The input hook enables an application to monitor the system-message queue or an application-message queue. The system calls an input-hook function whenever WinGetMsg or WinPeekMsg is about to return a message. Typically, an application uses the input hook to monitor mouse and keyboard input and other messages posted to a queue. The following code shows the syntax for an input-hook function:

BOOL EXPENTRY InputHook(HAB hab, PQMSG pQmsg, ULONG fs)

The pQmsg parameter is a pointer to a QMSG data structure that contains information about the message.

The fs parameter of InputHook can contain the following flags from WinPeekMsg, indicating whether or not the message is removed from the queue:

        PM_NOREMOVE
        PM_REMOVE

If an input-hook function returns TRUE, the system does not pass the message to the rest of the hook chain or to the application. If the function returns FALSE, the system passes the message to the next hook in the chain or to the application if no other hooks exist.

An input-hook function can modify a message by changing the contents of the QMSG data structure, then returning FALSE to pass the modified message to the rest of the chain. The following problems can occur when a hook modifies a message:

• If the caller uses WinPeekMsg or WinGetMsg with a message filter range (msgFilterFirst through msgFilterLast), the message is checked before the hook functions are called, not after. If the input-hook function modifies the msg field of the QMSG data structure, the caller can receive messages that are not in the range of the message filter of the caller.

• If the input-hook function changes a WM_CHAR message from one character into another-for example, if the function modifies all Tab messages into F6 messages-an application that depends on the key state is unable to interpret the result. (When the Tab key is translated into the F6 key, the application receives the F6 keystroke and enters a process loop, waiting for the F6 key to be released; the application calls WinGetKeyState with the HWND_DESKTOP and VK_F6 arguments).