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 15:02:28

Jürgen Hunold wrote:
> Rob Stewart wrote:
>
> > #define BOOST_SYMBOL_EXPORT
> > #define BOOST_SYMBOL_IMPORT
> > #define BOOST_SYMBOL_VISIBLE
> >
> > #ifdef BOOST_HAS_DECLSPEC
> > #  undef BOOST_SYMBOL_EXPORT
> > #  define BOOST_SYMBOL_EXPORT __declspec(dllexport)
> > #  undef BOOST_SYMBOL_IMPORT
> > #  define BOOST_SYMBOL_IMPORT __declspec(dllimport)
> > #elif defined BOOST_HAS_VISIBILITY
> > #  undef BOOST_SYMBOL_VISIBLE
> > #  define BOOST_SYMBOL_VISIBLE __attribute__(visibility("default"))
> > #  ifdef BOOST_DEFAULT_VISIBILITY_IS_HIDDEN
> > #     undef BOOST_SYMBOL_EXPORT
> > #     define BOOST_SYMBOL_EXPORT BOOST_SYMBOL_VISIBLE
> > #  endif
> > #endif
> >
> > BOOST_HAS_VISIBILITY would be undefined for most compilers,
> > but would be
> > defined for GCC (when __GNUC__ is defined, perhaps after
> > some particular
> > release, but I don't know when it was introduced).
>
> I had the same idea, but decided to take small steps.
> First, get visibility in with a minimal diff, then add
> support to Boost.Build
> and then refactor. It is __GNUC_ > 4, by the way ;-))

That's correct for BOOST_DEFAULT_VISIBILITY_IS_HIDDEN (for -fvisibility and the pragma), but the attribute was available before that.

> > BOOST_DEFAULT_VISIBILITY_IS_HIDDEN must be defined for GCC,
> > when __GNUC__
> > >= 4, but only if using -fvisiblity=hidden.  I haven't
> > found a way to
> > detect that, so I guess it has to be assumed or defined via
> > the command line.
>
> No. Visibility can always be explicitly specified. So no
> BOOST_DEFAULT_VISIBILITY_IS_HIDDEN macro is needed.
> I would simply always #define BOOST_SYMBOL_EXPORT/IMPORT to
> the correct values.

Excellent point.

_____
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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