resolving link errors with ios workspace with static library - ios

Question

I've done this before, several years ago with Xcode 5 or 6. Now using Xcode 8 or 9, I seem to be missing someting.
No matter what I've done, when I try to build and link my project, I get the Linker message:
Undefined symbols for architecture arm64:
"_OBJC_CLASS_$_MyClass", referenced from:
objc-class-ref in ViewController.o
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
This is not a duplicate question. It is not an architecture issue.
None of the other results I've found on S-O or google resolve this issue.
I'm obviously missing some build settings, but cannot figure out which one(s).
Between build attempts I've cleaned the build folder, and deleted DerivedData. Quit and restarted Xcode. Created a new set of projects with a new workspace and walked through the steps outlined below.
I'm writing in objective-C for this project if that matters (I don't think it does)
My question is what did I forget?
What I did --
Create a project for my framework and static library
a) create the project for a cocoa-touch framework - let's call it mySDK which creates mySDK.framework
b) Add a target for a cocoa-touch static library - let's call it SDK which creates libSDK.a
c) optionally build both targets -- they build fine
d) close the project
Create a project for my application - myApp - then close the project
Create a workspace - myWorkspace -- and open the workspace
Drag both project files from #1 mySDK and #2 myApp into the workspace
Build the framework and static library (both build and create the targets successfully)
Add the static library to myApp
a) Select myApp and go to build phases
b) Open Link Binary with Libraries
c) Drag from the library build products libSDK.a into the link binaries pane, set to "Required"
The library now appears in the Frameworks group-area in the myApp
7) Select the myApp project and go to build-settings
Add to Header Search Paths = "$(SRCROOT)/../mySDK/mySDK"
This resolves compile errors in finding the headers
Now I ask, why do I need to do anything else, shouldn't Xcode figure the rest out?
Various solutions or tutorials I've reviewed suggest adding:
Other Linking Flags = -ObjC
Library Search Paths = $(BUILT_PRODUCTS_DIR)
Library Search Paths = "$(SRCROOT)/../mySDK"
Changing Skip Install from YES to NO
Installation Build Product Location = $(BUILT_PRODUCTS_DIR)
I've tried all of these in different combinations.
Once I added the library to myApp I see that the Frameworks/libSDK.a file changes from red to black when I build the library so something in the workspace/project knows I built the library and it sees it.
So why when I try build myApp, do I still see the liner error:
Undefined symbols for architecture arm64:
"_OBJC_CLASS_$_MyClass", referenced from:
objc-class-ref in ViewController.o
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Nothing I have done seems to clear that error. I am pulling my hair out trying to find a solution.
I have also checked that it is building all the appropriate architectures, confirmed it using 'lipo -info'
Xcode obviously knows about the library, why isn't it linking?

Answer 1

My app also compiled with no problem but would hit an error while linking to the static library. My library was based on C++.
I had 2 problems to fix in my case:
The headers lacked proper extern "C" declarations for C functions in a C++ file.
One .cpp file did not #include its matching header so the extern "C" would be properly applied to the compiled version.
The linker had issues because the compiler used a mangled name for the function due to the mistakes. Later confirmed this by looking in the .a file with a text editor. I found my function with "_Z11" prefixed and "v" suffixed.
There was another clue: the error was reporting the missing function name with an underscore at the front.
Undefined symbols for architecture arm64:
"_myCFunction", referenced from:
SwiftModule.swiftFunction () -> () in MySwiftClass.o
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see invocation)