value changes after adding optimization flag in Clang

Question

I have written a little program:

#include <stdio.h>

void tester() {
    int x = 69;
}

void x() {
    int x;
    printf("%d\n", x);
}

int main() {
    tester();
    x();

}

which I expect to print out 69. It prints out 69 when I compile without optimization, but with O1-O3, it prints out large numbers. Why is this?

Answer 1

those 2 x variable are totally different things, they just happen to have the same name. If you want it to be the same x then you must create a global variable

int x;
void tester() {
    x = 69;
}

void xx() {
    printf("%d\n", x);
}

int main() {
    tester();
    xx();
}