Re: [boost] [config] GCC symbol visibility across shared libraries

Subject: Re: [boost] [config] GCC symbol visibility across shared libraries
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2010-05-20 10:21:27

Alexander Arhipenko
> On Thu, May 6, 2010 at 7:42 PM, Jürgen Hunold wrote:
> >
> > The patch replaces
> > __declspec(dllexport) with BOOST_SYMBOL_EXPORT
> > __declspec(dllimport) with BOOST_SYMBOL_IMPORT
> > and removes visibility declarations from forward
> > declarations
>
> The first version of BOOST_SYMBOL_EXPORT/IMPORT macro
> had slightly different definition for gcc (something like this):
> #define   BOOST_SYMBOL_EXPORT  __attribute__((visibility("default"))
> #define   BOOST_SYMBOL_IMPORT
>
> Assume that I've built boost filesystem library with hidden
> visibility.
> After that I've used boost filesystem functionality in my custom
> shared library foo (also build with hidden visibility).
> Assume also that boost filesystem was used as an
> implementation detail.
> You will never find any references to filesystem in foo's
> public interface.

Each dynamic library should define its own import/export macro to adorn the symbols of that library that should be exported/visible.  Those macros should be based upon BOOST_SYMBOL_EXPORT and BOOST_SYMBOL_IMPORT and whether the dynamic library is being built or used:

#ifdef BUILDING_MYLIB
#  define MYLIB_EXPORT BOOST_SYMBOL_EXPORT
#else
#  define MYLIB_EXPORT BOOST_SYMBOL_IMPORT
#endif

Using MYLIB_EXPORT for all public symbols in MyLib, and defining BUILDING_MYLIB only when compiling MyLib, will do what you want.

Arguably, "EXPORT" in "MYLIB_EXPORT" is misleading, at least for MyLib's clients.  Perhaps "MYLIB_VISIBLE" is a better name, leaving BOOST_SYMBOL_VISIBLE, or whatever it may be called, for the non-import/export cases.

> 2)
> What do you think about having BOOST_SYMBOL_EXPORT/IMPORT
> always defined?

If that's not what's being done -- I haven't looked at the patch itself -- then I agree.  Those macros should always be defined and each library must decide which to use as shown above.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer, Core Software      using std::disclaimer;
Susquehanna International Group, LLP  http://www.sig.com

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