RE: add to *Null pointer does not throw an exception.

-----Original Message-----
From: rpg400-l-admin@midrange.com [mailto:rpg400-l-admin@midrange.com]On
Behalf Of Barbara Morris

>Well, it's not by explicit intent of RPG.  We don't generate any special
>code to get this behaviour.

Hi Barbara,

It seems pretty odd to me that a *Null pointer can be added to without an
exception being returned.  First of all, in MI a null pointer cannot be
added to. Shouldnt the AS400 behave consistently, esp since the underlying
pointers are the same?

Also, the offset of a pointer can be used as data.  Subtracting one pointer
from another returns a decimal difference in RPG. Consider that a pointer
can be stored in a permanent user space. One job could add to the pointer
and another job could use the resulting pointer.  So the add to the pointer
is not a superfluous operation.


>I don't know if that makes it a bug though.  It may be that pointer
>arithmetic is done just on the 16-byte variable itself, without the
>check for pointerness, for performance reasons, leaving the
>pointer-check for later when the pointer is dereferenced.

Adding too much to a pointer will cause an exception. So there is checking
of the pointer at "add to the pointer" time. The following code causes an
MCH0601 exception.

 d ActualData      s            256a
 d pBegin          s               *
 d pData           s               *
 d nDiff           s             10i 0

 c                   eval      pBegin = %addr(ActualData)
 c                   eval      pData = pBegin
 c                   do        1000000
 c                   eval      pData = pData + 32
 c                   eval      nDiff = pData - pBase
 c                   enddo


Steve Richter