[Oberon] ORD for typecasting Pointers

[Joerg]

Fonts should import Display!
The type Pattern should be in Display, as only Display knows the bit layout if you add colors to the system.

br
Jörg

> Am 05.09.2020 um 07:44 schrieb Andreas Pirklbauer <andreas_pirklbauer at yahoo.com>:
> 
> 
>> 
>> 
>> If not a bug, at best I would classify the ability to use ORD to typecast
>> a pointer to an INTEGER as a 'loophole'.Refer to the Section titled
>> 'Loopholes' in 'Good Ideas, Through the Looking Glass', Niklaus Wirth 2005.
>> 
>> https://people.inf.ethz.ch/wirth/Articles/GoodIdeas.pdf
>> 
> 
> Agreed. So if one can write a line like the following
> 
>    PROCEDURE GetPat*(fnt: Font; ch: CHAR; VAR dx, x, y, w, h, patadr: INTEGER);
>      ...
>    BEGIN …  patadr := ORD(fnt) + FontDescSize + pa
> 
> then that clearly is a loophole: one could accidentally pass an incorrect
> value for ‘FontDescSize' or ‘pa’, thereby making ‘patadr' point to a memory
> location *outside* the range of the pattern array (no more range checks).
> 
> The real issue is already in the parameter ‘patadr' of Fonts.GetPat, i.e. it
> really is ADDRESS, which is interpreted as such in e.g. CopyPattern.
> 
> So I have created a variant of Fonts which returns a pointer to a dynamically
> allocated (pattern) byte array (possible in Extended Oberon,  but not in Project
> Oberon 2013) instead of an integer (=memory location) ‘patadr'.
> 
> But then one would also need to modify all procedures that access the
> pattern, e.g. Display.CopyPattern, to use that pointer instead of an absolute
> address ‘patadr’. When this is done, one gains type safety, at the expense
> of sacrificing performance in CopyPattern etc (where performance is critical).
> 
> Conclusio?
> 
> So it may be better to use SYSTEM.ADR instead of ORD then in Fonts.
> Then, at least the “loophole is honestly displayed”, as N.Wirth would call it
> (see his paper, section on Loopholes).
> 
> 
> --
> Oberon at lists.inf.ethz.ch mailing list for ETH Oberon and related systems
> https://lists.inf.ethz.ch/mailman/listinfo/oberon