Re: AW: 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.


------------------------ Yahoo! Groups Sponsor --------------------~--> 
$9.95 domain names from Yahoo!. Register anything.
http://us.click.yahoo.com/J8kdrA/y20IAA/yQLSAA/i7folB/TM
--------------------------------------------------------------------~-> 

 
Yahoo! Groups Links

<*> To visit your group on the web, go to:
    http://groups.yahoo.com/group/Delphi-JEDI/

<*> To unsubscribe from this group, send an email to:
    [email protected]

<*> Your use of Yahoo! Groups is subject to:
    http://docs.yahoo.com/info/terms/