std::condition_variable::wait_for
| template< class Rep, class Period >
std::cv_status wait_for( std::unique_lock<std::mutex>& lock, |
(1) | (since C++11) |
| template< class Rep, class Period, class Predicate >
bool wait_for( std::unique_lock<std::mutex>& lock, |
(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)
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) |
Contents |
[edit] 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(); |
[edit] Return value
rel_time expired, std::cv_status::no_timeout otherwise.pred still evaluates to false after the rel_time timeout expired, otherwise true.[edit] 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) |
[edit] Notes
Even if notified under lock, overload (1) makes no guarantees about the state of the associated predicate when returning due to timeout.
[edit] Example
#include <iostream> #include <atomic> #include <condition_variable> #include <thread> #include <chrono> std::condition_variable 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
[edit] See also
| blocks the current thread until the condition variable is woken up (public member function) |
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| blocks the current thread until the condition variable is woken up or until specified time point has been reached (public member function) |