std::condition_variable_any::notify_one
From cppreference.com
< cpp | thread | condition variable any
void notify_one();
|
(since C++11) | |
If any threads are waiting on *this, calling notify_one
unblocks one of the waiting threads.
Contents |
[edit] Parameters
(none)
[edit] Return value
(none)
[edit] Exceptions
noexcept specification:
noexcept
[edit] Notes
The effects of notify_one()
/notify_all()
and wait()
/wait_for()
/wait_until()
take place in a single total order, so it's 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.
The notifying thread does not need to hold the lock on the same mutex as the one held by the waiting thread(s); in fact doing so is a pessimization, since the notified thread would immediately block again, waiting for the notifying thread to release the lock.
[edit] Example
Run this code
#include <iostream> #include <condition_variable> #include <thread> #include <chrono> std::condition_variable_any cv; std::mutex cv_m; int i = 0; bool done = false; void waits() { std::unique_lock<std::mutex> lk(cv_m); std::cout << "Waiting... \n"; cv.wait(lk, []{return i == 1;}); std::cout << "...finished waiting. i == 1\n"; done = true; } void signals() { std::this_thread::sleep_for(std::chrono::seconds(1)); std::cout << "Notifying falsely...\n"; cv.notify_one(); //Most probably, the waiting thread is awaken unnecessarily. std::unique_lock<std::mutex> lk(cv_m); i = 1; while (!done) { std::cout << "Notifying true change...\n"; lk.unlock(); //It is a good idea unlock before notifying. cv.notify_one(); std::this_thread::sleep_for(std::chrono::seconds(1)); lk.lock(); } } int main() { std::thread t1(waits), t2(signals); t1.join(); t2.join(); }
Possible output:
Waiting... Notifying falsely... Notifying true change... ...finished waiting. i == 1
[edit] See also
notifies all waiting threads (public member function) |