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DllEntryPoint info  Overview  Group

The DllEntryPoint function is an optional method of entry into a dynamic-link library (DLL). If the function is used, it is called by the system when processes and threads are initialized and terminated, or upon calls to the LoadLibrary and FreeLibrary functions. DllEntryPoint is a placeholder for the library-defined function name. The actual name must be specified at build time. For more information, see the documentation included with your development tools.

BOOL WINAPI DllEntryPoint(

    HINSTANCE hinstDLL,

// handle to DLL module

    DWORD fdwReason,

// reason for calling function

    LPVOID lpvReserved 

// reserved

   );

Parameters

hinstDLL
A handle to the DLL. The value is the base address of the DLL. The HINSTANCE of a DLL is the same as the HMODULE of the DLL, so hinstDLL can be used in subsequent calls to the GetModuleFileName function and other functions that require a module handle.
fdwReason
Specifies a flag indicating why the DLL entry-point function is being called. This parameter can be one of the following values:

Value

Meaning

DLL_PROCESS_ATTACH

Indicates that the DLL is attaching to the 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.

During initial process startup or after a call to LoadLibrary, the operating system scans the list of loaded DLLs for the process. For each DLL that has not already been called with the DLL_PROCESS_ATTACH value, the system calls the DLLís entry-point function. This call is made in the context of the thread that caused the process address space to change, such as the primary thread of the process or the thread that called LoadLibrary.

DLL_THREAD_ATTACH

Indicates that 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 the DLL_PROCESS_ATTACH value does not call the DLL entry-point function with the DLL_THREAD_ATTACH value.

Note that a DLLís entry-point function is called with this value only by threads created after the DLL is attached to the process. When a DLL is attached by LoadLibrary, existing threads do not call the entry-point function of the newly loaded DLL.

DLL_THREAD_DETACH

Indicates that a thread is exiting cleanly. If the DLL has stored a pointer to allocated memory in a TLS slot, it uses this opportunity to free the memory. The operating 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. There are cases in which the entry-point function is called for a terminating thread even if the DLL never attached to the thread ¾ for example, the entry-point function was never called with the DLL_THREAD_ATTACH value in the context of the thread in either of these two situations:

  • The thread was the initial thread in the process, so the system called the entry-point function with the DLL_PROCESS_ATTACH value.

  • The thread was already running when a call to the LoadLibrary function was made, so the system never called the entry-point function for it.

DLL_PROCESS_DETACH

Indicates that the DLL is detaching from the address space of the calling process as a result of either a clean process exit or of a call to FreeLibrary. 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. When a DLL detaches from a process as a result of process termination or as a result of a call to FreeLibrary, the operating system does not call the DLLís entry-point function with the DLL_THREAD_DETACH value for the individual threads of the process. The DLL is only given DLL_PROCESS_DETACH notification. DLLs can take this opportunity to clean up all resources for all threads attached and known to the DLL.

lpvReserved
Specifies further aspects of DLL initialization and cleanup.

If fdwReason is DLL_PROCESS_ATTACH, lpvReserved is NULL for dynamic loads and non-NULL for static loads.

If fdwReason is DLL_PROCESS_DETACH, lpvReserved is NULL if DllEntryPoint has been called by using FreeLibrary and non-NULL if DllEntryPoint has been called during process termination.

Return Values

When the system calls the DllEntryPoint function with the DLL_PROCESS_ATTACH value, the function returns TRUE if it succeeds or FALSE if initialization fails. If the return value is FALSE when DllEntryPoint is called because the process uses the LoadLibrary function, LoadLibrary returns NULL. If the return value is FALSE when DllEntryPoint is called during process initialization, the process terminates with an error. To get extended error information, call GetLastError.

When the system calls the DllEntryPoint function with any value other than DLL_PROCESS_ATTACH, the return value is ignored.

Remarks

The body of your DLL entry-point function should perform only simple initialization tasks, such as setting up thread local storage (TLS), creating synchronization objects, and opening files. You must not call LoadLibrary in the entry-point function, because you 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.

Calling Win32 functions other than TLS, synchronization, and file functions may result in problems that are difficult to diagnose. For example, calling User, Shell, and COM functions can cause access violation errors, because some functions in their DLLs call LoadLibrary to load other system components

See Also

FreeLibrary, GetModuleFileName, LoadLibrary, TlsAlloc, TlsFree 

See also:


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