[CMake] Transitive linking

[Alexander Neundorf]

On Saturday 19 November 2011, Michael Hertling 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, 

It matters a bit.
IIRC this increases startup time and some memory consumption (I don't remember 
the details, and it's not much, but it's not zero), and also adds unnecessary 
dependencies for packagers.

Alex