STK_AVAIL
if (STK_AVAIL(args) < sizeof (int))
error = copyinstr(sp, args->stk_strp, STK_AVAIL(args), &len);
if (len > STK_AVAIL(args))
if (STK_AVAIL(args) < pad)
ASSERT((ssize_t)STK_AVAIL(args) >= 0);