bcmp

#include <string.h>

int bcmp(const void *ptr1, const void *ptr2, int length);

Compare memory pointed to by ptr1 and ptr2 for at most length bytes.

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

not ANSI, not POSIX

void f(char *s1, char *s2)
{
  int l = bcmp(s1, s2, strlen(s1));
  printf("Difference: %s, %s\n", s1+strlen(s1)-l, s2+strlen(s1)-l);
}