LLVM and GCC, different output same code

Question

This is a sample code just to show a different output from the LLVM compiler and the GCC. I wonder why? The answer should be very simple, but I can't see it. (Xcode 4.6.1)

The code:

#include <stdio.h>

#define MAX(a,b) ( (a) > (b) ? (a) : (b) )

int increment() {
    static int i = 42;
    i += 5;
    printf("increment returns %d\n",i);
    return i;
}

int main( int argc, char ** argv ) {
    int x = 50;
    printf("max of %d and %d is %d\n", x,increment(),MAX(x, increment()));
    printf("max of %d and %d is %d\n", x,increment(),MAX(x, increment()));
    return 0;
}

The LLVM Output:

increment returns 47
increment returns 52
increment returns 57
max of 50 and 47 is 57
increment returns 62
increment returns 67
increment returns 72
max of 50 and 62 is 72

GCC Output:

increment returns 47
increment returns 52
max of 50 and 52 is 50
increment returns 57
increment returns 62
increment returns 67
max of 50 and 67 is 62

Answer 1

The order of evaluation of the parameters is not defined specified. So this:

printf("max of %d and %d is %d\n", x,increment(),MAX(x, increment()));

causes undefined unspecified behavior . That's why you have different results on both compilers.

Another (potential) problem is: MAX - it could cause two calls to increment. Avoid using such macros.