Hide non-API functions

[urisimchoni]

Usage of -fvisibility=hidden is the recommended default for DSO's, and is harmless in statically-linked objects.

If the spdlog build is for a shared library, then API symbols have SPDLOG_API marking on them, and cause the symbol to be exported, with other symbols now hidden by default.

If the build is for a static library with position-independent code, then spdlog is to be embedded as implementation detail in another shared library or executable. In that case we don't want to export any symbol. Conveniently, SPDLPG_API in that case resolves to nothing, so with the default of hidden all symbols are made hidden.

[gabime]

Thanks. Why only in PIC ? I think it can be used without pic as well.

[urisimchoni]

Thanks. Why only in PIC ? I think it can be used without pic as well.

I suppose it's not really tied to PIC, although the context in which it matters is dynamically-linked executables, which require PIC. Without the condition, the -fvisibility=hidden flag is given to static builds as well, where it should have no effect, so I suppose I can remove the condition.

[gabime]

We need to be careful since the bundled fmt use default visibilty. Not sure what would happen. i.e. If users of spdlog will be able to use the bundled fmt lib. Could you check?

[urisimchoni]

If I understand correctly, the concern is that an aplication should be able to use fmt::XXX embedded in the shared libspdlog.so.
I ran the following quick check:

• Add the following lines to example.cpp:

    auto s = fmt::format("i={}",1);
    printf("%s\n", s.c_str());

• Build spdlog with cmake -DSPDLOG_BUILD_SHARED=ON -DSPDLOG_BUILD_PIC=ON ../spdlog && make -j
• Run the example

I can also see that libspdlog.so built that way exports some fmt symbols.