non-reentrant function

non-reentrant.c

#include <stdio.h>
#include <unistd.h>

int sum;

int calculate(int first, int second) {
    sum = first + second;
    sleep(3); // represent I/O intensive function call
    return sum;
}

int main()
{
    for (;;) {
        printf("Result from main: %d\n", calculate(3, 5));
        printf("---\n");
    }
    
    return 0;
}