This example uses the WinSetMsgInterest call to set the message interest of a window to only WM_SHOW messages.

#define INCL_WINMESSAGEMGR
#define INCL_WINHOOKS
#include <OS2.H>

HWND hwnd;
HAB  hab;
BOOL fSuccess;
   /* Hook Procedure Prototype */

BOOL MsgCtlHook(HAB hab,LONG  idContext,    /* this hook can */
               HWND hwnd, PSZ pszClassname,     /* be given any */
               ULONG  ulMsgclass,               /* name.        */
               LONG  idControl, PBOOL fSuccess);

main()
{
/* This function passes the hook procedure address to the system. */

WinSetHook(hab,
          (HMQ)0,
          MCHK_CLASSMSGINTEREST,
          (PFN)MsgCtlHook,
          (HMODULE)0); /* hook is into application queue. */

 /* This function sets the message interest of a window class. */
     WinSetMsgInterest(hwnd,
                       WM_SHOW,
                       SMI_AUTODISPATCH); /* interested in the */
                                 /* messages, but they are to     */
                                 /* be automatically dispatched   */
                                 /* to the window procedure.      */
}

BOOL MsgCtlHook(HAB hab, LONG idContext,       /* this hook can */
               HWND hwnd, PSZ pszClassname,    /* be given any  */
               ULONG ulMsgclass,               /* name.         */
               LONG idControl, PBOOL fSuccess)
{
/* ... */
}