windows - 我们可以从 ExitInstance 调用 FreeLibrary

Question

从MSDN文档中可以看出，在DllMain入口函数中不应该调用LoadLibrary/FreeLibrary。

The entry-point function should perform only simple initialization or termination tasks. It must not call the LoadLibrary or LoadLibraryEx function (or a function that calls these functions), because this may create dependency loops in the DLL load order. This can result in a DLL being used before the system has executed its initialization code. Similarly, the entry-point function must not call the FreeLibrary function (or a function that calls FreeLibrary) during process termination, because this can result in a DLL being used after the system has executed its termination code.

我的问题是:我们可以从 ExitInstance() 调用 FreeLibrary 吗？ 例如:

Test.exe - 主要可执行文件

HINSTANCE hDllMFC = LoadLibrary(L"TestApp.dll");
if (hDllMFC != NULL)
{
    FreeLibrary(hDllMFC);
}

while unload the hDllMFC, the call stack looks like:

TestApp.dll!CTestAppApp::ExitInstance() Line 42 C++
TestApp.dll!InternalDllMain() Line 155  C++
TestApp.dll!DllMain() Line 272  C++
TestApp.dll!__DllMainCRTStartup() Line 512  C
TestApp.dll!_DllMainCRTStartup() Line 477   C
ntdll.dll!LdrpUnloadDll()   Unknown
ntdll.dll!LdrUnloadDll()    Unknown
KernelBase.dll!FreeLibrary()    Unknown
Test.exe!wmain() Line 17    C++

TestApp.dll - 动态链接到 MFC 的常规 DLL

CTestApp theApp;
HINSTANCE hDllResource = NULL;

BOOL CTestApp::InitInstance()
{
    hDllResource = ::LoadLibrary(L"TestApp_Resource.dll");

    return CWinApp::InitInstance();
}

int CTestApp::ExitInstance()
{
    ::FreeLibrary(hDllResource);

    return CWinApp::ExitInstance();
}

TestApp_Resource.dll - 常规 DLL、资源

...

我认为我们不应该，但是从 CWinApp::ExitInstance() 的实现中，我们可以看出，它也在尝试卸载资源 dll。这是否意味着我们可以在 ExitIntance() 中调用 FreeLibrary？

int CWinApp::ExitInstance()
{
  //... 

 if (m_hLangResourceDLL != NULL)
  {
    ::FreeLibrary(m_hLangResourceDLL);
    m_hLangResourceDLL = NULL;
  }
  //...
}

我还找到了一份文档，确认在 Win95 中从 ExitInstance 调用 FreeLibrary 时存在错误。

错误:从 ExitInstance 调用 AfxFreeLibrary 时断言 http://support.microsoft.com/kb/187684

STATUS: Microsoft has confirmed this to be a bug in Windows 95. We are researching this bug and will post new information here in the Microsoft Knowledge Base as it becomes available.

Answer 1

如果实际上是从 DllMain 调用 ExitInstance(由堆栈跟踪确认)，则适用 DllMain 的所有规则，包括禁止递归加载或卸载其他 DLL。 (请注意，将您的 CWinApp 放在 DLL 中是非常不寻常的，MFC 可能还有其他问题，例如知识库文章中提到的问题。并不是说有或没有更多的问题潜伏，而是添加一个额外的注释注意。)