Re: Wrong class export/visibility setting for shared libs in many Boost libs

From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2021-02-02 14:27:25

On 02/02/2021 14:09, Peter Dimov via Boost wrote:
> Andrey Semashev wrote:
>> So, the bottom line, BOOST_PROGRAM_OPTIONS_DECL must be defined to:
>>
>> - BOOST_SYMBOL_EXPORT - on Windows, when the library is being built;
>> - BOOST_SYMBOL_IMPORT - on Windows, when the library is being consumed
>> by user;
>> - BOOST_SYMBOL_VISIBLE - in any other case.
> 
> BOOST_SYMBOL_EXPORT is defined to BOOST_SYMBOL_VISIBLE on non-Windows in
> order to make the third case unnecessary. Alexander suggests that on
> macOS (but not Linux), it's also necessary to define BOOST_SYMBOL_IMPORT
> to BOOST_SYMBOL_VISIBLE (instead of to nothing) on non-Windows. This
> seems sensible.

MachO has the same symbol visibility semantics as PE i.e. correct.

For pure compatibility with Linux reasons, Apple engineers have mapped
the GNU visibility attributes to the MachO equivalent of
__declspec(dllexport) and __declspec(dllimport). I argued with them at
the time that that was daft, but Linux compatibility without forcing
source changes was important to them.

I honestly don't know if you need to mark thrown types as dllexport in
MachO. I don't use Mac OS enough to know. But I would be very surprised
if you did, as MachO isn't broken here.

I also have anecdata that Louis found "exceptions work right over shared
libraries" on Mac OS despite them not working on Linux without
annotation. I remember him once reaching out to me to ask why.

Niall