Bdtjtk

I have the following scenario:

• I import two prebuilt libraries into my project (libA, libB)


• 
  libB has a dependency on libA
  


• The executable depends on both libA and libB
  

However, the relative linking order in my link.txt is incorrect

/usr/bin/c++     CMakeFiles/bin.dir/main.cpp.o  -o bin ../libA.a ../libB.a

I would expect libA.a to be listed after libB.a.
The CMakeLists.txt looks something along the following lines

cmake_minimum_required(VERSION 3.13)

project(cmake_test)

set(lib_dir ${CMAKE_CURRENT_SOURCE_DIR})



add_library(MY::libA IMPORTED INTERFACE)

set_target_properties(MY::libA PROPERTIES INTERFACE_LINK_LIBRARIES "${lib_dir}/libA.a")



add_library(MY::libB IMPORTED INTERFACE)

set_target_properties(MY::libB PROPERTIES INTERFACE_LINK_LIBRARIES "MY::libA;${lib_dir}/libB.a")



add_executable(bin ${CMAKE_CURRENT_SOURCE_DIR}/main.cpp)

target_link_libraries(bin PUBLIC MY::libB MY::libA)

Below a description of my attempts to solve the problem. Some without success and some with sucecss but using modifications that render the code usless for the production environment.

Successful attempts:

• Remove the depedency of bin on libA (i.e. replace the last line by target_link_libraries(bin PUBLIC MY::libB). This works but I cannot remove the dependency in real code.


• Replace the target type IMPORTED INTERFACE by IMPORTED STATIC. Use IMPORTED_LOCATION instead of INTERFACE_LINK_LIBRARIES and use target_link_libraries to express the dependency of libB on libA. In this case the link.txt yields: [...] -o bin ../libA.a ../libB.a ../libA.a. As soon as I revert the target type for libB the link order breaks down again. In the production environment, however, one of the targets is created by conan as IMPORTED INTERFACE.

Attempts without success (same behaviour as described):

• Create a separate IMPORTED target (use IMPORTED_LOCATION) for every lib and group them inside an INTERFACE target


• Sprinkle the code with ADD_DEPENDENCIES
  


• Remove libA from the INTERFACE_LINK_LIBRARIES in line 9 and use target_link_libraries(MY::libB INTERFACE MY::libA) instead. Same result.

Example code that shows the same failure using INTERFACES as a building block

cmake_minimum_required(VERSION 3.13)

project(cmake_test)

set(lib_dir ${CMAKE_CURRENT_SOURCE_DIR})



# libA

add_library(MY::libA_file1 IMPORTED STATIC)

set_target_properties(MY::libA_file1 PROPERTIES IMPORTED_LOCATION "${lib_dir}/libA.a")



add_library(libA INTERFACE)

target_link_libraries(libA INTERFACE MY::libA_file1)



# libB

add_library(MY::libB_file1 IMPORTED STATIC)

set_target_properties(MY::libB_file1 PROPERTIES IMPORTED_LOCATION "${lib_dir}/libB.a")



add_library(libB INTERFACE)

target_link_libraries(libB INTERFACE MY::libB_file1 libA)





add_executable(bin ${CMAKE_CURRENT_SOURCE_DIR}/main.cpp)

target_link_libraries(bin PUBLIC libA libB)

You incorrectly think about INTERFACE_LINK_LIBRARIES property as a "content" of the library's target, which is ordered by target_link_libraries call.

Using

target_link_libraries(MY::libB INTERFACE MY::libA)

you setup link dependency between library targets MY::libB and MY::libA. That is, "content" of MY::libB target should come before "content" of MY::libA target in the linking command line.

But INTERFACE_LINK_LIBRARIES property is NOT a "content" of the library target! It is just an additional link dependency.

As opposite, IMPORTED_LOCATION (for non-INTERFACE IMPORTED target) is a "content" of the library, and target_link_libraries affects on its ordering.

It seems that you cannot add link dependency for a library, using INTERFACE library target. You should use IMPORTED library target for that purpose:

# Collect libraries related to 'libA'

file(GLOB libs_A "${lib_dir}/libA*.a")

# For each library create IMPORTED target with IMPORTED_LOCATION property.

set(libs_A_targets)

foreach(lib_A ${libs_A})

    # Form a unique name for the IMPORTED target: subtarget_A_*

    string(REGEX REPLACE "^${lib_dir}/libA([^.]*).a$" "subtarget_A_\1" lib_A_target ${lib_A})

    # Create a target with this name

    add_library(${lib_A_target} STATIC IMPORTED)

    set_target_properties(${lib_A_target} PROPERTIES IMPORTED_LOCATION ${lib_A})

    # And add the target into the list

    list(APPEND libs_A_targets ${lib_A_target})

endforeach()



# In a similar way collect libraries for libB.

set(lib_B_targets ...)



# Now link each libB* library with each libA*.

foreach(lib_B_target ${libs_B_targets})

    target_link_libraries(${lib_B_target} INTERFACE ${libs_A_targets})

endforeach()



# Now interface libraries, which combine libA* and libB*, can be created

add_library(libA INTERFACE)

target_link_libraries(libA INTERFACE ${libs_A_targets})



add_library(libB INTERFACE)

target_link_libraries(libB INTERFACE ${libs_B_targets})



# Now these INTERFACE libraries can be linked into an executable in any order

add_executable(bin ${CMAKE_CURRENT_SOURCE_DIR}/main.cpp)

target_link_libraries(bin PUBLIC libA libB)