Portable way to print function pointers?

Question

I am wondering if this is a portable way to print function pointers.
Casting the function pointer directly to unsigned char * is non-portable.

/* writehex() prints the given byte as its hex representation to the given FILE * */
extern void writehex(FILE *, const unsigned char);

/* Theoretical printf() implementation. */
int printf(const char *format, ...) {

    ...

    void (*fptr)(void);
    const unsigned char *bytes = (const unsigned char *) &fptr;
    size_t i;

    fptr = va_arg(ap, ((void)(*)(void)));
    for (i = 0; i < sizeof(fptr); ++i)
        writehex(stdout, *bytes++);

    ...
}