int htoi (char s[]) {

    int i,j,n;

    i = j = n = 0;

    char hex[] = "0123456789abcdef"; /*the array of possible hexadecimal digits*/

    if (s[0] == '0' && (s[1] == 'x' || s[1] == 'X')) /*skip over the optional 0x or 0X*/

        i = 2;

    for (   ;s[i] != '\0'; i++) {

        for (j = 0; j <= 16; j++) {

            if ((s[i] == hex[j]) || (s[i] == toupper(hex[j])))

                n = 16*n + j;


                       }
                    }

    return n;

            }