This function calculates a rectangle that bounds the two source rectangles.

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

HAB       hab;       /*  Anchor-block handle. */
PRECTL    prclDest;  /*  Bounding rectangle. */
PRECTL    prclSrc1;  /*  First source rectangle. */
PRECTL    prclSrc2;  /*  Second source rectangle. */
BOOL      rc;        /*  Nonempty indicator. */

rc = WinUnionRect(hab, prclDest, prclSrc1,
       prclSrc2);