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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: Importing C structs?
Date: 31 Jul 2003 20:16:21 +0100
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"Xenos" <dont.spam.me@spamhate.com> writes:

> We do this sort of think all the time.  Just create an Ada record
> that is properly rep. spec'd according to the way your C compiler
> does its structs.  If the parameter is a pointer to a struct use
> either System.Address or an access type.

I'm pretty sure GNAT will do the Right Thing here (ie pass the address
of the record parameter). I suppose there might be problems with very
small arrays on architectures where these are passed in registers? but
since the Ada and C compilers have the same backend you are quite
likely to be lucky. pragma Convention (C, ) means "do what the C
compiler expects", after all.

The trouble with rep clauses where the C side has none, or hasn't used
them, is that you only need to move to a different architecture
(PowerPC vs i86) for it all to go out of the window.

If you are sending the packets over a network, of course, it's more
difficult.

> Unfortunately, there is no portable way to do it.  But you usually
> don't run into trouble unless your structs would have misaligned
> fields.  The compiler avoids this by creating "holes."  Some C
> compilers give you limited control of the packing, but again, its
> not portable.

With GNAT there's -gnatR which tells you what the representation
actually is.