Testing cmake to build a library with linking another external library on multiple platforms Linux, MS Windows, macOS and others.

This is mainly a simple test project for my own to learn how cmake commonly builds on different platforms. Here I found some very useful articles:

• Creating and using shared libraries with dfferent compilers on different operating systems
• CMake link shared library on Windows
• Don't know how to load and use dll file in C
• Importing and Exporting public symbols

I first create the external library using bash:

~$ cd linux/  # This is the project source directory
linux$ cmake -S external_lib/ -B external_lib/build/
linux$ cmake --build external_lib/build/

I find the shared library at external_lib/build/. Now build the main program

linux$ cmake -S . -B build
linux$ cmake --build build

We find the shared main program at build/.

linux$ ./build/main
Hello World from external library :-)

With the Microsoft Visual C Compiler (MSVC) external symbols are not exported by default. We explicitly have to do it with program decorators __declspec(dllexport) on symbols in the library (.dll) and with __declspec(dllimport) on symbols in the main program (.exe), means where the functions from the library are used. I create the external library using the PowerShell:

PS > cd ms-windows\  # This is the project source directory
PS ms-windows> cmake -S .\external_lib\ -B .\external_lib\build\
PS ms-windows> cmake --build .\external_lib\build\ --config Release

I find the shared library (.dll and corresponding .lib) at .\external_lib\build\Release\. Now build the main program

PS ms-windows> cmake -S . -B .\build\
PS ms-windows> cmake --build .\build\ --config Release

I find the shared main program at .\build\Release\ and need to copy the .dll to the main program, otherwise the main program will not find its library. In this case the main program will terminate silently. If you do not get the greeting then look for the .dll:

PS ms-windows> copy .\external_lib\build\Release\hello.dll .\build\Release\
PS ms-windows> .\build\Release\main.exe
Hello World from external library :-)

We have to ensure that both configurations above are running with one cmake setup. The problem is the different behavior with symbol exports of the different compiler. GCC on Linux exports all symbols by default but MSVC on MS Windows does not and need decorators as already described. We solve this with a preprocessor makro as defined in external_lib/hello.h and triggered with a target_compile_definitions() in external_lib/CMakeLists.txt.

Change to the subproject source directory cd multiplatform. Now we can use the commands as shown above for the respective platform with the same cmake multiplatform setup.