This function invokes the default window procedure.

#define INCL_WINWINDOWMGR /* Or use INCL_WIN, INCL_PM, Also in COMMON section */
#include <os2.h>

HWND       hwnd;       /*  Window handle. */
ULONG      ulMsgid;    /*  Message identity. */
MPARAM     mpParam1;   /*  Parameter 1. */
MPARAM     mpParam2;   /*  Parameter 2. */
MRESULT    mresReply;  /*  Message-return data. */

mresReply = WinDefWindowProc(hwnd, ulMsgid,
              mpParam1, mpParam2);