hexdigit_to_int
int val = (hexdigit_to_int(s[1]) * 100 +
hexdigit_to_int(s[2]) * 10 +
hexdigit_to_int(s[3]));
*t++ = hexdigit_to_int(src[0]) * 16 +
hexdigit_to_int(src[1]) ;
int hexdigit_to_int(char ch);