FindDL.cmake is not working when cross-compiling to WebAssembly via Emscripten

[osrf-migration]

Original report (archived issue) by Silvio Traversaro (Bitbucket: traversaro).

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When cross-compiling using Emscripten ( https://emscripten.org/ ) find_package(DL REQUIRED) should always be successful, as the functions usually contained in the dlfcn.h header are already linked without the need of linking any additional library (such as dl in Linux).

However, instead the FindDL.cmake script currently provided in ignition-cmake fails, for two reasons:

• It searches for a dl library in any non-Windows system, while it should observe the CMAKE_DL_LIBS standard CMake variable and only search for a library if CMAKE_DL_LIBS is non-empty.

• For some reason, the script fails to find the dlfcn.h system header provided by emscripten

The solution for the first problem is trivial, but I am not sure what is the reason behind the second problem.

[osrf-migration]

Original comment by Silvio Traversaro (Bitbucket: traversaro).

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• Edited issue description

[osrf-migration]

Original comment by Silvio Traversaro (Bitbucket: traversaro).

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• Edited issue description

[osrf-migration]

Original comment by Silvio Traversaro (Bitbucket: traversaro).

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If someone is interested in reproducing this problem, you can check the colcon instructions provided in https://gist.github.com/traversaro/4e6b094206001d9aa444e518afdb884d .

[osrf-migration]

Original comment by Silvio Traversaro (Bitbucket: traversaro).

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The first problem could be solved by changing the part related to find_library(DL_LIBRARIES dl) with:

  if(CMAKE_DL_LIBS)
    find_library(DL_LIBRARIES ${CMAKE_DL_LIBS})
    if(DL_LIBRARIES)

      if(NOT DL_FIND_QUIETLY)
        message(STATUS "Looking for libdl - found")
      endif()

    else(DL_LIBRARIES)

      if(NOT DL_FIND_QUIETLY)
        message(STATUS "Looking for libdl - not found")
      endif()

      set(DL_FOUND false)

    endif()
    mark_as_advanced(DL_LIBRARIES)
  endif()