IO::generic_readable#read_nonblock

static VALUE
strio_read_nonblock(int argc, VALUE *argv, VALUE self)
{
    VALUE opts = Qnil, val;

    rb_scan_args(argc, argv, "11:", NULL, NULL, &opts);

    if (!NIL_P(opts)) {
        argc--;
    }

    val = strio_read(argc, argv, self);
    if (NIL_P(val)) {
        if (!NIL_P(opts) &&
              rb_hash_lookup2(opts, sym_exception, Qundef) == Qfalse)
            return Qnil;
        else
            rb_eof_error();
    }

    return val;
}