_USRDLL _AFXDLL _WINDLL 三种dll编译宏的具体含..

Building Your DLL

When compiling regular DLLs that statically link to MFC, the symbols "_USRDLL" and "_WINDLL" must be defined. Your DLL code must also be compiled with the following compiler switches:

• /D_WINDLL signifies the compilation is for a DLL
• /D_USRDLL specifies you are building a regular DLL

When compiling regular DLLs that dynamically link to MFC, you must define the above symbols and use the above compiler switches. Additionally, the symbol "_AFXDLL" must be defined and your DLL code must be compiled with:

• /D_AFXDLL specifies that you are building a regular DLL that dynamically links to MFC

The interfaces (APIs) between the application and the DLL must be explicitly exported. It is recommended that you define your interfaces to be low bandwidth, sticking to C interfaces where possible. More direct C interfaces are easier to maintain than more complex C++ classes.

Place your APIs in a separate header that can be included by both C and C++ files (that way you won't limit your DLL customers to C++ programmers). See the header ScreenCap.h in the MFC Advanced Concepts sample DLLScreenCap for an example. To export your functions, enter them in the EXPORTS section of your module definition file (.DEF) or include __declspec(dllexport) on your function definitions. Use __declspec(dllimport) to import these functions into the client executable.

You must add the AFX_MANAGE_STATE macro at the beginning of all the exported functions in regular DLLs that dynamically link to MFC to set the current module state to the one for the DLL. This is done by adding the following line of code to the beginning of functions exported from the DLL:

AFX_MANAGE_STATE(AfxGetStaticModuleState( ))

WinMain -> DllMain

The MFC library defines the standard Win32 DllMain entry point that initializes your CWinApp derived object as in a normal MFC application. Place all DLL-specific initialization in the InitInstance member function as in a normal MFC application.

Note that the CWinApp::Run mechanism doesn't apply to a DLL, since the application owns the main message pump. If your DLL brings up modeless dialogs or has a main frame window of its own, your application's main message pump must call a DLL-exported routine that calls CWinApp::PreTranslateMessage.

See the DLLScreenCap sample for use of this function.