template< class Lock, class Rep, class Period > std::cv_status wait_for( Lock& lock, const std::chrono::duration<Rep, Period>& rel_time); | (1) | (since C++11) |
template< class Lock, class Rep, class Period, class Predicate > bool wait_for( Lock& 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. (until C++14) 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.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.
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 BasicLockable requirements, 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:
|
Return value
1)
std::cv_status::timeout
if the relative timeout specified by rel_time
expired, std::cv_status::no_timeout
otherwise. 2)
false
if the predicate pred
still evaluates to false
after the rel_time
timeout expired, otherwise true
.Exceptions
May throw | (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
#include <iostream> #include <atomic> #include <condition_variable> #include <thread> #include <chrono> std::condition_variable_any cv; std::mutex cv_m; std::atomic<int> i{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
blocks the current thread until the condition variable is woken up (public member function) | |
blocks the current thread until the condition variable is woken up or until specified time point has been reached (public member function) |
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