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



PROLOG
       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
       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. They shall be called with mutex locked
       by the calling thread or undefined behavior results.

       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_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.

       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 predi-
       cate 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_CAN-
       CEL_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 can-
       cellation 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  concur-
       rently  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.

       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 nonetheless 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.

RETURN VALUE
       Except  in the case of [ETIMEDOUT], 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 modi-
       fying the state of the mutex specified by mutex or the condition	 vari-
       able specified by cond.

       Upon  successful	 completion, a value of zero shall be returned; other-
       wise, an error number shall be returned to indicate the error.

ERRORS
       The pthread_cond_timedwait() function shall fail if:

       ETIMEDOUT
	      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.

EXAMPLES
       None.

APPLICATION USAGE
       None.

RATIONALE
   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 predi-
       cate state change.

       Some implementations, particularly on a multi-processor, may  sometimes
       cause  multiple	threads to wake up when the condition variable is sig-
       naled 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 equiva-
       lent of a "while loop" that checks the predicate.

   Timed Wait Semantics
       An  absolute time measure was chosen for specifying the timeout parame-
       ter 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	 time-
       out  on	top of a function that specifies relative timeouts.  For exam-
       ple, assume that clock_gettime() returns the current time and cond_rel-
       ative_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 irrel-
       evant 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 oper-
       ator, it is expected that implementations process any timed wait expir-
       ing 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 call-
       ing  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  interfac-
       ing  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 condi-
       tion 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 pro-
       cessed 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 sit-
       uation. 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 unimpor-
       tant.  Since there needs to be at least one context switch per Ada ren-
       dezvous, the efficiency of waiting on a condition  variable  is	impor-
       tant. 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 decou-
       pled from condition wait. This was rejected because it is the  combined
       nature of the operation that, in fact, facilitates realtime implementa-
       tions. Those implementations can atomically move a high-priority thread
       between the condition variable and the mutex in a manner that is trans-
       parent to the caller. This can prevent extra context switches and  pro-
       vide  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 condi-
       tion 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 pro-
       ceed  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) setmystate(&t);
	      (void) pthread_mutex_unlock(&t.mn);

       By making the timeout parameter absolute, it does not need to be recom-
       puted  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_signal() , pthread_cond_broadcast() , the Base Definitions
       volume of IEEE Std 1003.1-2001, <pthread.h>

COPYRIGHT
       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 original 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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