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Run-Time Library Behavior 

The C/C++ run-time library code performs the DLL startup sequence, eliminating the need to link with a separate module as was necessary in Windows 3.x. Included in the C/C++ run-time library code is the DLL entry-point function called _DllMainCRTStartup. The _DllMainCRTStartup function does several things, including calling _CRT_INIT, which initializes the C/C++ run-time library and invokes C++ constructors on static, non-local variables. Without this function, the run-time library would be left in an uninitialized state. _CRT_INIT is available for both a statically linked CRT or linking to the CRT DLL Msvcrt.dll, from a user DLL.

While it is possible to specify another entry-point function using the /ENTRY: linker option, it is not recommended because your entry-point function would have to duplicate everything that _DllMainCRTStartup does. When building DLLs in Visual C++, _DllMainCRTStartup is linked in automatically and you do not need to specify an entry-point function using the /ENTRY: linker option.

In addition to initializing the C run-time library, _DllMainCRTStartup calls a function called DllMain. Depending on the kind of DLL you are building, Visual C++ provides DllMain for you and it gets linked in so that _DllMainCRTStartup always has something to call. In this way, if you do not need to initialize your DLL, there is nothing special you have to do when building your DLL. If you need to initialize your DLL, where you add your code depends on the kind of DLL you are writing. For more information, see Initializing a DLL.

The C/C++ run-time library code calls constructors and destructors on static, non-local variables. For example, in the following DLL source code, Equus and Sugar are two static, non-local objects of class CHorse, defined in Horses.h. There is no function in source code that contains calls to a constructor function for CHorse or to the destructor function because these objects are defined outside of any function. Therefore, calls to these constructors and destructors must be performed by the run-time code. The run-time library code for applications also performs this function.

#include "horses.h"

CHorse  Equus( ARABIAN, MALE );
CHorse  Sugar( THOROUGHBRED, FEMALE );

BOOL    WINAPI   DllMain (HANDLE hInst, 
                            ULONG ul_reason_for_call,
                            LPVOID lpReserved)
...

Each time a new process attempts to use the DLL, the operating system creates a separate copy of the DLL's data: this is called process attach. The run-time library code for the DLL calls the constructors for all of the global objects, if any, and then calls the DllMain function with process attach selected. The opposite situation is process detach: the run-time library code calls DllMain with process detach selected and then calls a list of termination functions, including atexit functions, destructors for the global objects, and destructors for the static objects. Note that the order of events in process attach is the reverse of that in process detach.

The run-time library code is also called during thread attach and thread detach, but the run-time code does no initialization or termination on its own.

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