Why C Function Macros Are Bad

badmacro.c

#include <stdio.h>

#define IS_DIGIT(C) ((C) >= '0' && (C) <= '9')

int main()
{
    int i = 0, n = 0;
    char *s = "a!b+c(75"; // two digits, right?

    while (s[i] != '\0') {
        if (IS_DIGIT(s[i++])) {
            n++;
        }
    }
    printf("%d digits found in '%s'\n", n, s); // guess what!

    return 0;
}