[CMake] Dependency cycle - why?

[Robert Maynard]

Hi,

The goal that you have is fully supported by CMake. You have just run
into a bug in CMake, and you should report this to
https://gitlab.kitware.com/cmake/cmake/issues .
Basically at a very high level the name out the add_executable target
`callback_generator` is the same as the internal name that CMake is
using for the add_custom_command. This than causes some logic in CMake
(cmTargetTraceDependencies) to incorrectly build another link between
the add_custom_command and your add_executable.

The easiest way to solve this issue is name the add_executable target
to have a different name than the output of your custom command minus
file extension. So maybe something like  `callback_generator_exec`

On Fri, May 24, 2019 at 2:02 PM Bill Somerville <bill at classdesign.com> wrote:
>
> On 13/05/2019 12:03, Bill Somerville wrote:
>
> Hi folks,
>
> I am struggling to understand what the problem is with this:
>
> find_program (GO go)
> set (GOPATH "${CMAKE_CURRENT_BINARY_DIR}/go")
> file (MAKE_DIRECTORY ${GOPATH})
>
> set (sources ${CMAKE_CURRENT_SOURCE_DIR}/callback_generator.go)
> set (build_command ${GO} build)
> set (output callback_generator${CMAKE_EXECUTABLE_SUFFIX})
> add_custom_command (OUTPUT ${output}
>   COMMAND ${CMAKE_COMMAND} -E env GOPATH=${GOPATH} CGO_CFLAGS=${CMAKE_CGO_CFLAGS} ${build_command} -o ${output} ${CMAKE_GO_FLAGS} ${sources}
>   DEPENDS ${sources}
>   VERBATIM)
> add_custom_target (callback_generator_target DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/${output})
> add_executable (callback_generator IMPORTED)
> set_target_properties (callback_generator
>   PROPERTIES
>   IMPORTED_LOCATION ${CMAKE_CURRENT_BINARY_DIR}/${output}
>   )
> add_dependencies (callback_generator callback_generator_target)
>
> add_custom_target (server ALL DEPENDS callback_generator)
>
> which gives this error:
>
> >cmake --version
> cmake --version
> cmake version 3.14.3
>
> >cmake -G "MinGW Makefiles" ..\..
> cmake -G "MinGW Makefiles" ..\..
> -- Configuring done
> CMake Error: The inter-target dependency graph contains the following strongly connected component (cycle):
>   "callback_generator_target" of type UTILITY
>     depends on "callback_generator_target" (strong)
> The component contains at least one cycle consisting of strong dependencies (created by add_dependencies) that cannot be broken.
>
> The set_target_properties () call seems to be part of the problem but I don't understand why.
>
> Regards
> Bill Somerville.
>
> Hi again,
>
> answering my own post since no one has replied. Clearly my question was too specific. Let me try again with a more general question. I have tools that can generate executables (programs, static libraries, and shared libraries, including DLL import libraries on MS Windows). These tools are not directly supported by CMake but they are both needed within a project that uses them and installs them as the project artefacts. How do I integrate, for example a shared library, into a CMake project as if it were produced by a supported toolchain like C or C++? The library is produced by an add_custom_target() command (not add_custom_command() since the underlying tools do their own dependency checking so it is right to run the target commands every build). I need to have a target that I can use in target_link_libraries() and install(TARGETS ...) just like one would with a CMake generated library made with add_library() and programs made with add_executable().
>
> If it helps the tools that are not supported by CMake are the Go language build tools. The project builds some Go programs as utilities but the main project artefact is a shared library with a C language API built using the cgo tools. There are many of C and C++ test programs and example programs as well in the project which need to link against the project shared library etc.
>
> Regards
> Bill Somerville.
>
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