This example gets the dimensions of the window and calls WinInvalidateRect to invalidate the window. The application will be sent a WM_PAINT message with the entire window as the update rectangle.

#define INCL_WINWINDOWMGR
#include <OS2.H>

HAB   hab;
HWND  hwnd;
RECTL rcl;

 WinQueryWindowRect(hwnd, &rcl);
 WinInvalidateRect(hwnd,        /* window to invalidate    */
                   &rcl,    /* invalid rectangle       */
                   FALSE);      /* do not include children */