I have a c++ code as follows that uses condition-variable
for synchronization.
#include <iostream>
#include <condition_variable>
int n = 4;
enum class Turn { FOO, BAR };
Turn turn = Turn::FOO;
std::mutex mut;
std::condition_variable cv;
void foo() {
for (int i = 0; i < n; i++) {
std::unique_lock<std::mutex> lock(mut);
// wait for signal from bar & turn == FOO
cv.wait(lock, [] {return turn == Turn::FOO; });
std::cout << "foo" << std::endl;
// unlock & signal bar
lock.unlock();
turn = Turn::BAR;
cv.notify_one();
}
}
void bar() {
for (int i = 0; i < n; i++) {
std::unique_lock<std::mutex> lock(mut);
// wait for signal from foo & turn == BAR
cv.wait(lock, [] {return turn == Turn::BAR; });
std::cout << "bar" << std::endl;
// unlock & signal foo
lock.unlock();
turn = Turn::FOO;
cv.notify_one();
}
}
int main() {
std::thread thread_1(foo);
std::thread thread_2(bar);
thread_2.join();
thread_1.join();
return 0;
}
The output observed:
Question:
How would the cv.wait(lock, [] {return turn == Turn::FOO; });
inside the foo()
get triggered in the beginning?
From what I read, the wait()
call with the predicate would be equivalent to:while (!pred()) { wait(lock); }
. The predicate is true at the beginning (the initial value of turn
is Turn::FOO
), but how would the wait call get a notify? Regarding wait()
, I see this:
Atomically unlocks lock, blocks the current executing thread, and adds it to the list of threads waiting on *this. The thread will be unblocked when notify_all() or notify_one() is executed. It may also be unblocked spuriously. When unblocked, regardless of the reason, lock is reacquired and wait exits.
But I don't see the other thread's (the one running bar()
) to have it's notify_one()
executed since turn
is still FOO
.
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