std::condition_variable::wait_until

 

C++

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std::condition_variable

Member functions

condition_variable::condition_variable

condition_variable::~condition_variable

Notification

condition_variable::notify_one

condition_variable::notify_all

Waiting

condition_variable::wait

condition_variable::wait_for

condition_variable::wait_until

Native handle

condition_variable::native handle

 

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

The clock tied to timeout_time is used, which means that adjustments of the clock are taken into account. Thus, the maximum duration of the block might, but might not, be less or more than timeout_time - Clock::now() at the time of the call, depending on the direction of the adjustment. The function also may block for longer than until after timeout_time has been reached due to scheduling or resource contention delays.

Calling this function if lock.mutex() is not locked by the current thread is undefined behavior.

Calling this function if lock.mutex() is not the same mutex as the one used by all other threads that are currently waiting on the same condition variable is undefined behavior.

If these functions fail to meet the postconditions (lock.owns_lock()==true and lock.mutex() 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 std::unique_lock<std::mutex>, 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)

Example

Run this code

#include <iostream>
#include <atomic>
#include <condition_variable>
#include <thread>
#include <chrono>
 
std::condition_variable cv;
std::mutex cv_m;
std::atomic<int> i = ATOMIC_VAR_INIT(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 + std::chrono::milliseconds(idx*100), [](){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(std::chrono::milliseconds(120));
    std::cerr << "Notifying...\n";
    cv.notify_all();
    std::this_thread::sleep_for(std::chrono::milliseconds(100));
    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)