Hook functions manage system hooks, which are shared resources that install a specific type of filter function. A filter function is an application-supplied callback function, specified by the SetWindowsHook function, that processes events before they reach any application's message loop. Windows sends messages generated by a specific type of event to filter functions installed by the same type of hook. The following list briefly describes each hook function:
| Function | Description | |
| CallMsgFilter | Passes a message and other data to the current message-filter function. | |
| DefHookProc | Calls the next filter function in a filter-function chain. | |
| SetWindowsHook | Installs a system and/or application filter function. | |
| UnhookWindowsHook | Removes a Windows filter function from a filter-function chain. |