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

["Steve Richter" <srichter@xxxxxxxxxxxxx>]

So is the "no exception when adding to a *Null pointer" behavior by design
of RPG or a bug?  I dont find a mention in the RPG reference manual.

thanks,

Steve Richter

-----Original Message-----
From: rpg400-l-admin@midrange.com [mailto:rpg400-l-admin@midrange.com]On
Behalf Of Barbara Morris
Sent: Wednesday, October 16, 2002 12:37 PM
To: rpg400-l@midrange.com
Subject: Re: add to *Null pointer does not throw an exception.


Jim Langston wrote:
> ...
> Do we know for a fact how a Null pointer is handled in RPG?  Do we know,
for a fact, wither it is pointing to 0, or does it have a Null bit set?
> ...

There isn't any special RPG-ish way of dealing with pointers.  On the
as/400 (*), a pointer is a combination of 2 things: a 16 byte value with
offsets etc, PLUS a bit somewhere in the system that says that
particular 16 bytes really is a pointer.  A pointer is null if that bit
is not set.  You set the bit on by doing a pointer operation on the 16
byte value (say assigning to the pointer).  You set the bit off by
explicitly assigning *NULL or by changing the 16 bytes in a non-pointer
way (say you have the pointer in a data structure, and you assign to a
character overlay of the pointer, although this may or may not
invalidate the pointer if what you assign has a pointer in it).

(*) This whole thing may not apply to teraspace pointers.


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