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) |
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)
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.
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 postcondition (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 |
rel_time | - | an object of type std::chrono::duration representing the maximum time to spend waiting. Note that rel_time must be small enough not to overflow when added to std::chrono::steady_clock::now() . |
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
std::cv_status::timeout
if the relative timeout specified by rel_time
expired, std::cv_status::no_timeout
otherwise.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) |
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
#include <iostream> #include <atomic> #include <condition_variable> #include <thread> #include <chrono> using namespace std::chrono_literals; std::condition_variable cv; std::mutex cv_m; int i; void waits(int idx) { std::unique_lock<std::mutex> lk(cv_m); if(cv.wait_for(lk, 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); { std::lock_guard<std::mutex> lk(cv_m); 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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