[fpc-pascal] Converting C's bit field struct

"Vinzent Höfler" JeLlyFish.software at gmx.net
Wed Aug 13 23:36:19 CEST 2008

-------- Original-Nachricht --------
> Datum: Wed, 13 Aug 2008 22:03:23 +0100
> Von: Jonas Maebe <jonas.maebe at elis.ugent.be>
> An: FPC-Pascal users discussions <fpc-pascal at lists.freepascal.org>
> Betreff: Re: [fpc-pascal] Converting C\'s bit field struct
> On 13 Aug 2008, at 11:59, Vinzent Höfler wrote:
> > Should be something like that:
> >
> > type
> >  Unsigned_7  = 0 .. (1 shl 7)  - 1;
> >  Unsigned_20 = 0 .. (1 shl 20) - 1;
> >
> > type
> >  page_t = bitpacked record
> Yes and no: the layout of bitpacked records is by no means guaranteed  
> to be the same as that of a C record, regardless of the definition of  
> the C record.

Well, this is obviously a hardware structure and the actual layout should be checked regardless of the definition in C (IIRC, the C-standard doesn't guarantee any particular layout on such records neither).

> The internal format of bitpacked records is opaque and  
> must not be used in any case where data has to be interchanged or  
> where a particular layout is desired.

There was a reason why I brought up the portability issue. Honestly, what do you propose to mimic such stuff? I remember, we already had a discussion about that a couple of months ago. ;)

I don't want to bring up Ada again, but ... some years ago I implemented such nasty stuff for an embedded system. You might take a look for the implementation of system structures in that language http://www.gmx.de/mc/h9DZhQD3l4X4hfKafRIagg6e3PI6TW . Well, apart from Endianess issues, successful compilation virtually guarantees the layout.

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