DLL主要在Windows Vs. __attribute __((构造函数))Linux上的入口点

Question

考虑代码

可执行程序:

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的

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,在使用attribute((constructor))定义的Initializer之后调用.

为什么？

Answer 1

之前 没有调用Moo构造函数DllMain,它是从中 调用的DllMain.确切地说,它是从真实DllMain调用的,Windows首先调用的函数.这真正DllMain调用C++构造函数,然后调用您的C++ DllMain.这个真实的原因DllMain恰恰是初始化构造函数,这在C之前是不需要的

Linux(GCC/ELF)完全没有这个概念; 它只有构造函数.您的手动ctor和Moo的C++ ctor也是一样的.