Rob carried the standard well for the “spirit of UNIX,”

if (isatty(1)) {

	if (ioctl(1,JMUX,0) >= 0) {
		struct winsize win;
		if (ioctl(1,JWINSIZE,&win) >= 0) {
			screenwidth = win.bytesx;
			if (screenwidth == 0)
				screenwidth++;
		}
	}

	qflg = Cflg = 1;
	(void) gtty(1, &sgbuf);
	if ((sgbuf.sg_flags & XTABS) == 0)
		usetabs = 1;
}