[CMake] Consuming results of ExternalProject_Add

[Jean-Christophe Fillion-Robin]

Hi Andrey,

On Tue, Feb 17, 2015 at 5:52 PM, Andrey Pokrovskiy <wonder.mice at gmail.com>
wrote:

>
> I don't see how External_OpenSSL.cmake is used in Slicer (probably
> some weird scheme is involved).
>

We used "Artichoke" that provides a set of convenience function to managed
"superbuild" based project.
See [1] and [2] for mode details.

[1] http://public.kitware.com/pipermail/cmake/2014-June/057735.html
[2] https://github.com/commontk/Artichoke



> But from what I see, it supposed to be used like that:
>
>   include(External_OpenSSL)
>   add_executable(my_executable main.cpp ...)
>   target_include_directories(my_executable ${OPENSSL_INCLUDE_DIR})
>   target_link_libraries(my_executable ${OPENSSL_LIBRARIES})
>
> That means that I will need to include External_OpenSSL in every
> CMakeLists.txt that uses OpenSSL. That doesn't sounds right to me.
>


One way of using external projects is to consider it automate the following
steps:

 (1) manual build of OpenSSL
 (2) configuration of your project Foo passing either
OPENSSL_CRYPTO_LIBRARY/OPENSSL_SSL_LIBRARY options on unix or
LIB_EAY_DEBUG/LIB_EAY_RELEASE/SSL_EAY_DEBUG/SSL_EAY_RELEASE on windows, and
then build of the project


And since there are no need to manually rebuild openssl for each
CMakeLists.txt in your project Foo, there are no need to include
"External_OpenSSL" in every CMakeLists.txt.

Reading [3] should also be helpful to understand the concepts.

[3]
http://www.kitware.com/media/html/BuildingExternalProjectsWithCMake2.8.html







>
> And if not, then something adds OPENSSL_INCLUDE_DIR and
> OPENSSL_LIBRARIES to CACHE which is also doesn't sounds right.
>
> Also this will not setup build dependency between target and OpenSSL.
> I know, I could use additional add_dependencies() for that, but that's
> clumsy. 3 lines instead of just:
>
>   target_link_libraries(my_executable crypto ssl)
>

After doing:

  find_package(OpenSSL)

in your project, you could then do:

  target_link_libraries(my_executable ${OPENSSL_LIBRARIES})




>
> CMake already has a concept of "libraries" (added with add_library).
> There should be a way of saying "Hey, this static library comes from
> that external project. It also requires such and such include
> directories".


In fact, all FindXXXX.cmake module are slowly been converted to create
CMake imported targets.

Since the current FindOpenSSL.cmake does not create imported targets, it
could be updated to do so. For an example, see [4]

[4]
http://www.cmake.org/cmake/help/v3.1/manual/cmake-developer.7.html#a-sample-find-module
[5] https://github.com/Kitware/CMake/commit/bcb0e38


Hth
Jc




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