StringIO#ungetc

static VALUE
strio_ungetc(VALUE self, VALUE c)
{
    struct StringIO *ptr = readable(self);
    rb_encoding *enc, *enc2;

    check_modifiable(ptr);
    if (NIL_P(ptr->string)) return Qnil;
    if (NIL_P(c)) return Qnil;
    if (RB_INTEGER_TYPE_P(c)) {
        int len, cc = NUM2INT(c);
        char buf[16];

        enc = rb_enc_get(ptr->string);
        len = rb_enc_codelen(cc, enc);
        if (len <= 0) {
            rb_enc_uint_chr(cc, enc); /* to raise an exception */
            UNREACHABLE;
        }
        rb_enc_mbcput(cc, buf, enc);
        return strio_unget_bytes(ptr, buf, len);
    }
    else {
        StringValue(c);
        if (RSTRING_LEN(c) == 0) return Qnil;
        enc = rb_enc_get(ptr->string);
        enc2 = rb_enc_get(c);
        if (enc != enc2 && enc != rb_ascii8bit_encoding()) {
            c = rb_str_conv_enc(c, enc2, enc);
        }
        strio_unget_string(ptr, c);
        return Qnil;
    }
}