c1 = tolower (*s1); c2 = tolower (*s2);
c1 = tolower ((unsigned char) *s1); c2 = tolower ((unsigned char) *s2);
int strncasecmp (char *s1, char *s2, size_t n){  register int c1, c2, l = 0;    while (*s1 && *s2 && l < n)  {    c1 = tolower (*s1);    c2 = tolower (*s2);    if (c1 != c2)      return (c1 - c2);    s1++;    s2++;    l++;  }  if (l == n)    return (int) (0);  else    return (int) (*s1 - *s2);}