Windows Vs 上的 DLL Main __attribute__((constructor)) Linux 上的入口点

Question

考虑代码

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

输出不同：

在窗户上

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

在 Windows 上，Moo 构造函数在 DLLMain 之前调用，而在 linux 上，它在使用属性（（构造函数））定义的 Initializer 之后调用。

为什么？

Answer 1

有办法：

StartupCleanup.cpp：

// Redefine the same StartupCleanup class as it is in DllMain.cpp
// Definition of constructor and destructor must stay in DllMain.cpp
// And including here any headers which may define normal static or global constructors/destructors is strictly forbidden!
struct StartupAndCleanup
{
    /**/  StartupAndCleanup();
    /**/ ~StartupAndCleanup();
};

// It ensures this instance is the first to be constructed *BEFORE* any normal static or global constructors calls
// and the last to be destructed *AFTER* all normal destructors calls.
// The key to do so is using #pragma init_seg(lib), but that key applies for all the static and global constructors/destructors in the same .obj file! 
#pragma warning(push)
#pragma warning(disable:4073)
#pragma init_seg(lib)
#pragma warning(pop)

// this function is just to keep linker from discarding startupAndCleanup.obj when linking to an executable or dll
void needStartupAndCleanup()
{
}

static StartupAndCleanup startupAndCleanup;

DllMain.cpp：

...
// Definition of this class should be the same as in StartupAndCleanup.cpp!
struct StartupAndCleanup
{
    /**/  StartupAndCleanup();
    /**/ ~StartupAndCleanup();
};

StartupAndCleanup::StartupAndCleanup()
{
    // Do your initialization here !
}

StartupAndCleanup::~StartupAndCleanup()
{
    // Do your termination here !
}

你的 DllMain 必须只是一个空壳，并且像 Linux 一样在那些构造函数和析构函数中进行通常的安全初始化和终止。

注意：小心！如果您打算同步它们，则不能在静态或全局构造函数/析构函数中创建/删除线程。 就是这样！

编辑：您还需要在知道它已链接的函数中调用 needStartupAndCleanup()，否则目标文件 StartupCleanup.obj 将被丢弃，并且那些全局构造函数/析构函数也将被丢弃。

Answer 2

Moo 构造函数在之前 DllMain 没有被调用，它被从 DllMain 调用。准确地说，它是从真正的DllMain 调用的，Windows 最先调用的函数。这个真正的DllMain 调用C++ 构造函数，然后调用你的C++ DllMain。这个真正的DllMain 的原因正是为了初始化构造函数，这在 C 之前是不需要的

Linux (GCC/ELF) 根本没有这个概念；它只有构造函数。您的手动 ctor 和 Moo 的 C++ ctor 的处理方式相同。