🚀 go-pugleaf

stk@snafu.de (Stefan Haller) wrote in message news:<1fwrc5z.1wbucc58vha4aN%stk@snafu.de>...
> We need to pass objects between a Mach-O application and a CFM shared
> library, built from the same source code.
> 
> It seems that the Mach-O and CFM compilers behave differently with
> regard to the alignment of derived classes, so that the object layout is
> different.  Obviously this is a serious problem for us.
> 
> The following sample code illustrates the problem.  I don't understand
> what the padding rules are here; CFM doesn't *always* pad so that the
> derived class starts on the next 8-byte boundary, because if you comment
> out the "int i" in Base, the offset of b2 is 1 on both compilers.

Actually, a minimal padding can be a good thing if you need to a lot
of small objects.  Anyway, I see your problem is similar to one I have
been having some time ago (but not directly related, I had stack
alignment issues in my own code generated at runtime when interfacing
to CW functions).

> Is this a bug in one of the compilers?

No, the ABI is different, and it is actually documented what is being
used. Not in any of the manuals as I just noticed, also I thought I
had read it there. There are more options for alignment, and they are
not documented in the compiler manual yet, it seems:
You find them in "CW Mac PPC Notes 3.0.txt" in the Release Notes
folder. If you play with them a bit, in particular the "power_gcc"
options, I guess you get what you want assuming Apple and Metrowerks
follow the same revision gcc ABI for Mach-O to be compatible with each
other.

    Thorsten