CallWndProc

The CallWndProc hook procedure is an application-defined or library-defined callback function used with the SetWindowsHookEx function.

• Windows 95/98 and Windows NT 3.51: The system calls this function whenever the thread calls the SendMessage function. The WH_CALLWNDPROC hook is called in the context of the thread that calls SendMessage, not the thread that receives the message.
• Windows NT 4.0 and later: The system calls this function before calling the window procedure to process a message sent to the thread.

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

LRESULT CALLBACK CallWndProc(
  int nCode,      // hook code
  WPARAM wParam,  // current-process flag
  LPARAM lParam   // message data
);

Parameters

nCode

[in] 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 must return the value returned by CallNextHookEx.

wParam

[in] Specifies whether the message was sent by the current thread. If the message was sent by the current thread, it is nonzero; otherwise, it is zero.

lParam

[in] Pointer to a CWPSTRUCT structure that contains details about the message.

Return Values

If nCode is less than zero, the hook procedure must return the value returned by CallNextHookEx.

If nCode is greater than or equal to zero, it is highly recommended that you call CallNextHookEx and return the value it returns; otherwise, other applications that have installed WH_CALLWNDPROC hooks will not receive hook notifications and may behave incorrectly as a result. If the hook procedure does not call CallNextHookEx, the return value should be zero.

Remarks

The CallWndProc hook procedure can examine the message, but it cannot modify it. After the hook procedure returns control to the system, the message is passed to the window procedure.

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

Requirements

  Windows NT/2000: Requires Windows NT 3.1 or later.
  Windows 95/98: Requires Windows 95 or later.
  Header: Declared in Winuser.h; include Windows.h.

See Also

Hooks Overview, Hook Functions, CallNextHookEx, CWPSTRUCT, SendMessage, SetWindowsHookEx