stringizing #a in define, why is it bad

Question

#include <stdio.h>
#define print_int(a) printf("%s : %d\n",#a,(a))
int main(void) {
    int y = 10;
    print_int(y);
    return 0;
}

i am taking a class and have been asked to explain why this is bad... So i guess stringizing #a is the problem. It does work, so why is it dangerous?

Answer 1

because it bypasses type safety. What happens when someone hates you and goes print_int("5412");

#include <stdio.h>
#define print_int(a) printf("%s : %d\n",#a,(a))
int main(void) {
    print_int("1123123");
    return 0;
}

outputs

$ gcc test.c 
test.c: In function ‘main’:
test.c:4: warning: format ‘%d’ expects type ‘int’, but argument 3 has type ‘char *’
$ ./a.out 
"1123123" : 3870