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This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface
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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;
otherwise, 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 ``atomically 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 subsequent 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 predicate should be re-evaluated upon such return.
When a thread waits on a condition variable, having specified a particular 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 concurrently 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_timedwait() 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 for [ETIMEDOUT], [ENOTRECOVERABLE], and [EOWNERDEAD], 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 modifying the state of the mutex specified by mutex or the condition variable
specified by cond.
Upon successful completion, a value of zero shall be returned; otherwise, 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 previous 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 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.
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
inconsistent state is [EOWNERDEAD]. Applications that do not check the error returns, due to ruling out
the possibility of such errors arising, 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 argument to pthread_cond_timedwait() or
pthread_cond_wait() does not refer to an initialized condition variable, or detects that the value
specified 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 predicate 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 signaled 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 equivalent of a ``while loop'' that
checks the predicate.
Timed Wait Semantics
An absolute time measure was chosen for specifying the timeout parameter 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 timeout on top of a function that
specifies relative timeouts. For example, assume that clock_gettime() returns the current time and
cond_relative_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 irrelevant 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 operator, it is expected that
implementations process any timed wait expiring 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 calling 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
interfacing 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 condition 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 processed 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
situation. 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 unimportant. Since there needs to be at
least one context switch per Ada rendezvous, the efficiency of waiting on a condition variable is
important. 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 decoupled from condition wait. This was
rejected because it is the combined nature of the operation that, in fact, facilitates realtime
implementations. Those implementations can atomically move a high-priority thread between the condition
variable and the mutex in a manner that is transparent to the caller. This can prevent extra context
switches and provide 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 condition 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
proceed 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 recomputed 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_broadcast()
The Base Definitions volume of POSIX.1‐2017, Section 4.12, Memory Synchronization, <pthread.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .
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