ungetbyte

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

    check_modifiable(ptr);
    switch (TYPE(c)) {
      case T_NIL:
        return Qnil;
      case T_FIXNUM:
      case T_BIGNUM: ;
        /* rb_int_modulo() not visible from exts */
        VALUE v = rb_funcall(c, rb_intern("modulo"), 1, INT2FIX(256));
        unsigned char cc = NUM2INT(v) & 0xFF;
        c = rb_str_new((const char *)&cc, 1);
        break;
      default:
        SafeStringValue(c);
    }

    const char *cp = RSTRING_PTR(c);
    long cl = RSTRING_LEN(c);
    if (cl == 0) return Qnil;
    strio_unget_bytes(ptr, cp, cl);
    RB_GC_GUARD(c);
    return Qnil;
}