[CMake] Link against exe on Windows fails - .a file not found

[Michael Ho]

Ah yes, I forgot Windows doesn't export symbols by default.
Unfortunately adding set_target_properties(${PROJECT_NAME} PROPERTIES WINDOWS_EXPORT_ALL_SYMBOLS 1) didn't seem to help however - I get the same file not found error.

Something interesting I noticed however is that if I copy and rename ws2editor/CMakeFiles/ws2editor.dir/objects.a to ws2editor/libws2editor.dll.a, the plugin links just fine and even works when I run the application. I suppose I could add this as some post build step, but it certainly feels like a bit of a hacky solution. This only seems to work for the Unix Makefiles generate however (Which should be fine, although I had been using ninja at the moment so certainly not ideal).

Thanks
- Mike

(Note: sorry for the mostly duplicate message - I wasn't aware that you should use "Reply All" in mailing lists)

________________________________
From: Jano Svitok <jan.svitok at gmail.com>
Sent: 14 May 2018 00:34
To: Michael Ho
Cc: cmake at cmake.org
Subject: Re: [CMake] Link against exe on Windows fails - .a file not found

On Sun, May 13, 2018 at 1:41 PM, Michael Ho <CraftedCart at outlook.com<mailto:CraftedCart at outlook.com>> wrote:
>
> Hey there.
>
> I'm trying to make a plugin system for my application, and thus each plugin will need to link with my main application. This is fine on Linux and macOS, as I can just set the ENABLE_EXPORTS property of my executable target (set_target_properties(${PROJECT_NAME} PROPERTIES ENABLE_EXPORTS 1), and plugins will link just fine. On Windows however, clang++ yells at me that it can't find libws2editor.dll.a.
>
> clang++.exe: error: no such file or directory: 'ws2editor/libws2editor.dll.a'
>
> Upon looking over the CMake docs, it says "For DLL platforms an import library will be created for the exported symbols and then used for linking." - This import library file (presumably the .dll.a) never seems to be created though.
>
>
> After a bit more digging around I came across https://github.com/Kitware/CMake/blob/master/Tests/Plugin/CMakeLists.txt - When I clone the CMake repo and try and compile that test myself, it also fails with the same error when using clang + ninja! When building with MSVC/msbuild however it compiles just fine (My project fails to configure when generating files for MSVC however, and ideally I'd like to use the same build config across Win/Mac/Linux).
>
> So how can I get the CMake build on Windows to create this file, or otherwise link with an executable?

You need to 1. mark some/all symbols to export 2. actually export them
ENABLE_EXPORTS does #2. For #1, you need to either mark each symbol to export as __declspec(dllexport) or mark all symbols with WINDOWS_EXPORT_ALL_SYMBOLS (see https://blog.kitware.com/create-dlls-on-windows-without-declspec-using-new-cmake-export-all-feature/, check also comments for possible drawbacks).

https://github.com/Kitware/CMake/blob/master/Tests/Plugin/include/example.h shows how to use __declspec(dllexport): for each dll you define a macro (EXAMPLE_EXPORT here) that becomes either export (when #included from dll) or import (when #included from code linked to dll). Every class/function/variable you want to use from the plugin must be marked this way (example_exe_function). For class members, it's enough to mark the class.
Finally there's a #define (example_exe_EXPORTS here) that controls how is the macro defined. See https://cmake.org/cmake/help/git-master/prop_tgt/DEFINE_SYMBOL.html (I didn't know this until now, we used to manually define the symbol!)

Jano
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