[CMake] problem with export(TARGETS ...)

[Clinton Stimpson]

On Thursday, January 27, 2011 03:23:44 pm Brad King wrote:
> On 01/27/2011 03:08 PM, Clinton Stimpson wrote:
> > add_library(A SHARED a.cpp)
> > add_library(B SHARED b.cpp)
> > target_link_libraries(B A)
> > set_target_properties(B PROPERTIES LINK_INTERFACE_LIBRARIES "")
> > export(TARGETS B FILE B-exports.cmake)
> > 
> > that gives me an error
> > "export called with target "B" which requires target "A" that is not in
> > the
> > 
> >   export list."
> 
> Although A is not in B's link interface it is in its link implementation.
> When an application links to shared library B the linker wants to find
> B's runtime dependencies.  See "man ld" for the "-rpath-link" option.
> CMake needs to export enough information so that when the application
> imports target B and does
> 
>   target_link_libraries(myexe B)
> 
> then it can provide enough information to the linker to find library A.
> This is conservative to ensure things work whether or not B's RPATH
> is set (and therefore independent of RPATH settings).
> 
> Temporarily change the above example export() call to
> 
>   export(TARGETS B A FILE B-exports.cmake)
> 
> Then inspect the generated B-exports.cmake.  It will have:
> 
>   SET_TARGET_PROPERTIES(B PROPERTIES
>     IMPORTED_LINK_DEPENDENT_LIBRARIES_NOCONFIG "A"
>     ...
>     )
> 
> to tell the importing CMake that it needs to ensure the linker can
> find A when the application links to B even though A will not appear
> on the link line.
> 
> > If I change A to be a static library, the error goes away.
> 
> This is because A will be linked into B so when linking to B there is
> no need for the linker to look for A.
> 
> > If I change A to be a static library, and don't set the
> > LINK_INTERFACE_LIBRARIES property on B, I get the error.
> 
> This is because A remains in B's link interface and thus when an
> application links to B CMake passes A on the link line too.
> 
> > I don't want to export private shared libraries.  Is there some
> > other property like LINK_INTERFACE_LIBRARIES that I need to set?
> 
> Currently there is no way to do this due to the above requirements.
> If A is not used by other shared libraries then you can make it
> static and compile with -fPIC.  Otherwise you'll just have to
> document that it is not part of your interface.
> 
> Moving forward, we could consider adding a new property for IMPORTED
> targets to mark them as "private".  This would tell CMake that it
> is imported only for purposes like the above and should not be
> referenced directly by the importing project.
> 
> -Brad


Thanks, that explains it.  But I did have one issue about the 
usability/complexity of this export stuff.  These "private" libraries include 
some 3rd party libraries.  I'd like to avoid modifying other people's 
CMakeLists.txt files to put "EXPORT MyExportGroup" into install(TARGET ...) 
commands, to be able to export my libraries.

What's the reason for that being required?
From my point of view, it seems it would be cleaner and simpler it weren't 
required.  What I see is cmake complaining about it not being in the export 
group, when it could probably include it for me.  Perhaps the auto included 
ones could be marked as private?

-- 
Clinton Stimpson
Elemental Technologies, Inc
Computational Simulation Software, LLC
www.csimsoft.com