CMake library linking order

Question

I have following libraries lib_A, lib_B, lib_C, lib_D. I am doing something like this in my CMake files (order is important):

1. ADD_LIBRARY(lib_A)
2. ADD_LIBRARY(lib_B)
3. ADD_LIBRARY(lib_C)
4. ADD_LIBRARY(lib_D)
5. TARGET_LINK_LIBRARIES(lib_B lib_C)
6. TARGET_LINK_LIBRARIES(lib_A lib_B)
7. ADD_EXECUTABLE(Exec)
8. TARGET_LINK_LIBRARIES(exec lib_A)
9. TARGET_LINK_LIBRARIES(exec lib_D)

This results in following linker command.

linker -llib_A -llib_D -llib_B -llib_C

Q1. Why are lib_B and lib_C after lib_D?

Q2. When I change CMake a little bit and do something like this:

8. TARGET_LINK_LIBRARIES(lib_A lib_D)
9. TARGET_LINK_LIBRARIES(exec lib_A)

then linking order is like this:

linker -llib_A -llib_B -llib_C -llib_D

Here, lib_B and lib_C are before lib_D. It means that target_link_libraries works differently for executable targets and library targets. Am I right?

The problem here is that lib_B and lib_C also depend on lib_D, but I don't want to make target_link_libraries(lib_B lib_D) and target_link_libraries(lib_C lib_D), because I have more of such cases, and I would have to do it manually for each library. Of course doing like in Q2 solves the problem but:

Q3 - Is this order guaranteed somehow by CMake or it is just a fortuity?

Thank you

Answer 1

Just link lib_B and lib_C to lib_D, it's worth the effort (I can tell by experience), otherwise you will have big troubles, for example if you try to install your program. lib_C and lib_D should have all their symbols solved when you are finished creating their library files, before linking them to any other library.

By the way, you can compact your target_link_libraries in one line per target, like:

TARGET_LINK_LIBRARIES(exec lib_A lib_D)

And, if exec doesn't depend directly on lib_D, you can avoid linking it if you have correctly linked lib_A to lib_D.

Anyway, regarding Q1: even if the order is guaranteed by CMake, it is not guaranteed the way your linker will treat it, you are going to suffer pain if you rely on that