This function inverts a rectangular area.

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

HPS       hps;       /*  Presentation-space handle. */
PRECTL    prclRect;  /*  Rectangle to be inverted. */
BOOL      rc;        /*  Success indicator. */

rc = WinInvertRect(hps, prclRect);