std::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.
wait(lock);
{
if(pred())
returntrue;
wait(lock);
}
return pred();
Right after wait returns, lock is locked by the calling thread. If this postcondition cannot be satisfied[1], calls std::terminate.
- ↑This can happen if the re-locking of the mutex throws an exception.
Contents |
[edit]Parameters
| 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 of BasicLockable. | ||
-Predicate must meet the requirements of FunctionObject. | ||
| -pred() must be a valid expression, and its type and value category must meet the BooleanTestable requirements. | ||
[edit]Return value
[edit]Exceptions
[edit]Notes
The returned value of overload (3) indicates whether pred evaluated to true, regardless of whether there was a stop requested or not.
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.
[edit]Example
#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
[edit]Defect reports
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 to bool was too weak to reflect the expectation of implementations | requirements strengthened |
| LWG 2135 | C++11 | the behavior was unclear if lock.lock() throws an exception | calls std::terminate in this case |
[edit]See also
| blocks the current thread until the condition variable is awakened or after the specified timeout duration (public member function) | |
| blocks the current thread until the condition variable is awakened or until specified time point has been reached (public member function) | |
C documentation for cnd_wait | |
[edit]External links
| The Old New Thing article: Spurious wake-ups in Win32 condition variables. |
