HP-UX actually had an executable attribute bit that would control whether dereferencing a pointer to address 0 would accvio or result in the value 0. I learned

HP-UX actually had an executable attribute bit that would control whether dereferencing a pointer to address 0 would accvio or result in the value 0. I learned this fact when at least one library (OpenSSL? Kerberos?) shipped with the OS relied on the non-crashing behavior, unconditionally dereferencing an optional pointer-to-struct argument. That was a fun debugging session.