* HBYTES LENGTH OVERRUN block too long
* HBYTES LENGTH RANGE input length or offset is -ve or silly
* HBYTES LENGTH UNDERRUN block too short (or offset too big)
+ * HBYTES LENGTH MISMATCH when blocks must be exactly same length
* HBYTES SYNTAX supposed hex block had wrong syntax
* HBYTES VALUE OVERFLOW value to be conv'd to hex too big/long
* SOCKADDR AFUNIX LENGTH path for AF_UNIX socket too long