DLLPROCESSDETACH

DLL_PROCESS_DETACH (often referred to as DLL_PROCESS_DETACH in documentation) is a constant used as the fdwReason parameter in the DllMain entry point of a Windows dynamic-link library. It signals that the DLL is being unloaded from the current process. This can occur when the process is terminating or when the application explicitly calls FreeLibrary to unload the DLL.

When DLL_PROCESS_DETACH is received, the DLL should perform any necessary per-process cleanup, such as releasing resources that were acquired during DLL_PROCESS_ATTACH or during the lifetime of the process. Common tasks include freeing allocated memory, closing handles, and undoing global initializations. It is important to keep DllMain lightweight during this stage and to avoid complex operations, especially calls into other modules or threads, to prevent deadlocks or crashes during process shutdown.

Detachment can occur for two scenarios: explicit unloading via FreeLibrary, or process termination where the operating system unloads all loaded DLLs. Because the exact timing and context can vary, code in DllMain should avoid creating new threads, calling into other DLLs, or performing lengthy work. If more extensive cleanup is required, it is often safer to defer work to a dedicated shutdown routine or a separate thread that runs after DllMain has returned, or to rely on process-wide atexit-like mechanisms.

In summary, DLL_PROCESS_DETACH is a key lifecycle signal indicating a DLL is being removed from a process, and it is the appropriate place to release process-scoped resources while preserving safe, minimal operations within DllMain.