StringIO#ungetbyte

static VALUE
strio_ungetbyte(VALUE self, VALUE c)
{
    struct StringIO *ptr = readable(self);

    check_modifiable(ptr);
    if (NIL_P(c)) return Qnil;
    if (RB_INTEGER_TYPE_P(c)) {
        /* rb_int_and() not visible from exts */
        VALUE v = rb_funcall(c, '&', 1, INT2FIX(0xff));
        const char cc = NUM2INT(v) & 0xFF;
        strio_unget_bytes(ptr, &cc, 1);
    }
    else {
        long cl;
        SafeStringValue(c);
        cl = RSTRING_LEN(c);
        if (cl > 0) {
            strio_unget_bytes(ptr, RSTRING_PTR(c), cl);
            RB_GC_GUARD(c);
        }
    }
    return Qnil;
}