Re: Detect '-fvisibility=hidden' during pre process or compile time

[Alexander Monakov <amonakov@ispras.ru>]

On Fri, 22 Apr 2022, Max Sagebaum wrote:

> Hello @ all,
> 
> Short question: Is there a way to detect the option '-fvisibility=hidden'
> during the preprocessor or the compile time, such that a warning to the user
> can be issued, that the library might not support it?

No (note that visibility can change during optimization, like when during LTO
the compiler learns that external references will not be possible).

> Long question: We develop a header only library where we have a static
> structure member inside of a class. We require that this static member is seen
> by all operations on this class.
> 
> If our library is included with '-fvisibility=hidden' then we get multiple
> symbols of the same static member. We tried to fix this by declaring the
> static member with '__attribute__((visibility("default")))' but for members
> which are structs this does not seem to work.  I filed an bug report about
> this and got the answer, that  this is not a bug but the behavior is as
> desired.
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104631

Well, it is not a matter of a scalar vs a struct type. The issue is that the
containing class is a template, and when you instantiate it, template parameter
has hidden visibility. I've posted a short example to the above bug.


> Since we can not change the visibility of the static member and do not want to
> change the visibility of the while library. (Since there is the real use case,
> that our library is only used in a different library and all the symbols do
> not need to be exposed.)

You can change the visibility of the member if you don't require that its type
is a template parameter.

Alexander