t to the server, and that we have also received one back. A return value of 0 means that we have sent a &quot;close_notify&quot; alert to the server, but we have not yet received one back. Usually in this scenario you would call <a href="../man3/SSL_shutdown.html">SSL_shutdown(3)</a> again which (with a blocking socket) would block until the &quot;close_notify&quot; is received. However in this case we already know that the server has sent us a &quot;close_notify&quot; because of the SSL_ERROR_ZERO_RETURN t