Re: Import a type from C

["Eric G. Miller" <egm2.NOSPAM@jps-NOSPAM.net>]

Rupert Pigott wrote:
> "Stephen Leake" <stephen.a.leake.1@gsfc.nasa.gov> wrote in message
> news:u7kfqjnvv.fsf@gsfc.nasa.gov...
> 
>>"Frank J. Lhota" <NOSPAM.lhota.adarose@verizon.net> writes:
>>
>>
>>>    pragma Convention( C, My_Data );
>>>
>>>The Convention pragma can be used to specify that this record type
> 
> should be
> 
>>>laid out in the same way that a C compiler would.
>>
>>Note that "a C compiler" really means "the C compiler that the Ada
>>compiler knows about, with its default command line args". For
>>example, GNAT will assume Gnu C, _without_ -fpacked. ObjectAda
>>probably assumes MS C, again without the "packed" flag. That has
>>caused me some major problems, when linking against C code compiled
>>_with_ the -fpacked flag.
>>
>>So I always use a rep spec, instead of relying on Convention (C). Then
>>I write some C code that checks the struct size, to be sure there is
>>no padding.
> 
> 
> Perhaps I'm overly pessimistic here... IIRC there is no guarantee
> of the order of members within a structure in C either. One of those
> little gotchas (along with no alignment control) that **** portable
> code and force you to go the long way around...

You're high 8^)  The order of members in a C struct is always as specified
in the definition.  Furthermore, a pointer to a struct is always
guaranteed to resolve to a pointer to the first member.  The alignment
of any subsequent members is implementation defined.  Portable C code
would never pack a struct to begin with (an inherently unportable practice).

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