Issue
Long story short, I rewrote the Godot build system to cmake (only windows part), mostly because I wanted to learn it, but I have trouble compiling Godot with mingw. When I'm trying to compile it, at first everything goes fine, up until the point of linking final exe, where I get a lot of "undefined reference to" errors. It looks like main libraries (core/scene/editor/..) can't see functions from each other. MSVC build is working fine, and scons version is also compiling under mingw, so I clearly just missed something in my cmake version.. I tried to remove some compile/linking options throughout my cmake scripts as a test, but nothing changed. I don't really know how even debug this problem, so if someone could kick me in the right direction I would be really glad for it.
Solution
Ok, I finally got time to came back to this.
The problem
So basically the problem was in circular dependency on godot libs. I didn't thought this was the problem because I'm spoiled by MSVC (which doesn't depend on link order). Also, I tried to replicate circular dependency in mingw on a test project with much smaller scale. The project was a 30 libs with two functions in each, first function printing string, and another calling all first functions from all 30 libs, so there is 30 libs of circular dependency. Strangely enough, the project linked no problems, and printed 30^2 strings..
The solution
The solution is to use -Wl,--start-group/-Wl,--end-group linking flags around all libraries. There is two ways you can do it.
First way, is to add all your libraries to the some list of sorts. This could be global property, or property on some target (not just a simple variable), so it could be accessed from other subdirectories. After you formed your list of libraries you simply link it to the executable as follows
# getting all your libs from the global property..
get_property(__LIBS_LIST GLOBAL PROPERTY EXE_LIBS_LIST)
# linking all libraries to the exe..
target_link_libraries(my-exe PRIVATE -Wl,--start-group ${__LIBS_LIST} -Wl,--end-group)
This is the easiest solution, but be cautious about dependencies on libraries which you link to your exe, because it seems that when CMake creates link line for your exe, it first lists all libraries which are linked directly to your exe, and only after it places libraries which came from dependencies of libraries linked directly. Basically if your target dependency tree looks something like this: exe // your main exe file - lib_A // lib A linked directly to the main exe - lib_AA // lib AA linked to the lib_A - lib_AAA // lib AAA linked to the lib_AA - lib_B // lib B linked directly to the main exe - lib_BB // lib BB linked to the lib_B - lib_BBB // lib BBB linked to the lib_BB your link order for the exe will look something like this: // first libs linked directly to the exe lib_A lib_B // only after recursively initial libs dependencies lib_AA lib_AAA lib_BB lib_BBB
That's meen, that if you will link your libs like target_link_libraries(my-exe PRIVATE -Wl,--start-group ${__LIBS_LIST} -Wl,--end-group), --start-group and --end-group will guard only the libraries linked directly to your exe. I didn't found this described in documentation, but I found SO question which talks pretty much about same behaviour (CMake library linking order). Also, as I tested this on mingw, it didn't mattered how exactly libs lib_AA/lib_AAA/lib_BB/lib_BBB were linked, via PRIVATE or via INTERFACE, the results were the same.
Second way, is to exploit recursivity of link expanding for dependencies of libraries linked directly to the exe. From my example you can see, that dependencies of lib_A (lib_AA/lib_AAA) weren't mixed with dependencies of lib_B (lib_BB/lib_BBB). So basiacaly what we can do, is to create INTERFACE library and connect to it -Wl,--start-group immediately after that. Then add any number of libraries to it's interface and link global-libs to your exe (order does not matter). And in very end, close the group in your global-libs library
add_library(global-libs INTERFACE)
target_link_libraries(global-libs INTERFACE -Wl,--start-group)
# ...
# linking another libs, and linking global-libs to exe
# ...
target_link_libraries(global-libs INTERFACE -Wl,--end-group)
This will ensure, that all libraries connected to global-libs will be surrounded by -Wl,--start-group/-Wl,--end-group.
Now, theoretically, CMake should handle circular dependency by itself, by placing libraries in link line multiple times (how many times controlled by LINK_INTERFACE_MULTIPLICITY). But this method didn't worked for me (mb I just missed something..). Plus, you need declare dependencies between cmake targets, and with -Wl,--start-group/-Wl,--end-group you can just set one specific interface library as a holder for all libs with circular dependencies..
Answered By - Vlad
