std::condition_variable::wait_for

 

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 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)

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 relative timeout rel_time expires. It may also be unblocked spuriously. When unblocked, regardless of the reason, lock is reacquired and wait_for() exits. If this function exits via exception, lock is also reacquired.

2) Equivalent to

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

This overload may be used to ignore spurious awakenings.

A steady clock is used to measure the duration. This function may block for longer than timeout_duration 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.

Parameters

lock	-	an object of type std::unique_lock<std::mutex>, which must be locked by the current thread
rel_time	-	an object of type std::chrono::duration representing the maximum time to spend 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 relative timeout specified by rel_time expired, std::cv_status::no_timeout overwise.

2) false if the predicate pred still evaluates to false after the rel_time timeout expired, otherwise true.

Exceptions

May throw std::system_error, may also propagate exceptions thrown by lock.lock() or lock.unlock().

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);
    if(cv.wait_for(lk, 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();
}

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_until	blocks the current thread until the condition variable is woken up or until specified time point has been reached (public member function)