In 4.3BSD, at least, FIONBIO is implemented as case FIONBIO: ... fset(fd, FNDELAY, *(int *)data) break; and FIOASYNC as ... fset(fp, FASYNC, *(int *)data) so the answer is No, no difference at all. -- In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163) Domain: chris at mimsy.umd.edu Path: uunet!mimsy!chris