pthread_cond_timedwait(3p) — Linux manual page



PROLOG         top

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

NAME         top

       pthread_cond_timedwait, pthread_cond_wait — wait on a condition

SYNOPSIS         top

       #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         top

       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)

       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,

RETURN VALUE         top

       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

ERRORS         top

       These functions shall fail if:

              The state protected by the mutex is not recoverable.

              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

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

       The pthread_cond_timedwait() function shall fail if:

              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:

              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         top



       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         top

       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

       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

           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

       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

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

       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.



SEE ALSO         top


       The Base Definitions volume of POSIX.1‐2017, Section 4.12, Memory
       Synchronization, pthread.h(0p)

COPYRIGHT         top

       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 .

       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 .

IEEE/The Open Group               2017        PTHREAD_COND_TIMEDWAIT(3P)

Pages that refer to this page: pthread.h(0p)clock_nanosleep(3p)pthread_cancel(3p)pthread_condattr_getclock(3p)pthread_cond_broadcast(3p)pthread_cond_destroy(3p)pthread_mutexattr_gettype(3p)