Thread: Very weird issue with double to int conversion inside the loop

Hi,

I have spotted some piece of code which yields different result on addition of integers and doubles depending if it was performed in the loop or manually (+= vs + bare +).

Here is the (not very clean but short code):

Code:

#include <stdio.h>
#include <limits.h>
#include <unistd.h>
#include <stdlib.h>
#include <math.h>
#include <float.h>

/* Own power function, when used instead of pow()
the loop returns correct result.
*/
int pow2(int a, int b){


    int base = a;
    if (!b)
        return 1;


    while (b--) {
        a *= base;
    }


    return a/base;
}


int main (void) {
    
    int c,d,tmp,tmp2;
    for (d = 0, tmp = 0, tmp2 = tmp+1; tmp < 3; tmp++, tmp2++) {
        c = 9 * (int)pow(10, tmp) * tmp2;
        printf("TEST loop iter %d\n",c);
        d += c;
    }
    // add manually each iteration
    int test = (9 * (int)pow(10, 0) * 1) + (9 * (int)pow(10, 1) * 2) + (9 * (int)pow(10, 2) * 3);
    printf("Sum added manually: %d, sum in the loop: %d \n", test, d);


    return 0;
}

So first of all I have tested this code on both linux and windows machines and the result are different! On windows (mingw) the result is incorrect:
Windows:

Code:

gcc -Wall -O2 tmp.c -o tmp && ./tmp.exe
TEST loop iter 9
TEST loop iter 180
TEST loop iter 2673
Sum added manually: 2889, sum in the loop: 2862

Linux:

Code:

gcc -Wall -O2 tmp.c -o tmp -lm && ./tmp
TEST loop iter 9
TEST loop iter 180
TEST loop iter 2700
Sum added manually: 2889, sum in the loop: 2889

So the as you can see the addition performed manually yields different result on mingw than when it is performed inside the for loop. Why is that?