[CMake] Transitive linking

[James Bigler]

On Fri, Nov 18, 2011 at 5:51 PM, Michael Hertling <mhertling at online.de>wrote:

> On 11/18/2011 10:03 PM, James Bigler wrote:
> > 2011/11/18 Alexander Neundorf <a.neundorf-work at gmx.net>
> >
> >> On Friday 18 November 2011, James Bigler wrote:
> >>> I thought CMake knew how to not drag all the dependent libraries once
> you
> >>> linked an executable module.
> >>>
> >>> add_library(A STATIC a.cpp)
> >>> add_library(B SHARED b.cpp)
> >>> target_link_libraries(B A)
> >>> add_library(C SHARED c.cpp)
> >>> target_link_libraries(C B)
> >>>
> >>> add_executable(run run.cpp)
> >>> target_link_libraries(run C)
> >>>
> >>> At this point I'm seeing that C links against B and A when I think it
> >>> should only link against B since A shouldn't be needed to link against
> B.
> >>> In addition when compiling run, it links against B and A.
> >>>
> >>> /usr/bin/c++   -dynamiclib -Wl,-headerpad_max_install_names   -o
> >> libC.dylib
> >>> -install_name
> /Users/jbigler/tmp/code/cmake/translinking/build/libC.dylib
> >>> CMakeFiles/C.dir/c.cpp.o libB.dylib libA.a
> >>> /usr/bin/c++    -Wl,-search_paths_first
> -Wl,-headerpad_max_install_names
> >>> CMakeFiles/run.dir/run.cpp.o  -o run  libC.dylib libB.dylib libA.a
> >>>
> >>> Is this the expected behavior?
> >>
> >> Yes.
> >> If you want to limit this, use target_link_libraries( C
> >> LINK_INTERFACE_LIBRARIES ... ), with this you can specify the
> transitively
> >> linked libraries when linking against C.
> >>
> >> Alex
> >>
> >
> > OK, so propagating the libraries is the default behavior.  I tried to use
> > LINK_INTERFACE_LIBRARIES, but it only seemed to work for static
> libraries:
> >
> > add_library(A STATIC a.cpp)
> > add_library(B SHARED b.cpp)
> > target_link_libraries(B A)
> > target_link_libraries(B LINK_INTERFACE_LIBRARIES)
> > add_library(C SHARED c.cpp)
> > target_link_libraries(C B)
> > target_link_libraries(C LINK_INTERFACE_LIBRARIES)
> >
> > add_executable(run run.cpp)
> > target_link_libraries(run C)
> >
> > During build:
> >
> > Build B (this is fine)
> > /usr/bin/c++   -dynamiclib -o libB.dylib CMakeFiles/B.dir/b.cpp.o libA.a
> > Build C (this is fine too, no A in the list)
> > /usr/bin/c++   -dynamiclib -o libC.dylib CMakeFiles/C.dir/c.cpp.o
> libB.dylib
> > Build run (this is weird, it linked both B and C)
> > /usr/bin/c++    CMakeFiles/run.dir/run.cpp.o  -o run  libC.dylib
> libB.dylib
> >
> > Did I specify something wrong or does this property only consider static
> > libraries?
>
> On *nix, I can't confirm this:
>
> CMAKE_MINIMUM_REQUIRED(VERSION 2.8 FATAL_ERROR)
> PROJECT(TRANSLINK CXX)
> SET(CMAKE_VERBOSE_MAKEFILE ON)
> FILE(WRITE ${CMAKE_BINARY_DIR}/a.cxx "void a(){}\n")
> ADD_LIBRARY(A STATIC a.cxx)
> FILE(WRITE ${CMAKE_BINARY_DIR}/b.cxx "void b(){}\n")
> ADD_LIBRARY(B SHARED b.cxx)
> TARGET_LINK_LIBRARIES(B A)
> TARGET_LINK_LIBRARIES(B LINK_INTERFACE_LIBRARIES)
> FILE(WRITE ${CMAKE_BINARY_DIR}/c.cxx "void c(){}\n")
> ADD_LIBRARY(C SHARED c.cxx)
> TARGET_LINK_LIBRARIES(C B)
> TARGET_LINK_LIBRARIES(C LINK_INTERFACE_LIBRARIES)
> FILE(WRITE ${CMAKE_BINARY_DIR}/main.cxx "int main(){}\n")
> ADD_EXECUTABLE(main main.cxx)
> TARGET_LINK_LIBRARIES(main C)
>
> The main target's link command line reads:
>
> .../c++ .../main.cxx.o -o main ... libC.so ...  # No libB.so!
>
> Do you actually get different results with the above-noted project?
>
> In general, CMake's transitive handling of target dependencies causes
> no harm since no library is loaded unnecessarily, although there may be
> libraries which are specified unnecessarily. E.g., main would be linked
> against libB.so without immediately referring to the latter, i.e. it is
> formally overlinked, but whether libB.so is loaded on behalf of main or
> on behalf of libC.so does not matter, and overlinking w.r.t. a static
> library is not possible. Do you have particular requirements why you
> want to reduce a target's references to the immediate dependencies?
>
> Regards,
>
> Michael
> --
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I tried your script, and it seems to exhibit the same behavior as my script:

/usr/bin/c++    -Wl,-search_paths_first -Wl,-headerpad_max_install_names
CMakeFiles/main.dir/main.cxx.o  -o main  libC.dylib libB.dylib

Perhaps this is some Mac specific behavior that isn't showing up on other
systems.

The main reason I wanted it was to prevent build failures.  I specified a
library path (-L) in one subdirectory to correctly link in the set of
external static libraries.  CMake tried to drag those libraries on to the
"next" project which didn't have the library path and link failed.  This is
why I started to look into why those static libraries were linked in the
first place.  As far as I'm concerned once you link an executable module,
you don't need its dependencies anymore.  Those dependencies should be
satisfied within the library itself.  I'm not exactly sure why CMake
implements this behavior by default, since it seems to go against the
philosophy of shared library linkage.

BTW, since the LINK_INTERFACE_LIBRARIES seems to work for static libraries
it did solve my immediate problem, but the behavior for the shared
libraries seems wrong to me.

James
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