DllMain Callback Function

跟exe有个main或者WinMain入口函数一样，DLL也有一个入口函数，就是DllMain。
An optional entry point into a dynamic-link library (DLL). When the system starts or terminates a process or thread, it calls the entry-point function for each loaded DLL using the first thread of the process. The system also calls the entry-point function for a DLL when it is loaded or unloaded using the LoadLibrary and FreeLibrary functions.

Warning: There are serious limits on what you can do in a DLL entry point. To provide more complex initialization, create an initialization routine for the DLL. You can require applications to call the initialization routine before calling any other routines in the DLL.

BOOL WINAPI DllMain(
  __in  HINSTANCE hinstDLL,
  __in  DWORD fdwReason,
  __in  LPVOID lpvReserved
);

其第二个参数说明了这个DllMain是因为什么原因被调用的，可能的原因有4种：

• DLL_PROCESS_ATTACH(1): The DLL is being loaded into the virtual address space of the current process as a result of the process starting up or as a result of a call to LoadLibrary. DLLs can use this opportunity to initialize any instance data or to use the TlsAlloc function to allocate a thread local storage (TLS) index.The lpReserved parameter indicates whether the DLL is being loaded statically or dynamically.
• DLL_PROCESS_DETACH(0): The DLL is being unloaded from the virtual address space of the calling process because it was loaded unsuccessfully or the reference count has reached zero (the processes has either terminated or called FreeLibrary one time for each time it called LoadLibrary).The lpReserved parameter indicates whether the DLL is being unloaded as a result of a FreeLibrary call, a failure to load, or process termination.The DLL can use this opportunity to call the TlsFree function to free any TLS indices allocated by using TlsAlloc and to free any thread local data.Note that the thread that receives the DLL_PROCESS_DETACH notification is not necessarily the same thread that received the DLL_PROCESS_ATTACH notification.
• DLL_THREAD_ATTACH(2): The current process is creating a new thread. When this occurs, the system calls the entry-point function of all DLLs currently attached to the process. The call is made in the context of the new thread. DLLs can use this opportunity to initialize a TLS slot for the thread. A thread calling the DLL entry-point function with DLL_PROCESS_ATTACH does not call the DLL entry-point function with DLL_THREAD_ATTACH. Note that a DLL's entry-point function is called with this value only by threads created after the DLL is loaded by the process. When a DLL is loaded using LoadLibrary, existing threads do not call the entry-point function of the newly loaded DLL.
• DLL_THREAD_DETACH(3): A thread is exiting cleanly. If the DLL has stored a pointer to allocated memory in a TLS slot, it should use this opportunity to free the memory. The system calls the entry-point function of all currently loaded DLLs with this value. The call is made in the context of the exiting thread.

Ref: http://msdn.microsoft.com/en-us/library/ms682583(VS.85).aspx