std::condition_variable_any::wait_until

 

C++

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Technical Specifications

 

Thread support library

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thread (C++11)
this_thread namespace
get_id (C++11) yield (C++11)	sleep_for (C++11) sleep_until (C++11)
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mutex (C++11) recursive_mutex (C++11) shared_mutex (C++17)	timed_mutex (C++11) recursive_timed_mutex (C++11) shared_timed_mutex (C++14)
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Condition variables
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condition_variable_any (C++11)
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promise (C++11) future (C++11) shared_future (C++11) packaged_task (C++11) async (C++11)	launch (C++11) future_status (C++11) future_error (C++11) future_category (C++11) future_errc (C++11)

 

std::condition_variable_any

Member functions

condition_variable_any::condition_variable_any

condition_variable_any::~condition_variable_any

Notification

condition_variable_any::notify_one

condition_variable_any::notify_all

Waiting

condition_variable_any::wait

condition_variable_any::wait_for

condition_variable_any::wait_until

 

template< class Lock, class Clock, class Duration > std::cv_status wait_until( Lock& lock,                            const std::chrono::time_point<Clock, Duration>& timeout_time );	(1)	(since C++11)
template< class Lock, class Clock, class Duration, class Predicate > bool wait_until( Lock& lock,                  const std::chrono::time_point<Clock, Duration>& timeout_time,                  Predicate pred );	(2)	(since C++11)

wait_until causes the current thread to block until the condition variable is notified, a specific time is reached, or a spurious wakeup occurs, optionally looping until some predicate is satisfied.

1) Atomically releases lock, blocks the current executing thread, and adds it to the list of threads waiting on *this. The thread will be unblocked when notify_all() or notify_one() is executed, or when the absolute time point timeout_time is reached. It may also be unblocked spuriously. When unblocked, regardless of the reason, lock is reacquired and wait_until exits. If this function exits via exception, lock is also reacquired. (until C++14)

2) Equivalent to

while (!pred()) {
    if (wait_until(lock, abs_time) == std::cv_status::timeout) {
        return pred();
    }
}
return true;

This overload may be used to ignore spurious wakeups.

If these functions fail to meet the postcondition (lock is locked by the calling thread), std::terminate is called. For example, this could happen if relocking the mutex throws an exception,

(since C++14)

Parameters

lock	-	an object of type Lock that meets the requirements of BasicLockable, which must be locked by the current thread
timeout_time	-	an object of type std::chrono::time_point representing the time when to stop waiting
pred	-	predicate which returns ​false if the waiting should be continued. The signature of the predicate function should be equivalent to the following:  bool pred();​

Return value

1) std::cv_status::timeout if the absolute timeout specified by abs_time was reached, std::cv_status::no_timeout overwise.

2) false if the predicate pred still evaluates to false after the abs_time timeout expired, otherwise true. If the timeout had already expired, evaluates and returns the result of pred.

Exceptions

May throw std::system_error, may also propagate exceptions thrown by lock.lock() or lock.unlock().	(until C++14)
Any exception thrown by clock, time point, or duration during the execution (clocks, time points, and durations provided by the standard library never throw)	(since C++14)

Notes

The clock tied to timeout_time is used, which is not required to be a monotonic clock.There are no guarantees regarding the behavior of this function if the clock is adjusted discontinuously, but the existing implementations convert timeout_time from Clock to std::chrono::system_clock and delegate to POSIX pthread_cond_timedwait so that the wait honors ajustments to the system clock, but not to the the user-provided Clock. In any case, the function also may wait for longer than until after timeout_time has been reached due to scheduling or resource contention delays.

Even if the clock in use is std::chrono::steady_clock or another monotonic clock, a system clock adjustment may induce a spurious wakeup.

Example

Run this code

#include <iostream>
#include <atomic>
#include <condition_variable>
#include <thread>
#include <chrono>
using namespace std::literals;
 
std::condition_variable cv;
std::mutex cv_m;
std::atomic<int> i{0};
 
void waits(int idx)
{
    std::unique_lock<std::mutex> lk(cv_m);
    auto now = std::chrono::system_clock::now();
    if(cv.wait_until(lk, now + idx*100ms, [](){return i == 1;}))
        std::cerr << "Thread " << idx << " finished waiting. i == " << i << '\n';
    else
        std::cerr << "Thread " << idx << " timed out. i == " << i << '\n';
}
 
void signals()
{
    std::this_thread::sleep_for(120ms);
    std::cerr << "Notifying...\n";
    cv.notify_all();
    std::this_thread::sleep_for(100ms);
    i = 1;
    std::cerr << "Notifying again...\n";
    cv.notify_all();
}
 
int main()
{
    std::thread t1(waits, 1), t2(waits, 2), t3(waits, 3), t4(signals);
    t1.join(); 
    t2.join();
    t3.join();
    t4.join();
}

Possible output:

Thread 1 timed out. i == 0
Notifying...
Thread 2 timed out. i == 0
Notifying again...
Thread 3 finished waiting. i == 1

See also

wait	blocks the current thread until the condition variable is woken up (public member function)
wait_for	blocks the current thread until the condition variable is woken up or after the specified timeout duration (public member function)