Re: Can two files that have been compiled with different byte alignments be linked together?

Alberto Ganesh Barbati <>
Fri, 24 Aug 2007 08:54:50 CST
<swpzi.94183$> ha scritto:

Can two files that have been compiled with different byte alignments
be linked together?

As far as I understand it, byte alignment happens during compilation
so can I assume that linking two .obj files with differing byte
alignments into a single executable is possible?

You can *link* them, but don't expect the program to run correctly...

We are trying to get an older version of our application (byte
alignment 4) to talk to our current version (byte alignment 8).
Unfortunately we copy the entire structure instead of the pieces so we
need to come up with some way to translate our messages between the
two versions. Not only do we have different byte alignments, but we
also have differences in the structures themselves!

Any easy way to accomplish this? I have not found any yet.

C++03 provides no way to alignment control, so this is an
implementation-specific issue and this is the wrong place to ask this
question. A better place would be a newsgroup specific to your
compiler/implementation. Actually, C++0x is going to introduce some
support for alignment, but we don't have it now.

Anyway, although your compiler has a global alignment setting, it will
probably provide also a way to locally override such global setting. For
example, Visual Studio provides #pragma pack, while GCC has
__attribute__((aligned(n))). With those, you can easily override the
global setting when declaring, in the new version of you application,
the structs that are going to be shared between the two versions. For
example, in VS:

// file header.h

#pragma pack(push, 4)

struct MySharedStruct
  // whatever...

#pragma pack(pop)

(actually you don't need the #ifdef/#endif as in the old version the
packing is defined to be 4 anyway...)



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