DebugProc

The DebugProc hook procedure is an application-defined or library-defined callback function used with the SetWindowsHookEx function. The system calls this function before calling the hook procedures associated with any type of hook. The system passes information about the hook to be called to the DebugProc hook procedure, which examines the information and determines whether to allow the hook to be called.

The HOOKPROC type defines a pointer to this callback function. DebugProc is a placeholder for the application-defined or library-defined function name.

LRESULT CALLBACK DebugProc(
  int nCode,      // hook code
  WPARAM wParam,  // type of hook about to be called
  LPARAM lParam   // address of structure with debugging information
);
 

Parameters

nCode

Specifies whether the hook procedure must process the message. If nCode is HC_ACTION, the hook procedure must process the message. If nCode is less than zero, the hook procedure must pass the message to the CallNextHookEx function without further processing and should return the value returned by CallNextHookEx.

wParam

Specifies the type of hook about to be called. This parameter can be one of the following values:

Value	Description
WH_CALLWNDPROC	Installs a hook procedure that monitors messages sent to a window procedure. For more information, see the description of the CallWndProc hook procedure.
WH_CALLWNDPROCRET	Installs a hook procedure that monitors messages that have just been processed by a window procedure. For more information, see the description of the CallWndRetProc hook procedure.
WH_CBT	Installs a hook procedure that receives notifications useful to a computer-based training (CBT) application. For more information, see the description of the CBTProc hook procedure.
WH_DEBUG	Installs a hook procedure useful for debugging other hook procedures. For more information, see the description of the DebugProc hook procedure.
WH_GETMESSAGE	Installs a hook procedure that monitors messages posted to a message queue. For more information, see the description of the GetMsgProc hook procedure.
WH_JOURNALPLAYBACK	Installs a hook procedure that posts messages previously recorded by a WH_JOURNALRECORD hook procedure. For more information, see the description of the JournalPlaybackProc hook procedure.
WH_JOURNALRECORD	Installs a hook procedure that records input messages posted to the system message queue. This hook is useful for recording macros. For more information, see the description of the JournalRecordProc hook procedure.
WH_KEYBOARD	Installs a hook procedure that monitors keystroke messages. For more information, see the description of the KeyboardProc hook procedure.
WH_MOUSE	Installs a hook procedure that monitors mouse messages. For more information, see the description of the MouseProc hook procedure.
WH_MSGFILTER	Installs a hook procedure that monitors messages generated as a result of an input event in a dialog box, message box, menu, or scroll bar. The hook procedure monitors these messages only for the application that installed the hook procedure. For more information, see the description of the MessageProc hook procedure.
WH_SHELL	Installs a hook procedure that receives notifications useful to a shell application. For more information, see the description of the ShellProc hook procedure.
WH_SYSMSGFILTER	Installs a hook procedure that monitors messages generated as a result of an input event in a dialog box, message box, menu, or scroll bar. The hook procedure monitors these messages for all applications in the system. For more information, see the description of the SysMsgProc hook procedure.

lParam

Pointer to a DEBUGHOOKINFO structure that contains the parameters to be passed to the destination hook procedure.

Return Values

To prevent the system from calling the hook, the hook procedure must return a nonzero value. Otherwise, the hook procedure must call CallNextHookEx.

Remarks

An application installs this hook procedure by specifying the WH_DEBUG hook type and the pointer to the hook procedure in a call to the SetWindowsHookEx function.

QuickInfo

  Windows NT: Requires version 3.1 or later.
  Windows: Unsupported.
  Windows CE: Unsupported.
  Header: Declared in winuser.h.
  Import Library: User-defined.

See Also

Hooks Overview, Hook Functions, CallNextHookEx, CallWndProc, CallWndRetProc, CBTProc, DEBUGHOOKINFO, GetMsgProc, JournalPlaybackProc, JournalRecordProc, KeyboardProc, MessageProc, MouseProc, SetWindowsHookEx, ShellProc, SysMsgProc