pthread_cond_wait manpage

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       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.

       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. The application shall ensure that these
       functions are called with mutex locked by the  calling  thread;	other-
       wise,  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 ``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. 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 predi-
       cate should be re-evaluated upon such return.

       When a thread waits on a condition variable, having specified a partic-
       ular    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

       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 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. When  such	 timeouts  occur,  pthread_cond_timed-
       wait() 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

       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.

       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.

       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  pre-
	      vious 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:

	      The time specified by abstime  to	 pthread_cond_timedwait()  has

       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  ter-
	      minated  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.


       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 incon-
       sistent state is [EOWNERDEAD].  Applications  that  do  not  check  the
       error  returns,	due to ruling out the possibility of such errors aris-
       ing, 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.

       If an implementation detects that the value specified by the cond argu-
       ment  to pthread_cond_timedwait() or pthread_cond_wait() does not refer
       to an initialized condition variable, or detects that the value	speci-
       fied   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]

   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.

       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 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 example,
       assume that clock_gettime() returns the	current	 time  and  cond_rela-
       tive_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

       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);
	       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 || mypredicate(&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 time-
       out  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.



       The  Base Definitions volume of POSIX.1-2008, Section 4.11, Memory Syn-
       chronization, <pthread.h>

       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),	The  Open  Group  Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
       cal and Electronics Engineers,  Inc  and	 The  Open  Group.   (This  is
       POSIX.1-2008  with  the	2013  Technical Corrigendum 1 applied.) In the
       event of any discrepancy between this version and the original IEEE and
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       is the referee document. The original Standard can be  obtained	online
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IEEE/The Open Group		     2013	    PTHREAD_COND_TIMEDWAIT(3P)