ld not loading required object files

Question

I have a C++ project divided into multiple library targets and one executable target. I've been dynamically linking those libraries and am trying to switch to static linking but running into "undefined reference" errors.

One of those errors involved a class named Effect, which is in a library named shading. The library typography has a class named Type_Effect, which is derived from Effect. When processing typography the linker cannot find any symbols for the Effect class.

I verified that the libraries are being included like so:

.../shading/libshading.a .../typography/libtypography.a ...

I ran the linker with the -verbose command and part of the output for shading was:

attempt to open .../shading/libshading.a succeeded
(.../shading/libshading.a)Shader_Manager.cpp.obj
(.../shading/libshading.a)Shader.cpp.obj
(.../shading/libshading.a)Program.cpp.obj

Effect.cpp.obj was missing. I checked the shading.a file and it did include Effect.cpp and all of the related Effect symbols. Next, I added dummy code for creating an Effect object into my main executable cpp file, and sure enough the linker output now included:

([...]/shading/libshading.a)Effect.cpp.obj

Alternatively, preceeding linklibs.rsp with --whole-archive forced the linker to load Effect.cpp.obj, but loading everything introduces other problems beyond the scope of this question.

The evidence points to some breakdown in the linker determining which object files to include. The shading library is not directly used in my main executable target, so the presence of some shader objects being loaded indicates that at least some of the dependency tracing is working.

In the ld flag documentation for --whole-archive, it mentions how that flag

include[s] every object file in the archive in the link, rather than searching the archive for the required object files.

What is some of the logic behind that searching? What factors could make or break it?

This is not a duplicate of that question. It is the opposite. The question asks about symptoms, and the answer is about the cause of those symptoms. The linked question asks about a cause, and the answer lists the symptoms. With only the symptoms in front of me, there was no way on SO to find the linked question unless I already knew the answer to it. — silentorb, Sep 02 '16 at 16:33

score 1 · Accepted Answer · edited May 23 '17 at 12:08

The logic I was missing is that when static linking, ld only loads an object file from an archive when that object file contains the definitions to unresolved symbols ld already knows about from previously loaded object files.

Why does the order in which libraries are linked sometimes cause errors in GCC?

Thus, ld input needs to be ordered from dependents to dependencies. The typography library needed to be listed before the shading library. I had it backwards.

ld not loading required object files

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