pthread_cond_wait

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.


       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.


       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
       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
                  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)