Re: Typecasting - Gpc

31 Oct 2001


      I hope when you say "change p:=pchar(p)+2; to p:=pchar(p)+4;", you also
change "getmem(p,sizeof(mytype)+2);" to "getmem(p,sizeof(mytype)+4);"!
<g>
I'm sure we all agree that this is very ugly, dangerous coding, but that's
not the point. You are trying to "stretch the boundaries" of the compiler,
to see how it behaves and to find out if there are any areas which may
in practice cause problems with porting other code, right?
Well, to be honest, I can't see what the problem is here, although it must
have something to do with 32 bit alignment. As far as I can see, the two
lines "fillchar(p^,sizeof(mytype)+2,0);" and "word(p^):=sizeof(mytype);"
are irrelevant. All you are doing is allocating 2 extra bytes of memory than
your record type ("Mytype") needs, then offsetting a pointer to this by 2
bytes, thereby forcing the compiler to address the record (and in
particular,
the pointer "c" within the record) on a 16 bit boundary.
That's as far as I can analyse the situation, based simply on the source
code presented. Since I don't know the "inner workings" of the compiler
myself, perhaps someone else can comment further about this apparent
alignment issue.
Joe.
...
-----Original Message-----
From:	Anuradha [SMTP:anuradha@adcc.alcatel.be]
Sent:	Wednesday, October 31, 2001 10:19 AM
To:	gpc
Subject:	Re: Typecasting
Hello,
I am really sorry for having missed a ^ in the program but you see ,
the
program really worked in BP and not in GPC.But i do know that it is
stupid to
manipulate some parts of the pointer.kindly delete the previous mail.
It is this program that i have problems in.

program typcast1;
const ct :char='h';
type
  Mytype=record
    i : integer;
    c:^char;
  end;
var
 p:pointer;
begin
  getmem(p,sizeof(mytype)+2);
  fillchar(p^,sizeof(mytype)+2,0);
  word(p^):=sizeof(mytype);
  p:=pchar(p)+2;
  mytype(p^).c:=@ct;
  writeln(mytype(p^).c^);
end.

But if i change p:=pchar(p)+2; to p:=pchar(p)+4; then it works.Is it
something to do with the word-boundaries?
Thanks and regards,
Anuradha
Marco van de Voort wrote:
...
...
What is the purpose of these two lines :  ?
        word(p):=sizeof(mytype);
In real mode TP it sets the offset of the segment:offset pair to 1.
Probably it originally was meant as a simple inc(p);
The problem it expects that the offset part of a newly allocated block
is
...
always zero, which is afaik not true. (real mode TP allocates per half
segment).
...
    p:=pchar(p)+2;

inc(p,2);   or p:=@p[2];
The overall effect therefore is inc(p,3); or p:=pchar(p)[3];
--
S.Anuradha
Generic Data Tools,
Alcatel Chennai.
Alcanet : 2-757-7123