DLL Main on Windows Vs. __attribute__((constructor)) entry points on Linux

Question

Consider code

EXE:

int main ()
{

    printf("Executable Main, loading library\n");
#ifdef HAVE_WINDOWS
    HMODULE lib = LoadLibraryA ("testdll.dll"); 
#elif defined(HAVE_LINUX)
    void * lib  = dlopen("testdll.so", RTLD_LAZY);  
#endif 

    if (lib) {
        printf("Executable Main, Freeing library\n");
    #ifdef HAVE_WINDOWS
        FreeLibrary (lib); 
    #elif defined(HAVE_LINUX)
        dlclose(lib);   
    #endif 
    }
    printf("Executable Main, exiting\n");
    return 0;
}

DLL

struct Moo
{
    Moo() { printf("DLL Moo, constructor\n"); }
    ~Moo() { printf("DLL Moo, destructor\n"); }
};

Moo m;

#ifdef HAVE_WINDOWS
BOOL APIENTRY DllMain( HMODULE hModule,
                       DWORD  ul_reason_for_call,
                       LPVOID lpReserved)
{
    switch (ul_reason_for_call)
    {
    case DLL_PROCESS_ATTACH:
        printf("DllMain, DLL_PROCESS_ATTACH\n");
        break;
    case DLL_THREAD_ATTACH:
        printf("DllMain, DLL_THREAD_ATTACH\n");
        break;
    case DLL_THREAD_DETACH:
        printf("DllMain, DLL_THREAD_DETACH\n");
        break;
    case DLL_PROCESS_DETACH:
        printf("DllMain, DLL_PROCESS_DETACH\n");
        break;
    default:
        printf("DllMain, ????\n");
        break;
    }
    return TRUE;
}
#else
CP_BEGIN_EXTERN_C
__attribute__((constructor))
/**
 * initializer of the dylib.
 */
static void Initializer(int argc, char** argv, char** envp)
{
    printf("DllInitializer\n");
}

__attribute__((destructor))
/** 
 * It is called when dylib is being unloaded.
 * 
 */
static void Finalizer()
{
    printf("DllFinalizer\n");
}

CP_END_EXTERN_C
#endif

The output differs:

On windows

Executable Main, loading library
DLL Moo, constructor
DllMain, DLL_PROCESS_ATTACH
Executable Main, Freeing library
DllMain, DLL_PROCESS_DETACH
DLL Moo, destructor
Executable Main, exiting

Linux

Executable Main, loading library
DllInitializer
DLL Moo, constructor
Executable Main, Freeing library
DllFinalizer
DLL Moo, destructor
Executable Main, exiting

On windows, Moo constructor is called before DLLMain and whereas on linux it is called after Initializer defined using attribute((constructor)).

WHY?

Answer 1

Moo constructor isn't called before DllMain, it is called from DllMain. To be precise, its called from the real DllMain, the function Windows calls first. This real DllMain calls C++ constructors and then calls your C++ DllMain. The reason for this real DllMain is exactly to initialize constructors, something which wasn't needed in C before

Linux (GCC/ELF) doesn't have this concept at all; it only has constructors. Your manual ctor and the C++ ctor for Moo are treated the same.