std::condition_variable::wait() misunderstanding [duplicate] - c++

Question

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Why do you need a while loop while waiting for a condition variable
(4 answers)
Closed 1 year ago.
Why is it necessary to use while() with std::condition_variable::wait()
in this code:
#include <iostream> // std::cout
#include <thread> // std::thread
#include <mutex> // std::mutex, std::unique_lock
#include <condition_variable> // std::condition_variable
std::mutex mtx;
std::condition_variable cv;
bool ready = false;
void print_id (int id) {
std::unique_lock<std::mutex> lck(mtx);
while (!ready) cv.wait(lck);
// ...
std::cout << "thread " << id << '\n';
}
void go() {
std::unique_lock<std::mutex> lck(mtx);
ready = true;
cv.notify_all();
}
int main ()
{
std::thread threads[10];
// spawn 10 threads:
for (int i=0; i<10; ++i)
threads[i] = std::thread(print_id,i);
std::cout << "10 threads ready to race...\n";
go(); // go!
for (auto& th : threads) th.join();
return 0;
}
If, as I understand, std::condition_variable::wait() blocks the current thread, and that thread is waiting in the point of suspending until some another thread is calling a notify..() function

Answer 1

Why is it necessary to use while() with std::condition_variable::wait()
wait may return whether ready is true or not. If we want to proceed only when ready is true, then we must check it, and continue waiting in case it isn't. A loop is a convenient control structure for repeating an operation until a condition is satisfied.
why wait() may return seemingly spuriously
Because the standard says that it may:
Effects:
Atomically calls lock.unlock() and blocks on *this.
When unblocked, calls lock.lock( (possibly blocking on the lock), then returns.
The function will unblock when signaled by a call to notify_­one() or a call to notify_­all(), or spuriously.