       expiring at an intervening time as if that time had actually occurred.

   Cancellation and Condition Wait
       A condition wait, whether timed or not, is a cancellation point. That
       is, the functions pthread__cond__wait() or pthread__cond__timedwait() are
       points where a pending (or concurrent) cancellation request is noticed.
       The reason for this is that an indefinite wait is possible at these
       points--whatever event is being waited for, even if the program is
       totally correct, might never occur; for example, some input data being
       awaited might never be sent. By making condition wait a cancellation
       point, the thread can be canceled and perform its cancellation cleanup
       handler even though it may be stuck in some indefinite wait.

       A side-effect of acting on a cancellation request while a thread is
       blocked on a condition variable is to re-acquire the mutex before
       calling any of the cancellation cleanup handlers. This is done in order
       to ensure that the cancellation cleanup handler is executed in the same
       state as the critical code that lies both before and after the call to
       the condition wait function. This rule is also required when
       interfacing to POSIX threads from languages, such as Ada or C++, which
       may choose to map cancellation onto a language exception; this rule
       ensures that each exception handler guarding a critical section can
       always safely depend upon the fact that the associated mutex has
       already been locked regardless of exactly where within the critical
       section the exception was raised. Without this rule, there would not be
       a uniform rule that exception handlers could follow regarding the lock,
       and so coding would become very cumbersome.

       Therefore, since some statement has to be made regarding the state of
       the lock when a cancellation is delivered during a wait, a definition
       has been chosen that makes application coding most convenient and error
       free.

       When acting on a cancellation request while a thread is blocked on a
       condition variable, the implementation is required to ensure that the
       thread does not consume any condition signals directed at that
       condition variable if there are any other threads waiting on that
       condition variable. This rule is specified in order to avoid deadlock
       conditions that could occur if these two independent requests (one
       acting on a thread and the other acting on the condition variable) were
       not processed independently.

   Performance of Mutexes and Condition Variables
       Mutexes are expected to be locked only for a few instructions. This
       practice is almost automatically enforced by the desire of programmers
       to avoid long serial regions of execution (which would reduce total
       effective parallelism).

       When using mutexes and condition variables, one tries to ensure that
       the usual case is to lock the mutex, access shared data, and unlock the
       mutex. Waiting on a condition variable should be a relatively rare
       situation. For example, when implementing a read-write lock, code that
       acquires a read-lock typically needs only to increment the count of
       readers (under mutual-exclusion) and return. The calling thread would
       actually wait on the condition variable only when there is already an
       active writer. So the efficiency of a synchronization operation is
       bounded by the cost of mutex lock/unlock and not by condition wait.
       Note that in the usual case there is no context switch.

       This is not to say that the efficiency of condition waiting is
       unimportant. Since there needs to be at least one context switch per
       Ada rendezvous, the efficiency of waiting on a condition variable is
       important. The cost of waiting on a condition variable should be little
       more than the minimal cost for a context switch plus the time to unlock
       and lock the mutex.

   Features of Mutexes and Condition Variables
       It had been suggested that the mutex acquisition and release be
       decoupled from condition wait. This was rejected because it is the
       combined nature of the operation that, in fact, facilitates realtime
       implementations. Those implementations can atomically move a high-
       priority thread between the condition variable and the mutex in a
       manner that is transparent to the caller. This can prevent extra
       context switches and provide more deterministic acquisition of a mutex
       when the waiting thread is signaled. Thus, fairness and priority issues
       can be dealt with directly by the scheduling discipline.  Furthermore,
       the current condition wait operation matches existing practice.

   Scheduling Behavior of Mutexes and Condition Variables
       Synchronization primitives that attempt to interfere with scheduling
       policy by specifying an ordering rule are considered undesirable.
       Threads waiting on mutexes and condition variables are selected to
       proceed in an order dependent upon the scheduling policy rather than in
       some fixed order (for example, FIFO or priority). Thus, the scheduling
       policy determines which thread(s) are awakened and allowed to proceed.

   Timed Condition Wait
       The pthread__cond__timedwait() function allows an application to give up
       waiting for a particular condition after a given amount of time. An
       example of its use follows:


           (void) pthread_mutex_lock(&t.mn);
               t.waiters++;
               clock_gettime(CLOCK_REALTIME, &ts);
               ts.tv_sec += 5;
               rc = 0;
               while (! mypredicate(&t) && rc == 0)
                   rc = pthread_cond_timedwait(&t.cond, &t.mn, &ts);
               t.waiters--;
               if (rc == 0 || mypredicate(&t))
                   setmystate(&t);
           (void) pthread_mutex_unlock(&t.mn);

       By making the timeout parameter absolute, it does not need to be
       recomputed each time the program checks its blocking predicate. If the
       timeout was relative, it would have to be recomputed before each call.
       This would be especially difficult since such code would need to take
       into account the possibility of extra wakeups that result from extra
       broadcasts or signals on the condition variable that occur before
       either the predicate is true or the timeout is due.

FUTURE DIRECTIONS
       None.

SEE ALSO
       pthread__cond__broadcast()

       The Base Definitions volume of POSIX.1-2017, Section 4.12, Memory
       Synchronization, <pthread.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  In the event of any discrepancy between this version and the
       original IEEE and The Open Group Standard, the original IEEE and The
       Open Group Standard is the referee document. The original Standard can
       be obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017            PTHREAD_COND_TIMEDWAIT(3)
