race condition

[fewdiefie]

Hi! how do I make the following program print numbers in the correct order like this: 0 1 2 ... 100

#include <iostream>
#include <thread>

int x = 0;

void foo() {
    for (int i = 0; i < 100; i++) {
        x++;
    }
}

void bar() {
    for (int i = 0; i < 100; i++) {
        std::cout << x << std::endl;
    }
}

int main() {
    std::thread first(foo);
    std::thread second(bar);

    first.join();
    second.join();
}

UPDATE:
the following code worked for me but I'm sure there has to be a better way to do it

#include <iostream>
#include <thread>

int x = 0;

bool can_print = true;
bool can_increment = false;

void foo() {
    for (int i = 0; i < 100; i++) {
        while (!can_increment);
        x++;
        can_increment = false;
        can_print = true;
    }
}

void bar() {
    for (int i = 0; i < 100; i++) {
        while (!can_print);
        std::cout << x << std::endl;
        can_print = false;
        can_increment = true;
    }
}

int main() {
    std::thread first(foo);
    std::thread second(bar);

    first.join();
    second.join();
}

[kbw]

Apart from printing even numbers, it's not clear what you're trying to achieve as both versions of your code use threads.

Perhaps if you explain what your concern is, someone might be able to offer something helpful.

[fewdiefie]

idk what you're talking about, what I want to do is pretty obvious, however, if you don't understand it somehow compile and run the second program and ye shall see what I want.

[salem c]

https://www.cplusplus.com/reference/condition_variable/condition_variable/
After each x++, you do a set-and-notify.
Before each print (in the other thread), you do a test-and-wait.

[helios]

I believe that last code has undefined behavior. In particular, the behavior is unpredictable in NUMA architectures. If should work fine if the bools are changed to std::atomic<bool>.

[kbw]

idk what you're talking about

You can't post buggy code and expect someone to understand what your trying to do, if it was correct you might not have questions.

It's better to communicate what you want to do, then someone will be able to fix what you've posted.

The 2nd version of your code is wrong in principle. There are race conditions around the use of those global variables. You're lucky if you get the same results twice in a row.

[dhayden]

#include <iostream>
#include <thread>
#include <mutex>

int x = 0;
std::mutex fooLock, barLock;

void foo() {
    for (int i = 0; i < 100; i++) {
	fooLock.lock();
        x++;
	barLock.unlock();
    }
}

void bar() {
    for (int i = 0; i < 100; i++) {
	barLock.lock();
        std::cout << x << std::endl;
	fooLock.unlock();
    }
}

int main() {
    fooLock.lock();		// bar() goes first
    std::thread first(foo);
    std::thread second(bar);

    first.join();
    second.join();
}