Create a dynamic library with unresolved symbols at link time with mingw-w64

Question

I have a file mylib.c with the following content:

extern void some_function(void);

int add(int a, int b) {
    some_function();
    return a+b;
} 

I want to create a shared library from it. The function some_function should be linked dynamically by the consumer of the shared library. So while building the shared library, the symbol remains unresolved.

When I use gcc on Ubuntu via...

gcc mylib.c -o libmylib.so -shared

...the build succeeds.

However, I also need to cross-build the library for Windows. I am using mingw-w64 for this:

sudo apt install -y mingw-w64

x86_64-w64-mingw32-gcc mylib.c -o mylib.dll -shared

Now, I get this error:

/usr/bin/x86_64-w64-mingw32-ld: /tmp/ccWlwvfS.o:mylib.c:(.text+0xf): undefined reference to `some_function'
collect2: error: ld returned 1 exit status

After some research, I tried out various options to fix this. Below are some of them:

x86_64-w64-mingw32-gcc mylib.c -o mylib.dll -shared -Wl,--no-undefined

x86_64-w64-mingw32-gcc mylib.c -o mylib.dll -shared -Wl,-u,some_function

x86_64-w64-mingw32-gcc mylib.c -o mylib.dll -shared -Wl,-u,_some_function

x86_64-w64-mingw32-gcc mylib.c -o mylib.dll -shared -Wl,-U,some_function

x86_64-w64-mingw32-gcc mylib.c -o mylib.dll -shared -Wl,-U,_some_function

x86_64-w64-mingw32-gcc mylib.c -o mylib.dll -shared -Wl,--unresolved-symbols=ignore-all

None of them work, unfortunately.

How do I have to configure the linker so that it works? Or is there another workaround to get a DLL with link-time-undefined symbols from a Linux host?

Answer 1

When building Windows shared libraries (DLL files) with MinGW-w64 you should ether link specifying a .def file used to list the symbols you will be exporting, or use __declspec(dllexport) / __declspec(dllimport) on your exported/imported functions (the latter is most commonly used).

For a minimal example on how to do this please see: https://github.com/brechtsanders/ci-test