How can I make `target_link_libraries` dependencies transitive from OBJECT libraries in cmake?

Question

If I specify a dependency chain of OBJECT libraries in CMake, only the target_link_libraries dependencies of the very last one is used in the target executable.

Minimal Example:

main depends on objB, which depends on objA. Both objA and objB are OBJECT libraries in CMake.

I would expect main to link with both object files. It doesn't.

cmake_minimum_required(VERSION 3.13)
project(transitive-object-library-link-issue VERSION 1.0.0 LANGUAGES C)

add_library(objA OBJECT a.c)

add_library(objB OBJECT b.c)
target_link_libraries(objB PUBLIC objA)

# Should link with 'a.o', since 'objA' is "linked" by 'objB'
add_executable(main main.c)
target_link_libraries(main objB)

Note: A working set of files are available at https://github.com/scraimer/cmake-transitive-object-library-link-issue

If I change objA to a non-OBJECT library, the problem is eliminated by getting rid of the chain of OBJECT library dependencies. In other words, change the line

add_library(objA OBJECT a.c)

To be:

add_library(objA a.c)

So it's something specific to OBJECT libraries. How should I be specifying the dependencies to make main link with objA? (Without having to specify objA in every executable target that uses objB)

Answer 1

Object library targets are not real libraries, they are simply a collection of objects, but they are not really linked together until they are used to build a real target like an executable or shared/static library. Citing the linked documentation:

add_library(<name> OBJECT <src>...)

Creates an Object Library. An object library compiles source files but does not archive or link their object files into a library.

In despite of that, you can apply a target_link_library() command to an object library, but only to specify dependencies of their sources on other libraries. The linked documentation explains your problem:

The object library’s usage requirements are propagated transitively [...], but its object files are not.

Object Libraries may “link” to other object libraries to get usage requirements, but since they do not have a link step nothing is done with their object files.

So, the transitive propagation affects only to compile definitions and dependencies of other real libraries, not the objects themselves.