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Date: Sat, 05 Nov 2005 16:39:21 +0200
Subject: Re: Default rep specs for record types - documented??
Newsgroups: comp.lang.ada
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From: "Martin Krischik" <krischik@users.sourceforge.net>
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Date: 2005-11-05T16:39:21+02:00
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Am 05.11.2005, 05:33 Uhr, schrieb Anonymous Coward  
<bogus_addy@bogus_domain.net>:

> Perhaps, but for interfacing with C operations (which is my case), I
> would prefer to simply write "pragma convention (C, my_record)" and be
> able to expect the compiler to base the representational spec purely
> off the operational spec, just as the C compiler would for a structure
> that only has an operational spec.

Well, it should actualy work - unless you use an incompatible C compiler.  
You see: pragma convention (C... will only work with the appropiate sister  
C compiler. In case of GNAT it is the gcc.

Problem is that most C compiler will unpack records in order to speed up  
access to the data. That is so ever since we moved from 8 bit CPUs to  
16bit CPUs and accessing an even address became faster then accessing an  
odd address. On 32 bit CPUs data might even be unpacked to addresses which  
can be devided by 4. or 8 on 64 bit CPUs.

Or not at all since modern MMU (Memeory Management Units) and multiple  
cache buffer might make accessing an odd address just as fast then an even  
address reverting the hole trent again.

It's all a big maybe. And you might even get that trouble when you are  
using 2 diffrent C compilers. Like a 3rd party lib compiled with C  
compiler 1 optimizing for i386 and you use C compiler 2 optimizing for  
x86_64.

Most C compilers have "#pragma pack (...)" to solve the mess. And Ada has  
"for Record_Type'Alignment use ...;"

Martin