Re: [Boost-users] questions about visibility/dlls under mingw

Subject: Re: [Boost-users] questions about visibility/dlls under mingw
From: John Maddock (jz.maddock_at_[hidden])
Date: 2018-11-15 11:57:22

On 15/11/2018 00:04, Robert Ramey via Boost-users wrote:
> MINGW is the platform which runs gcc compilers under winsoze.
>
> I include all the visibility attribute in the "right" places with 
> serialization build and tests.  This is looking like it's passing on 
> all platforms EXCEPT MINGW.  I think MINGW uses the windows 
> attributes, but I'm wonder if our boost setup uses the gcc macros.  
> This would explain the bahavior I'm getting now.  But honestly I 
> really have no idea. here's the link to the appveyor results:
>
> https://ci.appveyor.com/project/RobertRamey/serialization/builds/20309631
>
> Perhaps this is a config issue? or I'm doing wrong?  I'm loath to much 
> with this just for this one platform.

I had a quick look, but I'm stuck.

* I can reproduce with mingw-gcc-8.1

* Focussing on just one failing test - 
test_no_rtti_polymorphic_text_archive I see:

..\..\..\bin.v2\libs\serialization\test\test_no_rtti_polymorphic_text_archive.test\gcc-8.1.0s17\debug\test_no_rtti.o: 
In
 Â function `save_derived(char const*)':
D:\data\boost\boost\libs\serialization\test/test_no_rtti.cpp:54: 
undefined reference to `__imp__ZN20polymorphic_derived2C1Ev'

And others, but focussing on the first error, 
__imp__ZN20polymorphic_derived2C1Ev is an exported default constructor 
for polymorphic_derived2.

* If I build with b2 -d2 and capture the command lines, I can preprocess 
polymorphic_derived2.cpp and check the declarations, and indeed I see:

class polymorphic_derived2 :
 Â Â Â  public polymorphic_base
{
 Â Â Â  friend class boost::serialization::access;
 Â Â Â  template<class Archive>
 Â Â Â  __attribute__((__dllexport__)) void serialize(
 Â Â Â Â Â Â Â  Archive &ar,
 Â Â Â Â Â Â Â  const unsigned int
 Â Â Â  );
 Â Â Â  __attribute__((__dllexport__)) const char * get_key() const;
public:
 Â Â Â  __attribute__((__dllexport__)) polymorphic_derived2();
 Â Â Â  __attribute__((__dllexport__)) ~polymorphic_derived2();
};

and

__attribute__((__dllexport__))
polymorphic_derived2::polymorphic_derived2(){}
__attribute__((__dllexport__))
polymorphic_derived2::~polymorphic_derived2(){}

So that all looks OK.  But,

* If I examine the object files with nm, I see symbols for 
polymorphic_derived2 *without* the leading "__imp_" - ie regular 
non-exported symbols.

At this point I'm stumped.  Anyone else?

John.

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