The journal playback hook enables an application to insert messages into the system-message queue. Typically, an application uses this hook to play back a series of mouse and keyboard events that were recorded earlier using the journal record hook. A journal playback hook function can be associated only with the system-message queue.

Regular mouse and keyboard input is disabled as long as a journal playback hook is installed. It is important to notice that, because mouse and keyboard input are disabled, this hook can easily hang the system. The following code shows the syntax for a journal playback hook function:

ULONG EXPENTRY JournalPlaybackHook(HAB hab, BOOL fSkip,
                                   PQMSG pQmsg)

The pQmsg parameter is a pointer to a QMSG data structure that the journal playback hook function fills in with the message to be played back. If the fSkip parameter is FALSE, the function fills in the QMSG data structure with the current recorded message. The function returns the same message each time it is called, until fSkip is TRUE. The same message is returned many times if an application is examining the queue but not removing the message. If fSkip is TRUE, the function advances to the next message without filling in the QMSG data structure, because the pQmsg parameter is NULL when fSkip is TRUE.

The journal playback hook returns a ULONG time-out value that tells the system how many milliseconds to wait before processing the current message from the playback hook. This enables the hook to control the timing of the events it plays back.

The time field of the QMSG data structure is filled in with the current time before the playback hook is called. The hook should use the time stored in this field, instead of the system clock, to set up delays between events.