This function makes a window become the system-modal window, or ends the system-modal state.

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

HWND    hwndDesktop;  /*  Desktop-window handle, or HWND_DESKTOP. */
HWND    hwnd;         /*  Handle of window to become system-modal window. */
BOOL    rc;           /*  Success indicator. */

rc = WinSetSysModalWindow(hwndDesktop, hwnd);