Why the linker is looking for non-needed libs - c++

Question

I have two C++ modules: A and B, where B links to a set of static libs lib*.a (I use * here to mean a set of lib files) and A links to B.
I have CMakeLists.txt for B:
add_library(B STATIC B.cpp)
target_include_directories(B PUBLIC ${CMAKE_CURRENT_SOURCE_DIR} "/path/to/headers/directory/for/lib*.a")
link_directories("Path/to/directory/contains/lib*.a")
target_link_libraries(B PRIVATE lib*)
and CMakeLists.txt for A:
add_library(A STATIC A.cpp)
target_include_directories(A PUBLIC ${CMAKE_CURRENT_SOURCE_DIR} "/path/to/headers/for/libB.a") # compiler outputs libB.a when target name is B
link_directories("Path/to/directory/contains/libB.a")
target_link_libraries(A PRIVATE B)
Everything works fine, until I try to make A a shared lib, then I got error from the linker saying cannot find -l*. I believe the reason is that when I set A to be shared, the compiler looks for shared lib, which is not available.
The thing I don't understand:
B is already a static lib, why the linker needs lib*?
why compiler looks for shared lib when I set A to be shared? I thought even a shared A can be linked to static libs
I do want to note that in A.cpp, I have #include B.hh at the top, and B.hh further includes the headers for lib*.a, that is why I have target_include_directories(B PUBLIC...)

Answer 1

Your initial setup was already not working, but you did not see the error until switching to a shared library.
The reason for this is that all your target_link_libraries dependencies on A and B were not actually processed at all so long as both were static libraries. This has to do with what a static library is: Just a bunch of object files packed together into a single file. A static library is not linked, hence any build dependencies that concern the linker (such as linking in additional static libraries as dependencies) will not get resolved. The build system will keep track of them, but if they are missing, you will not notice unless you actually pass those libraries to something that is actually being linked - such as an executable or, in your case now, a shared library.
Since it can be a bit embarassing to have invisible unresolved dependencies like this, idiomatic CMake will have you model things differently. You never pull in static dependencies directly, instead you use an approach like the following: At the lowest layer, you have a find_library call that finds any external pre-built dependencies on the system (the lib* in your example). You then wrap that up into an imported target so that you never have to touch the raw library path again. To smoothen things out, you can hide all of this inside a find package script, so that your code only ever sees a find_package call that provides a lib* target and don't worry about the internals.
You then model all of your library interdepencies through target dependencies only. Such an approach has several advantages that make it more robust: If CMake can't find the lib*, it will tell you right away when the find_package call fail, instead of waiting until you're trying to link like it does now. By only specifying target-based dependencies, you ensure that the build is actually consuming the libraries that it was configured with (which is very much not ensured with the link_directories based approach you're having now).