This function provides the default behavior for a secondary window.

#define INCL_SW
#include <os2.h>

HWND       hwnd;  /*  Dialog window handle. */
ULONG      msg;   /*  Message identity. */
MPARAM     mp1;   /*  Parameter 1. */
MPARAM     mp2;   /*  Parameter 2. */
MRESULT    rc;    /*  Returns MRESULT. */

rc = WinDefSecondaryWindowProc(hwnd, msg,
       mp1, mp2);