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Linux 2.6 - man page for pthread_cond_timedwait (linux section 3posix)


       pthread_cond_timedwait, pthread_cond_wait - wait on a condition

       #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);

       The  pthread_cond_timedwait() and pthread_cond_wait() functions shall block on a condition
       variable. They shall be called with mutex locked by the calling thread or undefined behav-
       ior results.

       These functions atomically release mutex and cause the calling thread to block on the con-
       dition 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

       When  using condition variables there is always a Boolean predicate involving shared vari-
       ables associated with each condition wait that is true if the thread should proceed.  Spu-
       rious  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.

       The effect of using  more  than	one  mutex  for  concurrent  pthread_cond_timedwait()  or
       pthread_cond_wait()  operations	on  the  same condition variable is undefined; that is, a
       condition variable becomes bound to a unique mutex when a thread waits  on  the	condition
       variable, and this (dynamic) binding shall end when the wait returns.

       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 (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	cancella-
       tion  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, sys-
       tem 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

       If the Clock Selection option is supported, 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.  When such timeouts occur, pthread_cond_timedwait()  shall  nonethe-
       less  release  and  re-acquire the mutex referenced by mutex. 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.

       Except in the case of [ETIMEDOUT], all these error checks shall act as if they  were  per-
       formed  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.

       The pthread_cond_timedwait() function shall fail if:

	      The time specified by abstime to pthread_cond_timedwait() has passed.

       The pthread_cond_timedwait() and pthread_cond_wait() functions may fail if:

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

       EINVAL Different  mutexes  were	supplied  for  concurrent   pthread_cond_timedwait()   or
	      pthread_cond_wait() operations on the same condition variable.

       EPERM  The mutex was not owned by the current thread at the time of the call.

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

       The following sections are informative.



   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.

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

       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 associ-
       ated 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  speci-
       fies  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 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 possi-
       ble 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  per-
       form 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  han-
       dlers.  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 excep-
       tion; 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 can-
       cellation 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 execu-
       tion (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  condi-
       tion variable only when there is already an active writer. So the efficiency of a synchro-
       nization 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 transpar-
       ent  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  par-
       ticular condition after a given amount of time. An example of its use follows:

	      (void) pthread_mutex_lock(&t.mn);
		  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);
		  if (rc == 0) 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.


       pthread_cond_signal()  ,  pthread_cond_broadcast()  ,  the  Base  Definitions  volume   of
       IEEE Std 1003.1-2001, <pthread.h>

       Portions  of  this  text  are  reprinted  and  reproduced in electronic form from IEEE Std
       1003.1, 2003 Edition, Standard for Information Technology  --  Portable	Operating  System
       Interface  (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 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 orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003			PTHREAD_COND_TIMEDWAIT(P)

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