Building a shared library using gcc on Linux and MinGW on Windows

Question

I'm having trouble with generating a build setup that allows shared libraries to be built in both Linux and Windows using gcc and MinGW, respectively. In Linux, a shared library doesn't have to resolve all dependencies at compile time; whereas, this appears to the case in Windows. Here is the problem setup: ``` $ cat foo.h #ifndef FOO_H #define FOO_H void printme(); #endif ``` ``` $ cat foo.c #include "foo.h" #include <stdio.h> void printme() { printf("Hello World!
"); } ``` ``` $ cat bar.h #ifndef BAR_H #define BAR_H void printme2(); #endif ``` ``` $ cat bar.c #include "bar.h" #include "foo.h" void printme2() { printme(); printme(); } ``` ``` $ cat main.c #include "bar.h" int main(){ printme2(); } ``` ``` $ cat Makefile .c.o: gcc -fPIC -c $< all: foo.o bar.o main.o gcc -shared foo.o -o libfoo.so gcc -shared bar.o -o libbar.so gcc main.o -Wl,-rpath=. -L . -lbar -lfoo -o main ``` Now, in Linux, this compiles and runs just fine: ``` $ make gcc -fPIC -c foo.c gcc -fPIC -c bar.c gcc -fPI…

Accepted Answer

On Windows, you need to create an import library for the DLL. An import library looks like a static library, in that it defines all of the needed symbols, but it doesn't have the actual function implementations, it just has stubs. The import library will resolve the "undefined reference" errors while avoiding static linking.

To create an import library with MinGW, follow the instructions here. The key is that when building the DLL, you must pass the option `-Wl,--out-implib,libexample_dll.a` to the linker to generate the import library `libexample_dll.a`.

Then, when you compile your main executable, you use the `-lexample_dll` option (along with `-L.`) to link against the import library. So with your code, I think this should work:

```
all: foo.o bar.o main.o
    gcc -shared foo.o -o libfoo.dll -Wl,--out-implib,libfoo.a
    gcc -shared bar.o foo.o -o libbar.dll -Wl,--out-implib,libbar.a
    gcc main.o  -Wl,-rpath=. -L. -lbar -lfoo -o main
```

Also, note that on Windows, the calling convention for exported functions in DLL is almost always `__stdcall`, not the default `__cdecl`, so if you want your DLLs to be usable by other software, I'd recommend making them `__cdecl`. But that's not strictly requires, as long as both the code in the DLL and the header files agree on what the calling convention is.

Building a shared library using gcc on Linux and MinGW on Windows

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