[cmake-developers] Importing platform SDK libraries without a full path

[Brad King]

Hi Folks,

Last year FindOpenGL gained imported targets:

 FindOpenGL: Provide imported targets for GL and GLU
 http://cmake.org/gitweb?p=cmake.git;a=commitdiff;h=b7804cb6

only to be reverted prior to release:

 FindOpenGL: Revert support for imported targets
 http://cmake.org/gitweb?p=cmake.git;a=commitdiff;h=02e34de2

due to this issue:

 FindOpenGL: need name-only imported targets on Windows
 http://www.cmake.org/Bug/view.php?id=15267

The problem is that we need a way to link to 'opengl32.lib' through
an imported target without knowing the full path to the library file
and instead ask the linker to search for it in the platform SDK
directory at link time.  Without a full path we cannot set the
IMPORTED_LOCATION property of an imported OpenGL::GL target.  We
cannot lift the requirement that IMPORTED_LOCATION be a full path
because that is fundamental to the design of imported targets and
is needed for things like $<TARGET_FILE:...> to work correctly.

The only kind of imported target that does not require an
IMPORTED_LOCATION is an INTERFACE library, so on Windows the OpenGL::GL
target will have to be an imported INTERFACE library.  We could use
INTERFACE_LINK_LIBRARIES to specify 'opengl32.lib', but that would not
preserve the inline library ordering as using IMPORTED_LOCATION does.

Instead I propose a new IMPORTED_LINK_ITEM[_<CONFIG>] property to take
the place of IMPORTED_LOCATION[_<CONFIG>] for an imported INTERFACE
library.  It will allow one to specify exactly one raw item to be
placed on the link line for the interface library since it has no
location.  For the above case this item would be just 'opengl32'.
This could also be used for cases like FindThreads and '-pthread'.

I've drafted an implementation of the approach here:

 Allow imported INTERFACE libraries to specify a link item
 http://cmake.org/gitweb?p=cmake.git;a=commitdiff;h=a6edb953

and used it to provide the imported OpenGL::GL target:

 FindOpenGL: Provide imported targets for GL and GLU
 http://cmake.org/gitweb?p=cmake.git;a=commitdiff;h=65d0930e

I'd appreciate feedback from others on this approach.

Thanks,
-Brad


P.S.  My original attempt at allowing link item specification was
much more general.  It was an INTERFACE_LINK_ITEMS property on
INTERFACE libraries that could be specified on in-project targets
and exported during installation:

 Allow INTERFACE libraries to specify link items
 http://cmake.org/gitweb?p=cmake.git;a=commitdiff;h=e4adad1e

I reverted it from 'next' and never merged it to 'master' due to
overlap with possible LINK_OPTIONS/ARCHIVE_OPTIONS work discussed
previously.  Instead I propose the more narrow IMPORTED_LINK_ITEM
approach to resolve the problem for FindOpenGL without tackling the
larger design goals yet.  Basically IMPORTED_LINK_ITEM completes
the design of IMPORTED_LOCATION as might have been done if interface
libraries existed when imported targets were first created.