[CMake] How to specify debug version of CRT library for Visual Studio generator?

[Robert Maynard]

Just a heads up, CMake 3.15 is introducing policy 91 which removes the
runtime library from the default set of flags, and instead has targets
establish what runtime they want.

For more information see:
https://cmake.org/cmake/help/v3.15/prop_tgt/MSVC_RUNTIME_LIBRARY.html

On Tue, Jun 18, 2019 at 10:06 AM Eric Dönges <doenges at mvtec.com> wrote:
>
> On 18.06.19 12:53, David Aldrich wrote:
> > I have a simple CMake project that builds an executable using Visual
> > Studio 2017:
>
>
> >
> > ################ Files ################
> > #   --   Add files to project.   --   #
> > #######################################
> >
> > file(GLOB SRC_FILES
> >     ${CPP_DIR_1}/*.cpp
> >     ${CPP_DIR_2}/*.cpp
> >     ${CPP_DIR_3}/*.cpp
> >     ${CPP_DIR_4}/*.cpp
> >     ${HEADER_DIR_1}/*.h
> > )
> >
> > # Add executable to build.
> > add_executable(${PROJECT_NAME}
> >    ${SRC_FILES}
> > )
> >
> > if(MSVC)
> >    target_link_libraries(${PROJECT_NAME} ws2_32.lib )
> > endif(MSVC)
> >
> > #=====================================
> >
> > The Release build succeeds but the Debug build fails with linker errors
> > such as:
> >
> > [build] CPlaneTest.obj : error LNK2001: unresolved external symbol
> > __imp___CrtDbgReport
> >
> > I think this is because the linker is not using the debug version of CRT
> > (C Run-time Library).
> >
> > Should CMake select the debug build of CRT automatically or do I need to
> > specify it manually? If so, should I do so using CMAKE_EXE_LINKER_FLAGS?
> >
>
> CMake will select the correct CRT automatically if you let it (unless
> you want the static CRT, in which case you have to override CMake's
> default settings). You are setting your CMAKE_CXX_FLAGS_DEBUG incorrectly:
>
> > if(MSVC)
> >    #set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} /D _DEBUG /W3
> > /MD /Od /Zi /EHsc")
> >    set(CMAKE_CXX_FLAGS_RELEASE "${CMAKE_CXX_FLAGS_RELEASE} /W3 /GL /Od
> > /Oi /Gy /Zi /EHsc")
> >    set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_RELEASE} /D _DEBUG /W3
> > /GL /Od /Oi /Gy /Zi /EHsc")
> > endif(MSVC)
>
> In case of the setting you've commented out, you are explicitly telling
> CMake to use /MD. CMAKE_CXX_FLAGS_DEBUG should already contain /MDd, but
> since you append the /MD, that is what the compiler will actually use.
>
> For the setting that is not commented out, you set CMAKE_CXX_FLAGS_DEBUG
> to the contents of CMAKE_CXX_FLAGS_RELEASE - which is certainly not what
> you want (and also specifies /MD).
>
> I would suggest looking at what flags CMake sets by default (look at the
> Windows-MSVC.cmake file in CMake's 'Modules/Platform' directory) and
> only setting those flags that CMake doesn't already. For version 3.14,
> CMake should be setting the following flags for CMAKE_CXX_FLAGS_DEBUG by
> default (assuming you are using MVSC >= 1310, no Clang toolset):
>
>         /MDd /Zi /Ob0 /Od /GR /EHSC
>
> So in your case, it would probably be enough to
>
>         string(APPEND CMAKE_CXX_FLAGS_DEBUG " /D_DEBUG /W3")
>
> As a final recommendation, use string(APPEND <var> ...) (or list(APPEND
> <list> ...), if the variable is interpreted as a list) when appending
> values to existing variables. This makes your intent clearer.
>
>
>  - when appending compiler flags, use the "string(APPEND ...)" command
> to make is clearer what you are doing).
> --
>
> Powered by www.kitware.com
>
> Please keep messages on-topic and check the CMake FAQ at: http://www.cmake.org/Wiki/CMake_FAQ
>
> Kitware offers various services to support the CMake community. For more information on each offering, please visit:
>
> CMake Support: http://cmake.org/cmake/help/support.html
> CMake Consulting: http://cmake.org/cmake/help/consulting.html
> CMake Training Courses: http://cmake.org/cmake/help/training.html
>
> Visit other Kitware open-source projects at http://www.kitware.com/opensource/opensource.html
>
> Follow this link to subscribe/unsubscribe:
> https://cmake.org/mailman/listinfo/cmake