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| Member functions | ||||
| Notification | ||||
| Waiting | ||||
condition_variable_any::wait | ||||
template<class Lock> void wait( Lock& lock); | (1) | (since C++11) |
template<class Lock,class Predicate> void wait( Lock& lock, Predicate pred); | (2) | (since C++11) |
template<class Lock,class Predicate> bool wait( Lock& lock,std::stop_token stoken, Predicate pred); | (3) | (since C++20) |
wait causes the current thread to block until the condition variable is notified or a spurious wakeup occurs.pred can be optionally provided to detect spurious wakeup.
Right afterwait returns,lock is locked by the calling thread. If this postcondition cannot be satisfied[1], callsstd::terminate.
Contents |
| lock | - | an lock which must be locked by the calling thread |
| stoken | - | a stop token to register interruption for |
| pred | - | the predicate to check whether the waiting can be completed |
| Type requirements | ||
-Lock must meet the requirements ofBasicLockable. | ||
-Predicate must meet the requirements ofFunctionObject. | ||
| -pred() must be a valid expression, and its type and value category must meet theBooleanTestable requirements. | ||
The returned value of overload(3) indicates whetherpred evaluated totrue, regardless of whether there was a stop requested or not.
The effects ofnotify_one()/notify_all() and each of the three atomic parts ofwait()/wait_for()/wait_until() (unlock+wait, wakeup, and lock) take place in a single total order that can be viewed asmodification order of an atomic variable: the order is specific to this individual condition variable. This makes it impossible fornotify_one() to, for example, be delayed and unblock a thread that started waiting just after the call tonotify_one() was made.
#include <chrono>#include <condition_variable>#include <iostream>#include <thread> std::condition_variable_any 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 cvint 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
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 2114 (P2167R3) | C++11 | convertibility tobool was too weak to reflect the expectation of implementations | requirements strengthened |
| LWG 2135 | C++11 | the behavior was unclear iflock.lock() throws an exception | callsstd::terminate in this case |
| blocks the current thread until the condition variable is awakened or after the specified timeout duration (public member function)[edit] | |
| blocks the current thread until the condition variable is awakened or until specified time point has been reached (public member function)[edit] | |
C documentation forcnd_wait | |
| The Old New Thing article: Spurious wake-ups in Win32 condition variables. |
