ISAPI filters expose their functionality to IIS by providing special entry-point functions. IIS calls these functions to initialize, notify, and terminate your filter.
Both GetFilterVersion and HttpFilterProc must be provided by every ISAPI filter. TerminateFilter, called by IIS just before the filter is unloaded, is considered optional, though you should consider providing this function if your filter needs to free any allocated or locked resources before being unloaded.
All function declarations and data structures are available in the ISAPI filter header file httpfilt.h.