[CMake] Can imported libraries depend on ExternalProject targets?

[Stefan Reuschl]

Am 22.06.2012, 20:21 Uhr, schrieb Kent Williams  
<nkwmailinglists at gmail.com>:

> 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.

The following once worked fine for me using CMake 2.8.8:

ExternalProject_Add( ep )
add_library( epLib IMPORTED )
set_target_properties( epLib PROPERTIES IMPORTED_LOCATION ... )
add_dependencies( epLib ep )

> 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.

No matter if this works, I would like to suggest another approach,
which I recently tested for the same use case and which might be
helpful:

The next problem of using ExternalProject this way is, that the user
of the external project need to know the location of public headers
and libraries...
The CMake way of exporting targets using projectConfig.cmake files won't
work because the externals project's config file will be created at build
time of the main project - while it is needed at configure time.

A workaround without superbuild which David suggested, is to download/
configure/build/install the external project at configure time of the
main project. After that, your main project can use find_package(ep).
This approach seems to work fine for me. A downside might be, that
the main project's configure step will take very long.
If the external project supports exporting targets from the build
tree, only the download and configure steps need to be executed at
configuration time, see:
http://www.cmake.org/Wiki/CMake/Tutorials/Exporting_and_Importing_Targets#Exporting_from_a_Build_Tree


>
> 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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>>
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