std::condition_variable::wait_for

template< class Rep, class Period > std::cv_status wait_for( std::unique_lock<std::mutex>& lock, const std::chrono::duration<Rep, Period>& rel_time );	(1)	(since C++11)
template< class Rep, class Period, class Predicate > bool wait_for( std::unique_lock<std::mutex>& lock, const std::chrono::duration<Rep, Period>& rel_time, Predicate pred );	(2)	(since C++11)

wait_for causes the current thread to block until the condition variable is notified, the given duration has been elapsed, or a spurious wakeup occurs. pred can be optionally provided to detect spurious wakeup.

1) Equivalent to return wait_until(lock, std::chrono::steady_clock::now() + rel_time);.

2) Equivalent to return wait_until(lock, std::chrono::steady_clock::now() + rel_time, std::move(pred));.

This overload may be used to ignore spurious awakenings while waiting for a specific condition to become true.

Right after wait_for returns, lock.owns_lock() is true, and lock.mutex() is locked by the calling thread. If these postconditions cannot be satisfied^[1], calls std::terminate.

If any of the following conditions is satisfied, the behavior is undefined:

lock.owns_lock() is false.
lock.mutex() is not locked by the calling thread.
If some other threads are also waiting on *this, lock.mutex() is different from the mutex unlocked by the waiting functions (wait, wait_for and wait_until) called on *this by those threads.

↑ This can happen if the re-locking of the mutex throws an exception.

Parameters

lock	-	a lock which must be locked by the calling thread
rel_time	-	the maximum duration to wait
pred	-	the predicate to check whether the waiting can be completed
Type requirements
- `Predicate` must meet the requirements of FunctionObject.
- pred() must be a valid expression, and its type and value category must meet the BooleanTestable requirements.

Return value

1) std::cv_status::timeout if rel_time has been elapsed since the beginning of this call, otherwise std::cv_status::no_timeout.

2) The latest result of pred() before returning to the caller.

Exceptions

1) Timeout-related exceptions.

2) Timeout-related exceptions, and any exception thrown by pred.

Notes

Even if notified under lock, overload (1) makes no guarantees about the state of the associated predicate when returning due to timeout.

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

Run this code

#include <chrono>
#include <condition_variable>
#include <iostream>
#include <thread>
 
std::condition_variable cv;
std::mutex cv_m; // This mutex is used for three purposes:
                 // 1) to synchronize accesses to i
                 // 2) to synchronize accesses to std::cerr
                 // 3) for the condition variable cv
int i = 0;
 
void waits()
{
    std::unique_lock<std::mutex> lk(cv_m);
    std::cerr << "Waiting... \n";
    cv.wait(lk, []{ return i == 1; });
    std::cerr << "...finished waiting. i == 1\n";
}
 
void signals()
{
    std::this_thread::sleep_for(std::chrono::seconds(1));
    {
        std::lock_guard<std::mutex> lk(cv_m);
        std::cerr << "Notifying...\n";
    }
    cv.notify_all();
 
    std::this_thread::sleep_for(std::chrono::seconds(1));
 
    {
        std::lock_guard<std::mutex> lk(cv_m);
        i = 1;
        std::cerr << "Notifying again...\n";
    }
    cv.notify_all();
}
 
int main()
{
    std::thread t1(waits), t2(waits), t3(waits), t4(signals);
    t1.join(); 
    t2.join(); 
    t3.join();
    t4.join();
}

Possible output:

Waiting...
Waiting...
Waiting...
Notifying...
Notifying again...
...finished waiting. i == 1
...finished waiting. i == 1
...finished waiting. i == 1

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2093	C++11	timeout-related exceptions were missing in the specification	mentions these exceptions
LWG 2114 (P2167R3)	C++11	convertibility to bool was too weak to reflect the expectation of implementations	requirements strengthened
LWG 2135	C++11	the behavior was unclear if lock.lock() throws an exception	calls std::terminate in this case

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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