ungetbyte

static VALUE
strio_ungetbyte(VALUE self, VALUE c)
{
    struct StringIO *ptr = readable(self);
    char buf[1], *cp = buf;
    long cl = 1;

    check_modifiable(ptr);
    if (NIL_P(c)) return Qnil;
    if (FIXNUM_P(c)) {
        buf[0] = (char)FIX2INT(c);
        return strio_unget_bytes(ptr, buf, 1);
    }
    else {
        SafeStringValue(c);
        cp = RSTRING_PTR(c);
        cl = RSTRING_LEN(c);
        if (cl == 0) return Qnil;
        strio_unget_bytes(ptr, cp, cl);
        RB_GC_GUARD(c);
        return Qnil;
    }
}