pthread_cond_wait (3p) - Linux Man Pages
pthread_cond_wait: wait on a condition
PROLOGThis manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.
pthread_cond_timedwait, pthread_cond_wait - wait on a condition
int pthread_cond_timedwait(pthread_cond_t *restrict cond,
int pthread_cond_wait(pthread_cond_t *restrict cond,
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.
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.
The pthread_cond_timedwait() function shall fail if:
The time specified by abstime to pthread_cond_timedwait()
The pthread_cond_timedwait() and pthread_cond_wait() functions may fail if:
- The value specified by cond, mutex, or abstime is invalid.
- Different mutexes were supplied for concurrent pthread_cond_timedwait() or pthread_cond_wait() operations on the same condition variable.
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].
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.
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.
COPYRIGHTPortions 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 .