For the return value, the docs say

	"The number of bytes remaining when the first mismatch occurred,
	or zero if all bytes were equal."

[This should be clarified with "including the mismatching postition itself"
or something to that effect.]

Codewise bcmp() is implemented as an alias of memcmp() which has a different
return value system, so only the zeroness of the return value can be counted
on.  I have yet to see a program that uses more than the zeroness, but who
knows what kind of code is out there?

[untested code follows.]

#include <string.h>

int
bcmp(const void *s1, const void *s2, size_t n)
{
  if (n != 0)
  {
    const unsigned char *p1 = s1, *p2 = s2;

    do {
      if (*p1++ != *p2++)
	return n;
    } while (--n != 0);
  }
  return 0;
}