static inline unsigned char __tolower(unsigned char c)
{
        if (isupper(c))
            c -= 'A'-'a';
        return c;
}


 static inline unsigned char __toupper(unsigned char c)
 {
    if (islower(c))
            c -= 'a'-'A';
    return c;
 }

c -= 'A'-'a'; ----> c = c ^ 0x20 ;  //using xor to convert to lower case to upper case and vice versa