Question

int rng(int min, int max){
    int result = min + ( ( (max - (min - 1) ) * rand() ) / (RAND_MAX + 1) );
    return result;
}

This won’t compile with ideone using c99 mode, but does so when I’m using gcc on CodeBlocks while on -std=c99. I figured using long int would do the trick, but apparently not so. I use ideone when I’m trying some stuff in school since they just have VC++ and I’m doing C C99. I wouldn’t want to not use this function when I need it, and so here I am asking for what is wrong.

Answer 1

Actually, on my compiler this produces warning: integer overflow in expression [-Woverflow].

The problem is caused by RAND_MAX + 1. RAND_MAX often happens to be the same as INT_MAX. Adding 1 to INT_MAX of course yields undefined behavior (except the compiler catches at compile time and replaces the code with something else).

If you want to see in action why it’s bad, do this:

scanf("%d", &dummy); /* So the compiler won't catch it. */
dummy += INT_MAX;

gcc -ftrapv -Wall -Wextra # ftrapv is the key point