This example shows different behaviour of cmake project build when building MODULEs on Linux and Mac OS

The example consists of three parts:

• AbstractPlugin OBJECT library defines a virtual class AbstractPlugin. It has one defined method preface and one pure virtual method run. The preface method required to be defined in the object library
• dynamic_lookup executable links with the AbstractPlugin library and therefore contains a definition for AbstractPlugin::preface method
• ModulePlugin MODULE library defines an implementation of AbstractPlugin. The resulting .so file contains the run definition and does not contain the preface method definition.

The CMakeLists.txt file describes an option DYNAMIC_LOOKUP (off by default) which adds a -undefined dynamic_lookup linker flag for the MODULE target.

dynamic_lookup executable lists preface symbol as defined:

$ nm -g build/dynamic_lookup | grep AbstractPlugin.preface | grep T
0000000100003010 T __ZN14AbstractPlugin7prefaceEv

and libModulePlugin module lists preface symbol as undefined:

$ nm -g build/libModulePlugin.so | grep AbstractPlugin.preface | grep U
                 U __ZN14AbstractPlugin7prefaceEv

Linux build (Linux Build CI) completes just fine: no need for extra flags on top of marking the libs as OBJECT and MODULE where needed:

$ cmake -B build
$ cmake --build build
[ 20%] Building CXX object CMakeFiles/AbstractPlugin.dir/AbstractPlugin.cpp.o
[ 20%] Built target AbstractPlugin
[ 40%] Building CXX object CMakeFiles/ModulePlugin.dir/ModulePlugin.cpp.o
[ 60%] Linking CXX shared module libModulePlugin.so
[ 60%] Built target ModulePlugin
[ 80%] Building CXX object CMakeFiles/dynamic_lookup.dir/main.cpp.o
[100%] Linking CXX executable dynamic_lookup
[100%] Built target dynamic_lookup

Mac OS build (Mac Build Without Flags CI) fails under the same conditions:

$ cmake -B build
$ cmake --build build
[ 20%] Building CXX object CMakeFiles/AbstractPlugin.dir/AbstractPlugin.cpp.o
[ 20%] Built target AbstractPlugin
[ 40%] Building CXX object CMakeFiles/ModulePlugin.dir/ModulePlugin.cpp.o
[ 60%] Linking CXX shared module libModulePlugin.so
Undefined symbols for architecture x86_64:
  "AbstractPlugin::preface()", referenced from:
      ModulePlugin::run() in ModulePlugin.cpp.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [libModulePlugin.so] Error 1
make[1]: *** [CMakeFiles/ModulePlugin.dir/all] Error 2
make: *** [all] Error 2

Mac OS build needs a separate linker flag -undefined dynamic_lookup in order to achive the same results (Mac Build With Flags CI):

$ cmake -B build -DDYNAMIC_LOOKUP=ON
$ cmake --build build
[ 20%] Building CXX object CMakeFiles/AbstractPlugin.dir/AbstractPlugin.cpp.o
[ 20%] Built target AbstractPlugin
[ 40%] Building CXX object CMakeFiles/ModulePlugin.dir/ModulePlugin.cpp.o
[ 60%] Linking CXX shared module libModulePlugin.so
[ 60%] Built target ModulePlugin
[ 80%] Building CXX object CMakeFiles/dynamic_lookup.dir/main.cpp.o
[100%] Linking CXX executable dynamic_lookup
[100%] Built target dynamic_lookup