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Subject: Re: [Boost-users] questions about visibility/dlls under mingw
From: John Maddock (jz.maddock_at_[hidden])
Date: 2018-11-20 17:35:20

On 19/11/2018 19:07, John Maddock wrote:
>> OK - due to some sleuthing on the part of one of the auxiliary
>> maintainers of the boost serialization library, it's seems that the
>> problem is that
>> BOOST_SYMBOL_IMPORT is not defined to be
>> __attribute__((__dllimport__)) under MINGW and that is the source of
>> the problem.
> That's not true, gcc.hpp has:
> #    define BOOST_SYMBOL_EXPORT __attribute__((__dllexport__))
> #    define BOOST_SYMBOL_IMPORT __attribute__((__dllimport__))
> when _Win32 is defined.
> And if I modify config_info.cpp to print these out then I see:
>     BOOST_SYMBOL_EXPORT =__attribute__((__dllexport__))
>     BOOST_SYMBOL_IMPORT =__attribute__((__dllimport__))
>     BOOST_SYMBOL_VISIBLE =__attribute__((__visibility__("default")))
> Which looks good to me.

I *think* I may have twigged on to what's happening here.

To recap, msvc when exporting a symbol from a dll mangles the name with
a __imp_ prefix.  Further it performs "early name mangling" - ie the
compiler does it and the mangled name for the export ends up in the
object file.

GCC normally does not mangle names when changing visibility, instead it
stores "something" in the object file to tell the linker to make the
symbol visible.  On windows it's the *linker* that takes this
information and creates an alias with the appropriately mangled name
when building the dll.  That means that the object files do not contain
records with the __imp_ prefix, only the finished dll.

However, when importing, since dllimport suggests import from a dll, gcc
mangles the name immediately so that the linker knows to look for a
__imp_ prefixed name.

Now look at what you're doing in these failing tests: you're mixing
object files *in the same executable* some of which are using dllexport
and some using dllimport and now we have a mismatch with dllimport
implying the __imp_ prefix but there are no matching __imp_ prefixed
records in the object files.

I think this is a bug in your tests, albeit a very obscure one. To fix
you would need to either:

* Use dllexport consistently throughout an executable: ie if a class is
dllexport in one object file in the dll or exe, then it should be
dllexport in *all the object files in that dll/exe*, including in object
files that only use those symbols not define them.

* Don't export these symbols: so far as I can tell, the exported symbols
are never imported from anything (since we never build a dll with them)
so exporting is completely redundant in this case. You may still need
BOOST_SYMBOL_VISIBLE on them on for *nix systems, not sure about that

* Or, put these exported symbols in a dll and link the test program to it.

Hopefully that makes sense, John.

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