CMake append objects from different CMakeLists.txt into one library

Question

I would like to create a single library from objects from multiple sub-directories, each one containing their own CMakeLists.txt with OBJECT library trick to have multiple targets with different compile options.

Here are the files:

project_dir
|--- subdir1
|    |--- src1.c
|    |--- CMakeLists.txt
|--- subdir2
|    |--- src2.c
|    |--- CMakeLists.txt
|--- CMakeLists.txt

Contents of all CMakeLists.txt

// project_dir/CMakeLists.txt
// what to put here? to make one single library (mainLib)


// project_dir/subdir1/CMakeLists.txt
add_library(lib11 OBJECT src1.c)
set_target_properties(lib11 PROPERTIES COMPILE_FLAGS "some-flags11")

add_library(lib12 OBJECT src1.c)
set_target_properties(lib12 PROPERTIES COMPILE_FLAGS "some-flags12")
// here I would like to add lib11:objects and lib12:objects to mainLib
// how should it be done?

// project_dir/subdir2/CMakeLists.txt
// *** similar to subdir1 but with src2.c that creates lib21 and lib22
// here I would like to add lib21:objects and lib22:objects to mainLib
// how should it be done?

Can it be done platform independently? Thanks.

EDIT: Based on CMake: how create a single shared library from all static libraries of subprojects?, I can change the cmake files to the following, but still doesn't solve my problem.

// project_dir/subdir1/CMakeLists.txt
add_library(lib11 OBJECT src1.c)
set_target_properties(lib11 PROPERTIES COMPILE_FLAGS "some-flags11")

add_library(lib12 OBJECT src1.c)
set_target_properties(lib12 PROPERTIES COMPILE_FLAGS "some-flags12")

add_library(lib1 STATIC $<TARGET_OBJECTS:lib11> $<TARGET_OBJECTS:lib12>)

// Above add_library cannot be OBJECT which would fix my problem.
// how to append the lib1 (lib11 and lib12) to mainLib?

EDIT: Updating my post with attempt to chain interface libraries as suggested in the answer.

add_library(lib11 OBJECT test1/sub1/src1.c)
set_target_properties(lib11 PROPERTIES COMPILE_FLAGS "-DF1")

add_library(lib12 OBJECT test1/sub1/src1.c)
set_target_properties(lib12 PROPERTIES COMPILE_FLAGS "-DF2")

add_library(lib1 INTERFACE)
target_sources(lib1 INTERFACE $<TARGET_OBJECTS:lib11>)
target_sources(lib1 INTERFACE $<TARGET_OBJECTS:lib12>)

add_library(lib21 OBJECT test1/sub2/src2.c)
set_target_properties(lib21 PROPERTIES COMPILE_FLAGS "-DF1")

add_library(lib22 OBJECT test1/sub2/src2.c)
set_target_properties(lib22 PROPERTIES COMPILE_FLAGS "-DF2")

add_library(lib2 INTERFACE)
target_sources(lib2 INTERFACE $<TARGET_OBJECTS:lib21>)
target_sources(lib2 INTERFACE $<TARGET_OBJECTS:lib22>)

add_library(PARENT INTERFACE)

target_link_libraries(PARENT INTERFACE lib1)
target_link_libraries(PARENT INTERFACE lib2)

add_library(CORE OBJECT src.c)
add_library(GPARENT STATIC $<TARGET_OBJECTS:CORE>)

target_link_libraries(GPARENT INTERFACE PARENT)

Answer 1

Several OBJECT libraries can be incorporated into one INTERFACE library:

# Create lib1 OBJECT library with some sources, compile flags and so.
add_library(lib1 OBJECT ...)

# Create lib2 OBJECT library with some sources, compile flags and so.
add_library(lib2 OBJECT ...)

# Create INTERFACE library..
add_library(libs INTERFACE)
# .. which combines OBJECT libraries
target_sources(libs INTERFACE $<TARGET_OBJECTS:lib1> $<TARGET_OBJECTS:lib2>)

Resulted library can be used with standard target_link_libraries:

add_library(mainLib <some-source>)
target_link_libraries(mainLib libs)

It is possible to combine INTERFACE libraries futher:

# Create libs_another INTERFACE library like 'libs' one
add_library(libs_another INTERFACE)
..

# Combine 'libs' and 'libs_another' together
add_library(upper_level_libs INTERFACE)
target_link_libraries(upper_level_libs INTERFACE libs libs_another)

Resulted libraries' "chain" could be of any length.

---

While this way for combination of OBJECT libraries looks hacky, it is actually allowed by CMake documentation:

Although object libraries may not be named directly in calls to the target_link_libraries() command, they can be "linked" indirectly by using an Interface Library whose INTERFACE_SOURCES target property is set to name $<TARGET_OBJECTS:objlib>.