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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. 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 "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.
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.
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_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.
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 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
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.
RATIONALE
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.
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) 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_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 .