This is an example C++ project using CMake that illustrates an issue with passing linker flags to be applied to specific libraries. What we really want is to be able to specify that certain external libraries are to be force-linked.
We have 4 fake imported static libraries that we want to link (that is pre-built 3rd party libraries). These are
declared in the main CMakeLists.txt. There are dependencies between them which are declared via
set_target_properties on each library. Since there are dependencies between them they should appear on the link line
in a specific order.
We have 2 libraries, lib_a and lib_b, that make use of some of these external libraries. They also require them to
be force linked. So they contain lines like:
target_link_libraries(lib_a PUBLIC -Wl,-force_load fake_4)
However, when we compile we see that some, but not all, libraries get the -Wl,-force_load applied. It seems CMake sees
that as a separate library it can place anywhere rather than a modifier specific libraries.
Other things we've tried:
- Use
target_link_libraries(lib_a PUBLIC "-Wl,-force_load fake_4"). This doesn't work because cmake doesn't expandfake_4to it's declared library path. - Use
target_link_libraries(lib_a PUBLIC "-Wl,-force_load $<TARGET_PROPERTY:fake_4,IMPORTED_LOCATION>"). This doesn't work because cmake doesn't know that this refers to thefake_4we declared in the rootCMakeLists.txtso it ignores the dependencies between our 4 fake imported libraries. - Use
target_link_options: this doesn't apply the link options to a specific library but rather adds them in an arbitrary position on the link line. - We can't put the
-Wl,-force_loadproperties on the libraries themselves as we have other executables that use them but do not require a force link.
To see what happens do the following from this directory:
$ mkdir build
$ cd build
$ cmake ..
$ VERBOSE=1 make
the link will fail as libfake_X.a aren't really static libraries but you can also see that the libraries are out of
order and not correctly force-linked though we've said they must be. Specifically, I get the following liker line:
Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ -isysroot
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.15.sdk
-mmacosx-version-min=10.14 -Wl,-search_paths_first -Wl,-headerpad_max_install_names CMakeFiles/exe.dir/main.cpp.o -o
exe ../lib_b/liblib_b.a -Wl,-force_load ../../imported_libs/libfake_1.a -Wl,-force_load ../../imported_libs/libfake_2.a
../lib_a/liblib_a.a -Wl,-force_load ../../imported_libs/libfake_4.a ../../imported_libs/libfake_3.a
../../imported_libs/libfake_2.a ../../imported_libs/libfake_1.a
here we can see:
libfake_3.ais not preceeded by a-Wl,-force_loadthoughlib_a/CMakeLists.txtsaid it was necessary.- The libraries are arranged in proper order on the link line however. But if you try experiments (1) or (2) above that is no longer the case.