Question

I was wondering whether there is anything wrong with passing a pointer to putchar, or any other standard function which can be implemented as a macro, to a function accepting a function pointer. Below is an example of what I’m doing.

#include <stdio.h>

static int print(const char *s, int (*printc)(int))
{
        int c;

        while (*s != '\0') {
                if ((c = printc(*s++)) < 0)
                        return c;
        }
        return printc('\n');
}

int main(void)
{
        print("Hello, world", putchar);
        return 0;
}

I don’t have any problems compiling this with GCC and Clang under GNU/Linux, as well as GCC under OpenBSD. I’m wondering whether it will have the same behaviour under every other standard compliant implementation, since putchar can be implemented as a macro. I’ve searched through the standard, particularly the sections on function pointers and putchar, and haven’t been able to find anything that specifies whether this is legal or not.

Thanks.

Answer 1

Following on from my comments-based discussion with @pmg, I found the relevant section of the standard (C99, 7.1.4 p.1):

… it is permitted to take the address of a library function even if it is also defined as
a macro.¹⁶¹⁾

---

_{161) This means that an implementation shall provide an actual function for each library function, even if it also provides a macro for that function.}