1PTHREAD_COND_TIMEDWAIT(3P) POSIX Programmer's ManualPTHREAD_COND_TIMEDWAIT(3P)
2
3
4

PROLOG

6       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
7       implementation of this interface may differ (consult the  corresponding
8       Linux  manual page for details of Linux behavior), or the interface may
9       not be implemented on Linux.
10

NAME

12       pthread_cond_timedwait, pthread_cond_wait — wait on a condition
13

SYNOPSIS

15       #include <pthread.h>
16
17       int pthread_cond_timedwait(pthread_cond_t *restrict cond,
18           pthread_mutex_t *restrict mutex,
19           const struct timespec *restrict abstime);
20       int pthread_cond_wait(pthread_cond_t *restrict cond,
21           pthread_mutex_t *restrict mutex);
22

DESCRIPTION

24       The pthread_cond_timedwait() and  pthread_cond_wait()  functions  shall
25       block  on a condition variable. The application shall ensure that these
26       functions are called with mutex locked by the  calling  thread;  other‐
27       wise,  an  error  (for  PTHREAD_MUTEX_ERRORCHECK and robust mutexes) or
28       undefined behavior (for other mutexes) results.
29
30       These functions atomically release mutex and cause the  calling  thread
31       to block on the condition variable cond; atomically here means ``atomi‐
32       cally with respect to access by another thread to the  mutex  and  then
33       the condition variable''. That is, if another thread is able to acquire
34       the mutex after the about-to-block thread has released it, then a  sub‐
35       sequent  call  to  pthread_cond_broadcast() or pthread_cond_signal() in
36       that thread shall behave as if it were issued after the  about-to-block
37       thread has blocked.
38
39       Upon  successful  return, the mutex shall have been locked and shall be
40       owned by the calling thread. If mutex is a robust mutex where an  owner
41       terminated  while  holding  the  lock and the state is recoverable, the
42       mutex shall be acquired even though the function returns an error code.
43
44       When using condition variables there  is  always  a  Boolean  predicate
45       involving  shared variables associated with each condition wait that is
46       true  if  the  thread  should  proceed.  Spurious  wakeups   from   the
47       pthread_cond_timedwait()  or  pthread_cond_wait()  functions may occur.
48       Since the return from pthread_cond_timedwait()  or  pthread_cond_wait()
49       does  not  imply anything about the value of this predicate, the predi‐
50       cate should be re-evaluated upon such return.
51
52       When a thread waits on a condition variable, having specified a partic‐
53       ular    mutex   to   either   the   pthread_cond_timedwait()   or   the
54       pthread_cond_wait() operation, a dynamic binding is formed between that
55       mutex and condition variable that remains in effect as long as at least
56       one thread is blocked on the condition variable. During this time,  the
57       effect  of  an attempt by any thread to wait on that condition variable
58       using a different mutex is undefined. Once  all  waiting  threads  have
59       been unblocked (as by the pthread_cond_broadcast() operation), the next
60       wait operation on that condition variable  shall  form  a  new  dynamic
61       binding  with  the  mutex specified by that wait operation. Even though
62       the dynamic binding between condition variable and mutex may be removed
63       or  replaced  between the time a thread is unblocked from a wait on the
64       condition variable and the time that it returns to the caller or begins
65       cancellation  cleanup, the unblocked thread shall always re-acquire the
66       mutex specified in the condition wait operation call from which  it  is
67       returning.
68
69       A  condition  wait (whether timed or not) is a cancellation point. When
70       the cancelability type of a thread is set to PTHREAD_CANCEL_DEFERRED, a
71       side-effect  of acting upon a cancellation request while in a condition
72       wait is that the mutex is (in effect) re-acquired  before  calling  the
73       first cancellation cleanup handler. The effect is as if the thread were
74       unblocked, allowed to execute up to the point  of  returning  from  the
75       call  to  pthread_cond_timedwait()  or pthread_cond_wait(), but at that
76       point notices the cancellation request and instead of returning to  the
77       caller  of  pthread_cond_timedwait() or pthread_cond_wait(), starts the
78       thread cancellation activities,  which  includes  calling  cancellation
79       cleanup handlers.
80
81       A  thread  that  has  been unblocked because it has been canceled while
82       blocked in a call to  pthread_cond_timedwait()  or  pthread_cond_wait()
83       shall  not  consume  any  condition signal that may be directed concur‐
84       rently at the condition variable if there are other threads blocked  on
85       the condition variable.
86
87       The   pthread_cond_timedwait()   function   shall   be   equivalent  to
88       pthread_cond_wait(), except that an error is returned if  the  absolute
89       time  specified  by  abstime  passes  (that  is,  system time equals or
90       exceeds abstime) before the condition cond is signaled or  broadcasted,
91       or if the absolute time specified by abstime has already been passed at
92       the time of the call. When  such  timeouts  occur,  pthread_cond_timed‐
93       wait() shall nonetheless release and re-acquire the mutex referenced by
94       mutex, and may consume a condition signal directed concurrently at  the
95       condition variable.
96
97       The condition variable shall have a clock attribute which specifies the
98       clock that shall be used to measure the time specified by  the  abstime
99       argument.  The pthread_cond_timedwait() function is also a cancellation
100       point.
101
102       If a signal is delivered to a thread waiting for a condition  variable,
103       upon  return from the signal handler the thread resumes waiting for the
104       condition variable as if it was not interrupted,  or  it  shall  return
105       zero due to spurious wakeup.
106
107       The  behavior  is undefined if the value specified by the cond or mutex
108       argument to these functions does not refer to an initialized  condition
109       variable or an initialized mutex object, respectively.
110

RETURN VALUE

112       Except  for [ETIMEDOUT], [ENOTRECOVERABLE], and [EOWNERDEAD], all these
113       error checks shall act as if they were  performed  immediately  at  the
114       beginning  of  processing  for  the  function  and shall cause an error
115       return, in effect, prior to modifying the state of the mutex  specified
116       by mutex or the condition variable specified by cond.
117
118       Upon  successful  completion, a value of zero shall be returned; other‐
119       wise, an error number shall be returned to indicate the error.
120

ERRORS

122       These functions shall fail if:
123
124       ENOTRECOVERABLE
125              The state protected by the mutex is not recoverable.
126
127       EOWNERDEAD
128              The mutex is a robust mutex and the process containing the  pre‐
129              vious owning thread terminated while holding the mutex lock. The
130              mutex lock shall be acquired by the calling thread and it is  up
131              to the new owner to make the state consistent.
132
133       EPERM  The  mutex  type  is  PTHREAD_MUTEX_ERRORCHECK or the mutex is a
134              robust mutex, and the current thread does not own the mutex.
135
136       The pthread_cond_timedwait() function shall fail if:
137
138       ETIMEDOUT
139              The time specified by abstime  to  pthread_cond_timedwait()  has
140              passed.
141
142       EINVAL The abstime argument specified a nanosecond value less than zero
143              or greater than or equal to 1000 million.
144
145       These functions may fail if:
146
147       EOWNERDEAD
148              The mutex is a robust mutex and the previous owning thread  ter‐
149              minated  while  holding  the mutex lock. The mutex lock shall be
150              acquired by the calling thread and it is up to the new owner  to
151              make the state consistent.
152
153       These functions shall not return an error code of [EINTR].
154
155       The following sections are informative.
156

EXAMPLES

158       None.
159

APPLICATION USAGE

161       Applications  that  have assumed that non-zero return values are errors
162       will need updating for use with robust mutexes, since  a  valid  return
163       for  a  thread acquiring a mutex which is protecting a currently incon‐
164       sistent state is [EOWNERDEAD].  Applications  that  do  not  check  the
165       error  returns,  due to ruling out the possibility of such errors aris‐
166       ing, should not use robust mutexes. If an application  is  supposed  to
167       work  with normal and robust mutexes, it should check all return values
168       for error conditions and if necessary take appropriate action.
169

RATIONALE

171       If an implementation detects that the value specified by the cond argu‐
172       ment  to pthread_cond_timedwait() or pthread_cond_wait() does not refer
173       to an initialized condition variable, or detects that the value  speci‐
174       fied   by   the   mutex   argument   to   pthread_cond_timedwait()   or
175       pthread_cond_wait() does not refer to an initialized mutex  object,  it
176       is  recommended  that  the  function should fail and report an [EINVAL]
177       error.
178
179   Condition Wait Semantics
180       It  is  important   to   note   that   when   pthread_cond_wait()   and
181       pthread_cond_timedwait() return without error, the associated predicate
182       may still be false.  Similarly, when  pthread_cond_timedwait()  returns
183       with  the timeout error, the associated predicate may be true due to an
184       unavoidable race between the expiration of the timeout and  the  predi‐
185       cate state change.
186
187       The application needs to recheck the predicate on any return because it
188       cannot be sure there is another thread waiting on the thread to  handle
189       the  signal, and if there is not then the signal is lost. The burden is
190       on the application to check the predicate.
191
192       Some implementations, particularly on a multi-processor, may  sometimes
193       cause  multiple  threads to wake up when the condition variable is sig‐
194       naled simultaneously on different processors.
195
196       In general, whenever a condition wait returns, the thread  has  to  re-
197       evaluate  the predicate associated with the condition wait to determine
198       whether it can safely proceed, should wait again, or should  declare  a
199       timeout.  A  return  from  the  wait does not imply that the associated
200       predicate is either true or false.
201
202       It is thus recommended that a condition wait be enclosed in the equiva‐
203       lent of a ``while loop'' that checks the predicate.
204
205   Timed Wait Semantics
206       An  absolute time measure was chosen for specifying the timeout parame‐
207       ter for two reasons. First, a  relative  time  measure  can  be  easily
208       implemented  on  top  of  a  function that specifies absolute time, but
209       there is a race condition associated with specifying an absolute  time‐
210       out on top of a function that specifies relative timeouts. For example,
211       assume that clock_gettime() returns the  current  time  and  cond_rela‐
212       tive_timed_wait() uses relative timeouts:
213
214
215           clock_gettime(CLOCK_REALTIME, &now)
216           reltime = sleep_til_this_absolute_time -now;
217           cond_relative_timed_wait(c, m, &reltime);
218
219       If  the  thread  is  preempted between the first statement and the last
220       statement, the thread blocks for too long. Blocking, however, is irrel‐
221       evant if an absolute timeout is used. An absolute timeout also need not
222       be recomputed if it is used multiple times in  a  loop,  such  as  that
223       enclosing a condition wait.
224
225       For cases when the system clock is advanced discontinuously by an oper‐
226       ator, it is expected that implementations process any timed wait expir‐
227       ing at an intervening time as if that time had actually occurred.
228
229   Cancellation and Condition Wait
230       A  condition  wait, whether timed or not, is a cancellation point. That
231       is, the functions pthread_cond_wait() or  pthread_cond_timedwait()  are
232       points where a pending (or concurrent) cancellation request is noticed.
233       The reason for this is that an indefinite wait  is  possible  at  these
234       points—whatever  event  is  being  waited  for,  even if the program is
235       totally correct, might never occur; for example, some input data  being
236       awaited  might  never  be sent. By making condition wait a cancellation
237       point, the thread can be canceled and perform its cancellation  cleanup
238       handler even though it may be stuck in some indefinite wait.
239
240       A  side-effect  of  acting  on a cancellation request while a thread is
241       blocked on a condition variable is to re-acquire the mutex before call‐
242       ing  any of the cancellation cleanup handlers. This is done in order to
243       ensure that the cancellation cleanup handler is executed  in  the  same
244       state  as the critical code that lies both before and after the call to
245       the condition wait function. This rule is also required when  interfac‐
246       ing  to  POSIX  threads  from  languages, such as Ada or C++, which may
247       choose to map cancellation onto a language exception; this rule ensures
248       that  each  exception  handler  guarding  a critical section can always
249       safely depend upon the fact that the associated mutex has already  been
250       locked  regardless  of  exactly  where  within the critical section the
251       exception was raised. Without this rule, there would not be  a  uniform
252       rule  that  exception  handlers could follow regarding the lock, and so
253       coding would become very cumbersome.
254
255       Therefore, since some statement has to be made regarding the  state  of
256       the  lock  when a cancellation is delivered during a wait, a definition
257       has been chosen that makes application coding most convenient and error
258       free.
259
260       When  acting  on  a cancellation request while a thread is blocked on a
261       condition variable, the implementation is required to ensure  that  the
262       thread  does  not consume any condition signals directed at that condi‐
263       tion variable if there are any other threads waiting on that  condition
264       variable.  This rule is specified in order to avoid deadlock conditions
265       that could occur if these two independent requests  (one  acting  on  a
266       thread  and  the  other acting on the condition variable) were not pro‐
267       cessed independently.
268
269   Performance of Mutexes and Condition Variables
270       Mutexes are expected to be locked only for  a  few  instructions.  This
271       practice  is almost automatically enforced by the desire of programmers
272       to avoid long serial regions of execution  (which  would  reduce  total
273       effective parallelism).
274
275       When  using  mutexes  and condition variables, one tries to ensure that
276       the usual case is to lock the mutex, access shared data, and unlock the
277       mutex. Waiting on a condition variable should be a relatively rare sit‐
278       uation. For example, when implementing a  read-write  lock,  code  that
279       acquires  a  read-lock  typically  needs only to increment the count of
280       readers (under mutual-exclusion) and return. The calling  thread  would
281       actually  wait  on the condition variable only when there is already an
282       active writer. So the efficiency  of  a  synchronization  operation  is
283       bounded  by  the  cost  of mutex lock/unlock and not by condition wait.
284       Note that in the usual case there is no context switch.
285
286       This is not to say that the efficiency of condition waiting is unimpor‐
287       tant.  Since there needs to be at least one context switch per Ada ren‐
288       dezvous, the efficiency of waiting on a condition  variable  is  impor‐
289       tant. The cost of waiting on a condition variable should be little more
290       than the minimal cost for a context switch plus the time to unlock  and
291       lock the mutex.
292
293   Features of Mutexes and Condition Variables
294       It  had been suggested that the mutex acquisition and release be decou‐
295       pled from condition wait. This was rejected because it is the  combined
296       nature of the operation that, in fact, facilitates realtime implementa‐
297       tions. Those implementations can atomically move a high-priority thread
298       between the condition variable and the mutex in a manner that is trans‐
299       parent to the caller. This can prevent extra context switches and  pro‐
300       vide  more deterministic acquisition of a mutex when the waiting thread
301       is signaled. Thus, fairness and  priority  issues  can  be  dealt  with
302       directly by the scheduling discipline.  Furthermore, the current condi‐
303       tion wait operation matches existing practice.
304
305   Scheduling Behavior of Mutexes and Condition Variables
306       Synchronization primitives that attempt to  interfere  with  scheduling
307       policy  by  specifying  an  ordering  rule  are considered undesirable.
308       Threads waiting on mutexes and condition variables are selected to pro‐
309       ceed  in  an  order dependent upon the scheduling policy rather than in
310       some fixed order (for example, FIFO or priority). Thus, the  scheduling
311       policy determines which thread(s) are awakened and allowed to proceed.
312
313   Timed Condition Wait
314       The  pthread_cond_timedwait() function allows an application to give up
315       waiting for a particular condition after a given  amount  of  time.  An
316       example of its use follows:
317
318
319           (void) pthread_mutex_lock(&t.mn);
320               t.waiters++;
321               clock_gettime(CLOCK_REALTIME, &ts);
322               ts.tv_sec += 5;
323               rc = 0;
324               while (! mypredicate(&t) && rc == 0)
325                   rc = pthread_cond_timedwait(&t.cond, &t.mn, &ts);
326               t.waiters--;
327               if (rc == 0 || mypredicate(&t))
328                   setmystate(&t);
329           (void) pthread_mutex_unlock(&t.mn);
330
331       By making the timeout parameter absolute, it does not need to be recom‐
332       puted each time the program checks its blocking predicate. If the time‐
333       out  was  relative,  it  would  have to be recomputed before each call.
334       This would be especially difficult since such code would need  to  take
335       into  account  the  possibility of extra wakeups that result from extra
336       broadcasts or signals on  the  condition  variable  that  occur  before
337       either the predicate is true or the timeout is due.
338

FUTURE DIRECTIONS

340       None.
341

SEE ALSO

343       pthread_cond_broadcast()
344
345       The  Base Definitions volume of POSIX.1‐2017, Section 4.12, Memory Syn‐
346       chronization, <pthread.h>
347
349       Portions of this text are reprinted and reproduced in  electronic  form
350       from  IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
351       table Operating System Interface (POSIX), The Open Group Base  Specifi‐
352       cations  Issue  7, 2018 Edition, Copyright (C) 2018 by the Institute of
353       Electrical and Electronics Engineers, Inc and The Open Group.   In  the
354       event of any discrepancy between this version and the original IEEE and
355       The Open Group Standard, the original IEEE and The Open Group  Standard
356       is  the  referee document. The original Standard can be obtained online
357       at http://www.opengroup.org/unix/online.html .
358
359       Any typographical or formatting errors that appear  in  this  page  are
360       most likely to have been introduced during the conversion of the source
361       files to man page format. To report such errors,  see  https://www.ker
362       nel.org/doc/man-pages/reporting_bugs.html .
363
364
365
366IEEE/The Open Group                  2017           PTHREAD_COND_TIMEDWAIT(3P)
Impressum