Re: [fpc-devel] {$mode tp} compatibility issue

[Mattias Gaertner]

On Tue, 5 Jun 2018 14:09:16 +0200
Klaus Hartnegg  wrote:

> Hi,
> 
> I just stepped into a trap when compiling Turbo Pascal source with 
> FreePascal, and wonder how FreePascal could prevent this.
> 
> The source effectively contains something like
> 
> type
>mytype = (a,b,c);
> 
> var
>t : mytype;
>f : text;
> 
> begin
>assign (f,'filename');
>reset (f);
>readln (f,byte(t));
> 
> Except that in the real thing the type declaration was in another unit,
> but that should not matter, right?

It does matter. See here:
https://www.freepascal.org/docs-html/prog/progsu59.html#x66-650001.2.59

Mattias
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Re: [fpc-devel] {$mode tp} compatibility issue

[Marco van de Voort]

In our previous episode, Klaus Hartnegg said:
> 
> Could FreePascal check that variables like "mytype = (a,b,c)" have size 
> 1 before using them in {$mode tp}? Otherwise they should be flaged as 
> incompatible type, to prevent such surprises.

Not really, better edit your project to make sure that everything is forced
TP if you work exclusively in TP mode.
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[fpc-devel] {$mode tp} compatibility issue

[Klaus Hartnegg]

Hi,

I just stepped into a trap when compiling Turbo Pascal source with 
FreePascal, and wonder how FreePascal could prevent this.


The source effectively contains something like

type
  mytype = (a,b,c);

var
  t : mytype;
  f : text;

begin
  assign (f,'filename');
  reset (f);
  readln (f,byte(t));

Except that in the real thing the type declaration was in another unit,
but that should not matter, right?

Well, sometimes I got t > high(mytype), eventhough the file contents 
guaranteed that this could not happen. It took me a while until I found 
the cause:


The last statement can only be compiled with {$mode tp}, so I had added 
that. But the unit, where mytype was declared, did not have {$mode tp}. 
Thus sizeof(t)=4, which does not resonate well with byte(t). The read 
statement modifies only one of the four bytes. In Turbo Pascal such 
variables have only one byte.


Miracle solved, but somehow this should not happen.

Could FreePascal check that variables like "mytype = (a,b,c)" have size 
1 before using them in {$mode tp}? Otherwise they should be flaged as 
incompatible type, to prevent such surprises.


thanks,
Klaus
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