[Insight-developers] Multiply defined symbols with MSVC and BUILD_SHARED_LIBS=ON

[Tom Vercauteren]

Hi all,

I am trying to help a fellow developer with building a project that
uses ITK on windows. The project needs shared libraries because of a
plugin mechanism.

Hence ITK was built with BUILD_SHARED_LIBS=ON. Building our project
fails at link time with the following error message:
  ITKNrrdIO.lib(miscAir.obj) : error LNK2005: _itk_airFree already
defined in mydll.lib(mydll.dll)
  fatal error LNK1169: one or more multiply defined symbols found

This arises each time we build an executable X that links with ITK and
with another shared libray A that also links with ITK.

I have filled a bug report here:
  http://www.itk.org/Bug/view.php?id=10699

Does anyone have a fix or a work-around for this problem?

Apparently, the same problem has been in ITK for a while but was
appearing with different symbols but my fellow developer apparently
managed to work-around it previously by carefully picking when to link
with what. I guess that this was only due to the fact that we were
never in a more complex scenario that we now see:
  X links with A and B. Both A and B link with some ITK library.

A wild guess from my part would be that this could be related to the
ITK_EXPORT macro. My basic understanding (I am not a regular windows
developer) is that visual c++ requires one specific export/import
macro per dll. ITK only uses ITKCommon_EXPORT and goes away with it by
only building one shared library (ITKCommon.dll) while the other are
built as static ones. This looks a bit weird to me. Could someone
explain me the rationale of this approach?

Cheers,
Tom