W3cubDocs

/C++

std::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 Pred >
bool wait_until( Lock& lock,
                 const std::chrono::time_point<Clock, Duration>& timeout_time,
                 Pred 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, timeout_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 timeout_time was reached, std::cv_status::no_timeout overwise.

2) false if the predicate pred still evaluates to false after the timeout_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)

2) Same as (1) but may also propagate exceptions thrown by pred

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.

The effects of notify_one()/notify_all() and each of the three atomic parts of wait()/wait_for()/wait_until() (unlock+wait, wakeup, and lock) take place in a single total order that can be viewed as modification order of an atomic variable: the order is specific to this individual condition_variable. This makes it impossible for notify_one() to, for example, be delayed and unblock a thread that started waiting just after the call to notify_one() was made.

Example

#include <iostream>
#include <atomic>
#include <condition_variable>
#include <thread>
#include <chrono>
using namespace std::chrono_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