@jcfr
Some systems and some language's dev packages don't contain the language library, and require the usage of dynamic look up in the executable for the language symbols. The manylinux docker image is one case of this. Previously the CMake variable "SITK_UNDEFINED_SYMBOLS_ALLOWED" was used to determine if the libraries were required.

https://github.com/SimpleITK/SimpleITK/blob/master/CMake/sitkLanguageOptions.cmake#L23-L27

I don't see a replacement for this variable. Any suggestions on how to update the behavior?

@jcfr Any feedback or suggestions on this issue?

When using manylinux, the following is done:

https://github.com/SimpleITK/SimpleITKPythonPackage/blob/426b971c730ac5ceaf81f87fac0142192b9031f1/scripts/internal/manylinux-build-wheels.sh#L26-L40

This allows to keep find_package(PythonLibs) happy while ensuring no target is linking against the python library. Simply using QUIET would not play the same role.

What is missing from the solution of marking a find language library as QUIET, and then not linking against it?

The prior solution does not include a check to ensure linking does not occur, but it does not require custom configuration when a system or language is encountered without the libraries. The problem ( or purposeful intent ) of language development packages not including libraries has occurred several time in the real world ( outside of building on manylinux). It is necessary to continue supporting these systems output the box.

An improved solution to the existing one is welcomed, if it meets the requirements.

solution of marking a find language library as QUIET, and then not linking against it?

Looking at your previous link, I missed the part where it doesn't link against it. It looks like this is done in function sitk_target_link_libraries_with_dynamic_lookup implemented in sitkTargetLinkLibrariesWithDynamicLookup.cmake#L81-L89

It turns out that the implementation of that same function in scikit-build also conditionally links against the python library. We are good on that front.

The problem is in two locations:

• (1) SimpleITK/Wrapping/Python/CMakeLists.txt where SITK_UNDEFINED_SYMBOLS_ALLOWED is not set anymore.

and

• (2) SimpleITKPythonPackage/CMakeLists.txt
  

Problem (1)

For the problem (1), changing that code from

if ( SITK_UNDEFINED_SYMBOLS_ALLOWED )
  set( _QUIET_LIBRARY "QUIET" )
else()
  set( _QUIET_LIBRARY "REQUIRED" )
endif()

to

check_dynamic_lookup(MODULE
  SHARED
  SITK_UNDEFINED_SYMBOLS_ALLOWED
  unused
  )
if ( SITK_UNDEFINED_SYMBOLS_ALLOWED )
  set( _QUIET_LIBRARY "QUIET" )
else()
  set( _QUIET_LIBRARY "REQUIRED" )
endif()

should address the issue.

Similarly to the other CMake modules, I will make sure it is documented on read the docs.

In the mean time, associated documentation is here: https://github.com/scikit-build/scikit-build/blob/d592c8084a7e0502d9da9e7aacf0e465be40cc67/skbuild/resources/cmake/targetLinkLibrariesWithDynamicLookup.cmake#L61-L92

Problem (2)

For the problem (2) in SimpleITKPythonPackage/CMakeLists.txt, the code:

  if(NOT DEFINED PYTHON_INCLUDE_DIR
     OR NOT DEFINED PYTHON_LIBRARY
     OR NOT DEFINED PYTHON_EXECUTABLE)

    find_package ( PythonLibs REQUIRED )
    find_package ( PythonInterp REQUIRED )

  endif()

should be changed into

  # TODO: Download  https://raw.githubusercontent.com/scikit-build/scikit-build/v0.4.0/skbuild/resources/cmake/targetLinkLibrariesWithDynamicLookup.cmake
  include(targetLinkLibrariesWithDynamicLookup)
  check_dynamic_lookup(MODULE
    SHARED
    SITK_UNDEFINED_SYMBOLS_ALLOWED
    unused
    )

  if(NOT DEFINED PYTHON_INCLUDE_DIR
       OR NOT DEFINED PYTHON_LIBRARY
       OR NOT DEFINED PYTHON_EXECUTABLE)
    
    if ( SITK_UNDEFINED_SYMBOLS_ALLOWED )
      set( _QUIET_LIBRARY "QUIET" )
    else()
      set( _QUIET_LIBRARY "REQUIRED" )
    endif()
    find_package ( PythonLibs ${_QUIET_LIBRARY})
    find_package ( PythonInterp REQUIRED )

  endif()

Todo:

• Generate documentation for targetLinkLibrariesWithDynamicLookup.cmake
• Improve check_dynamic_lookup so that the fourth argument link_flags_var is optional
• Even better, support signature check_dynamic_lookup(SITK_UNDEFINED_SYMBOLS_ALLOWED) that would be equivalent of calling check_dynamic_lookup(MODULE SHARED SITK_UNDEFINED_SYMBOLS_ALLOWED unused)

Thanks!

And it's more that just Python. All language library linking goes through sitk_target_link_libraries_with_dynamic_lookup.

scikit-build/scikit-build#230 is adding documentation along with support for the short signature check_dynamic_lookup(<ResultVar>)

CircleCI continuous build's configuration looks OK.

Merging into SimpleITK for nightly testing across platforms.