I have experimented to modify C++ example code of condition_variable so that the variable is notified before waiting is initiated. I end up with the code, where I do not notify at all but all the threads are awake.
#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(int id)
{
std::this_thread::sleep_for(std::chrono::seconds(2));
std::unique_lock<std::mutex> lk(cv_m);
for (int t=10; t!=0; t--)
{
std::cerr << id << " waiting " << t <<" seconds... \n";
std::this_thread::sleep_for(std::chrono::seconds(1));
}
// cv.wait(lk);
cv.wait(lk, []{return i == 1;});
for (int t=10; t!=0; t--)
{
std::cerr << id << " finish waiting " << t <<" seconds... \n";
std::this_thread::sleep_for(std::chrono::seconds(1));
}
}
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_one();
}
int main()
{
std::thread t1(waits, 0), t2(waits, 1), t3(waits, 2), t4(signals);
t1.join();
t2.join();
t3.join();
t4.join();
}
Interestingly without predicate calling just cv.wait(lk); in waits causes infinite waiting. So it seems when the predicate is satisfied, no waiting occurs? See https://onlinegdb.com/Sr4iaHu5T So is there some magic going on or this is expected? The consequence is that cv.wait(lk, []{return true;}); causes no waiting and is not equivalent to cv.wait(lk);.
Is this something I can rely on? I mean I could call cond_var.wait(lock, []() { return !packet_queue.empty(); }); and know that if the queue isn't empty it won't wait even without notification.
