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std::condition_variable::notify_all(3) | C++ Standard Libary | std::condition_variable::notify_all(3) |
NAME¶
std::condition_variable::notify_all - std::condition_variable::notify_all
Synopsis¶
void notify_all() noexcept; (since C++11)
Unblocks all threads currently waiting for *this.
Parameters¶
(none)
Return value¶
(none)
Notes¶
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.
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.
Example¶
// Run this code
#include <iostream>
#include <condition_variable>
#include <thread>
#include <chrono>
std::condition_variable 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 cv
int 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
See also¶
notify_one notifies one waiting thread
(public member function)
2022.07.31 | http://cppreference.com |