AW: AW: Converting C/C++ DLL to Delphi

Ok I get the point.
But do you like the idea of it? I certainly don't ;-)

In my view, this is one reason not to combine Delphi and C code for exchanging data between DLL's

I changed from Delphi to C#(.net), and I don't regret it.
Especially in calling C++ dll's.
Not to mention other topics.

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Von: [email protected] [mailto:[email protected]] 
Gesendet: Samstag, 11. September 2004 14:45
An: [email protected]
Betreff: Re: AW: [Delphi-JEDI] Converting C/C++ DLL to Delphi

> Delphi structures are incompatible to c structures.
> Especially when you use "packed structures" in c.
> 
> I tried a lot of stuff with it, and it was purely chaos.

Delphi has four ways of aligning {$a1},{$a2},{$a4} and {$a8}

If I understood the delphi help correctly, this means a _fixed_ alignment, iow 

{$a1} all fields aligned on 1 byte	(equal to declaring record packed)
{$a2} all fields aligned on 2 byte 

etc.

C alignment (at least e.g. gcc, don't know the Windows compilers that well) is a bit different. It typically aligns each element on the size of the element, so alignment is not constant throughout the record:

Assume we would have C alignment, then:

type myrec = record
		a : byte;    // size 1 aligned on 1. Is always.  offset 0
		b : word;    // size 2 must be aligned on 2.	 offset 2, one byte skipped.
		c : dword;   // size 4 must be aligned on 4.	 offset 4, none skipped
                a2: byte;    // size 1 aligned on 1.             offset 8, 
		e: int64;    // size 8 must be aligned on 8.	 offset 16, 7 skipped
		end;


The above records has 1+2+4+1+8=16 bytes worth of fields.

directive		size
packed or $a1		16
$a2			18	(easy to see, the bytes need to be aligned)
$a4			24	(byte + word fields need to be alligned to 4 bytes. 2x3+1x2)
$a8			30
<(gc)c alignment>		24	(see above comments in record def)

One can see that the C alignment doesn't match with any of the $a ones. The only with the correct size is $a4, and $a4 already aligns the second field differently.

However one can always lay this out by inserting dummy fields that take the place of alignment in a packed record.

E.g. again the above record, that can't be represented by Delphi (at least afaik):

type myrec = packed record
                a : byte;    
	        dummy1: byte;
                b : word;    
                c : dword;   
                a2: byte;    
		dummy2:array[0..6] of byte;			// 7 byttes;
                e: int64;    
                end;
  
The free unices (FreeBSD, linux) quite often manually align their structs this way, to be more compiler independant in their type declarations.


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