The check message filter hook is called whenever WinGetMsg, WinWaitMsg, or WinPeekMsg are used to filter message identities. This hook lets an application apply very specific message filtering, for example, based on the values of message parameters. This hook is called after window handle filtering and before message filtering. The following code shows the syntax for a check message filter hook function:

BOOL EXPENTRY CheckMsgFilterHook ( HAB   hab,
                                   PQMSG pQmsg,
                                   ULONG usFirst,
                                   ULONG usLast,
                                   ULONG fOptions);

The hab parameter is the anchor block handle.

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

The usFirst parameter is the first message identity specified on a call to the WinGetMsg, WinPeekMsg, or WinWaitMsg function.

The usLast parameter is the last message identity specified on a call to the WinGetMsg, WinPeekMsg, or WinWaitMsg function.

The fOptions parameter indicates whether or not the message is removed from the queue:

PM_NOREMOVE
PM_REMOVE

If the check message filter hook function returns TRUE, the message is accepted by the filtering. Any further check message filter hooks in the chain are ignored, any filtering specified by the WinGetMsg, WinPeekMsg, and WinWaitMsg functions are ignored, and processing of the message continues.

A hook that always returns TRUE effectively switches off message filtering.

If the check message filter hook function returns FALSE, the message is passed on to the next check message filter hook in the chain. If the end of the chain has been reached, the filtering specified by the WinGetMsg, WinPeekMsg, or WinWaitMsg functions is applied.