[CMake] Can imported libraries depend on ExternalProject targets?

[Kent Williams]

OK, I guess.

The only reason I bring this up is ITK. If you're familiar with the
ITK build process, it has a 'module' concept -- not a module in the
CMake sense (where it is a library intended for runtime loading), but
in the sense that the build process is modular.  Each of the ITK
libraries is a module, which is defined by a standardised directory
layout and cmake files.

I made an Module for DCMTK that satisfies the requirements of an ITK
module -- it builds DCMTK as an External Project, and uses
add_library(name <lib-type> IMPORTED) on each of the libraries DCMTK
creates, and connects the imported library with the actual library
file in the file system.

This all works fine EXCEPT for this one conundrum, you can't have the
imported libraries depend on the ExternalProject target, so if you
want to make sure the ExternalProject gets built before the targets
that try to link to them, you have to make the executable (or library)
target depend on the ExternalProject target to serialize the build of
the dependee before the depnder.

So really it would be easy to just say 'too bad, make ITK its own
external project, and build the prerequisite' or 'too bad, add the 3rd
party library source to ITK/Modules/ThirdParty the way we've always
done it," but it would be kind of awesome if CMake could handle
imported libraries depending on targets.

If you're curious about what I've done:
http://review.source.kitware.com/#/c/5989/

On Fri, Jun 22, 2012 at 12:29 PM, David Cole <david.cole at kitware.com> wrote:
> On Fri, Jun 22, 2012 at 11:33 AM, Kent Williams <nkwmailinglists at gmail.com>
> wrote:
>>
>> Say I have an ExternalProject that generates several libraries
>>
>> ExternalProject_Add(foo
>>  # the usual mumbo jumbo
>> )
>>
>> set(foo_LIBRARIES)
>>
>> # foo makes 3 libraries
>> foreach(lib a b c)
>>  # import the libraries
>>  add_library(${lib} STATIC IMPORTED)
>>
>>  # tell CMake where the library file is
>>  set_property(TARGET ${lib} PROPERTY
>>     IMPORTED_LOCATION
>>     ${imported_library_filename})
>>
>>  # add to the library list
>>  list(APPEND foo_LIBRARIES ${lib})
>>
>>  # this doesn't work apparently
>>  add_dependencies(${lib} foo)
>> endforeach()
>>
>> In order for parallel make to work, the foo ExternalProject must
>> complete successfully before any programs that link to
>> ${foo_LIBRARIES} are compiled and linked.
>>
>> I thought that making the imported library targets depend on the
>> ExternalProject target would behave in a transitive manner - i.e.
>>
>> add_executable(foo_user foo.cxx)
>> target_link_libraries(foo_user ${foo_LIBRARIES})
>>
>> But this is not the case. In a parallel build, the foo_user build
>> doesn't wait for the foo ExternalProject to finish.
>>
>> If I add
>>
>> add_dependencies(foo_user foo)
>>
>> Everything behaves fine.  But that requires explicitly add a
>> dependency on an ExternalProject target everywhere it's outputs are
>> used.
>>
>> Is this a bug? A feature request? Or is there another way to make this
>> work?
>
>
>
> It's neither a bug nor a feature request, it's just the way it works. The
> explicit dependency is the only way to connect up the outputs of one
> ExternalProject call to another, which by their nature are independent of
> one another unless explicitly connected via arguments / cache entries.
>
> If you want foo_user to depend on the actual libraries, then it should be
> its own project that does a find_package(foo) to get them. And then *also*
> have a SuperBuild that builds foo and then foo_user, where foo_user as an
> ExternalProject depends on foo as an ExternalProject.
>
> The best way to use ExternalProject is to have a SuperBuild project that
> builds *everything* as an ExternalProject.
>
> It's not easy (or advisable, in my thinking) to combine ExternalProject
> calls with non-ExternalProject CMake targets. That's why I recommend a
> SuperBuild, with exclusively ExternalProject targets, as the best bet.
>
>
> HTH,
> David
>
>
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