CallWndProc

The CallWndProc hook procedure is an application-defined or library-defined callback function used with the SetWindowsHookEx function. The system calls this function whenever the SendMessage function is called. Before passing the message to the destination window procedure, the system passes the message to the hook procedure. The hook procedure can examine the message; it cannot modify it.

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   // address of structure with message data
);
 

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 must return the value returned by CallNextHookEx.

wParam

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

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 the address of the hook procedure in a call to the SetWindowsHookEx function.

The WM_CALLWNDPROC hook is called in the context of the thread that calls SendMessage, not the thread that receives the message.

QuickInfo

  Windows NT: Requires version 3.1 or later.
  Windows: Requires Windows 95 or later.
  Windows CE: Unsupported.
  Header: Declared in winuser.h.
  Import Library: User-defined.

See Also

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