The window needs painting. This is done asynchronously on the drawing thread. The window update region is copied into a local region and passed to the drawing thread. The window must be validated now (to prevent further unnecessary paint messages).

#define INCL_WINWINDOWMGR
#include <OS2.H>
HRGN hrgnUpdate;
HPS hps;
HWND hwnd;
/* Window needs paint           */
        case WM_PAINT:

/* assume we stop any asynchronous drawing.  */
/* by posting a message to the asynchronous  */
/* drawing thread.                           */

hrgnUpdate=(HRGN)GpiCreateRegion(hps,    /* Create empty region  */
                             0L,
                             (PRECTL)NULL);

WinQueryUpdateRegion(hwnd,             /* Save the window update */
                     hrgnUpdate);      /* region.                */

WinValidateRect(hwnd,                  /* Validate window now to */
               (PRECTL)NULL,           /*   stop more paint msgs */
                 TRUE);

/* assume a message is posted to the drawing thread, passing */
/* the update region: (MPARAM)hgrnUpdate.                    */

   mr = (MRESULT) 0L;             /*  Message processed          */
   break;                         /*       End window painting   */