Shouldn't that be --j in your reverse function?

Anyhow, when asked to write those on a blackboard, I typically do this:

  size_t strlen(char* start) {
     char* end=start;
     while(*end) ++end;
     return (end-start);
  }

and

  void reverse(char* i) {
    char* j=(i+strlen(i)-1);
    for (; i < j; ++i,--j) {
      *i ^= *j;
      *j ^= *i;
      *i ^= *j;
    }