GetMsgProc

The GetMsgProc function is an application-defined or library-defined callback function used with the SetWindowsHookEx function. The system calls this function whenever the GetMessage function has retrieved a message from an application message queue. Before passing the retrieved message to the destination window procedure, the system passes the message to the hook procedure.

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

LRESULT CALLBACK GetMsgProc(
  int code,       // hook code
  WPARAM wParam,  // removal flag
  LPARAM lParam   // address of structure with message
);
 

Parameters

code

Specifies whether the hook procedure must process the message. If code is HC_ACTION, the hook procedure must process the message. If code 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 whether the message has been removed from the queue. This parameter can be one of the following values:

Value	Meaning
PM_NOREMOVE	Specifies that the message has not been removed from the queue. (An application called the PeekMessage function, specifying the PM_NOREMOVE flag.)
PM_REMOVE	Specifies that the message has been removed from the queue. (An application called GetMessage, or it called the PeekMessage function, specifying the PM_REMOVE flag.)

lParam

Pointer to an MSG 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_GETMESSAGE 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 GetMsgProc hook procedure can examine or modify the message. After the hook procedure returns control to the system, the GetMessage function returns the message, along with any modifications, to the application that originally called it.

An application installs this hook procedure by specifying the WH_GETMESSAGE hook type and the address of the hook procedure in a call to the SetWindowsHookEx function.

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, GetMessage, MSG, PeekMessage, SetWindowsHookEx