PTHREAD_COND_TIMEDWAIT(3P) POSIX Programmer's ManualPTHREAD_COND_TIMEDWAIT(3P)
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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. 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
returning.
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
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.
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.
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
These functions shall fail if:
ENOTRECOVERABLE
The state protected by the mutex is not recoverable.
EOWNERDEAD
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:
ETIMEDOUT
The time specified by abstime to pthread_cond_timedwait() has
passed.
EINVAL The abstime argument specified a nanosecond value less than zero
or greater than or equal to 1000 million.
These functions may fail if:
EOWNERDEAD
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.
EXAMPLES
None.
APPLICATION USAGE
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.
RATIONALE
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]
error.
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
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 || mypredicate(&t))
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 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.
FUTURE DIRECTIONS
None.
SEE ALSO
pthread_cond_broadcast()
The Base Definitions volume of POSIX.1-2008, Section 4.11, Memory Syn-
chronization, <pthread.h>
COPYRIGHT
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
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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the source
files to man page format. To report such errors, see https://www.ker-
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IEEE/The Open Group 2013 PTHREAD_COND_TIMEDWAIT(3P)
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