This function calculates a client rectangle from a frame rectangle, or a frame rectangle from a client rectangle.

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

HWND      hwndFrame;  /*  Frame-window handle. */
PRECTL    prcl;       /*  Window rectangle. */
BOOL      fClient;    /*  Frame indicator. */
BOOL      rc;         /*  Rectangle-calculated indicator. */

rc = WinCalcFrameRect(hwndFrame, prcl, fClient);