#49 Link failure when Qt6 is compiled with visibility protected

[Pedro Lopez-Cabanillas]

The origin of this breakage can be traced to this work: Remove copy relocation and optimize locally defined symbol access by Thiago, that finally made its way into Qt6. The Arch Linux Qt6 packager used the new build flag FEATURE_no_direct_extern_access=ON when building Qt6 which exposed the build problem on my Drumstick Libraries.

When using FEATURE_no_direct_extern_access=ON, the macro Q_DECL_EXPORT will be defined as __attribute__((visibility("protected"))) in Linux/GCC, potentially breaking existing public shared libraries using this macro.

This ticket comes from GitHub issue #12.

[Pedro Lopez-Cabanillas]

Commit [97d89a88f864993759484741ec667f2f535b412f] on develop branch

[Pedro Lopez-Cabanillas]

CMake has an alternative solution, which has been discarded:

https://cmake.org/cmake/help/latest/module/GenerateExportHeader.html

https://cmake.org/cmake/help/latest/prop_tgt/DEFINE_SYMBOL.html

set(CMAKE_CXX_VISIBILITY_PRESET hidden)
set(CMAKE_VISIBILITY_INLINES_HIDDEN 1)
add_library(somelib someclass.cpp)
generate_export_header(somelib)
install(TARGETS somelib DESTINATION ${LIBRARY_INSTALL_DIR})
install(FILES
    someclass.h
    ${PROJECT_BINARY_DIR}/somelib_export.h DESTINATION ${INCLUDE_INSTALL_DIR}
)

This means that four headers will be generated, and would need to be installed:

• drumsmtick_alsa_export.h
• drumsmtick_file_export.h
• drumsmtick_rt_export.h
• drumsmtick_widgets_export.h

This solution will be cross-platform, and contain some other macros as well, but will be disconnected from any future Qt6 advances.

Using Q_DECL_EXPORT and Q_DECL_IMPORT, whatever new development in Qt6 on this area will be adopted by Drumstick as well when Linux distros migrate.