c1 = tolower (*s1); c2 = tolower (*s2);
c1 = tolower ((unsigned char) *s1); c2 = tolower ((unsigned char) *s2);
int strcasecmp (char *s1, char *s2){  register int c1, c2;    while (*s1 && *s2)  {    c1 = tolower (*s1);    c2 = tolower (*s2);    if (c1 != c2)      return (c1 - c2);    s1++;    s2++;  }                                                                             return (int) (*s1 - *s2);}