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std::condition_variable_any::wait_until

From cppreference.com

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std::condition_variable_any
 
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)

Contents

[edit] 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();​

[edit] 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.

[edit] Exceptions

1)

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

[edit] 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.

[edit] Example

Run this code
#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

[edit] See also

 blocks the current thread until the condition variable is woken up
(public member function) [edit]
blocks the current thread until the condition variable is woken up or after the specified timeout duration
(public member function) [edit]
C documentation for cnd_timedwait
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