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

From: Alexander Grund (alexander.grund_at_[hidden])
Date: 2021-02-02 16:40:36

Am 02.02.21 um 17:25 schrieb Andrey Semashev via Boost:
> On 2/2/21 7:09 PM, Alexander Grund via Boost wrote:
>>
>> Peter and I found a simple way to reproduce the behavior I was seeing 
>> on Mac: Use libc++
>> `./b2 link=shared libs/program_options/test/ toolset=clang 
>> stdlib=libc++` is enough to reproduce the problem.
>>
>> I'm actually surprised that this hasn't come up before. No tests for 
>> all boost with libc++?
>
> I suspect, Mac OS (more precisely, its linker) is the crucial part, 
> not libc++. clang+libc++ on Linux is relatively well tested (or at 
> least used to be, when Travis was more usable), but Mac OS is less 
> tested because of slow turnaround.

The above command which tests Boost.ProgramOptions using clang + libc++ 
on Linux shows the same failure I see on OSX due to using DECL instead 
of VISIBLE on the exception class defined in the header. So this setting 
is obviously untested, which I find odd.

IIRC OSX uses libc++ too which explains the correlation.

>>> BOOST_SYMBOL_IMPORT doesn't need to expand to BOOST_SYMBOL_VISIBLE 
>>> when the marked symbols are only defined in the library, which is 
>>> the case normally. When the user consumes the library, no markup is 
>>> needed as the compiler will simply leave unresolved references to 
>>> those symbols, to be resolved by the linker.
>>> Â Basically, BOOST_SYMBOL_IMPORT is only there for compatibility with 
>>> Windows.
>> Agreed with the first part.
>> But quite many are defined in headers, especially the exceptions, so 
>> there it is required.
>
> For classes defined in a header, neither EXPORT nor IMPORT are needed. 
> Only VISIBLE is, if you need publicly visible RTTI.

Ok great, so we have a few actual bugs now: Not using VISIBLE for those.

Next issue are classes declared in the header and defined in a cpp file 
which should be (able to be) caught by consumers. Due to the definition 
in the CPP I think they need the DECL (->EXPORT/IMPORT) but also VISIBLE.

For comparison again: CMake defines the IMPORT macro to VISIBLE, so 
that's a datapoint at least: 
https://github.com/Kitware/CMake/blob/36bb0e32d7e3976eed424078cf5cac459651f0f1/Modules/GenerateExportHeader.cmake#L309-L311