Returns "on" if SSL compression is in use, else it returns "off".
           </para>
          </listitem>
         </varlistentry>
        <varlistentry>
         <term><literal>alpn</literal></term>
          <listitem>
           <para>
            Application protocol selected by the TLS Application-Layer
            Protocol Negotiation (ALPN) extension.  The only protocol
            supported by libpq is <literal>postgresql</literal>, so this is
            mainly useful for checking whether the server supported ALPN or
            not. Empty string if ALPN was not used.
           </para>
          </listitem>
         </varlistentry>
       </variablelist>
      </para>

      <para>
       As a special case, the <literal>library</literal> attribute may be
       queried without a connection by passing NULL as
       the <literal>conn</literal> argument.  The result will be the default
       SSL library name, or NULL if <application>libpq</application> was
       compiled without any SSL support.  (Prior
       to <productname>PostgreSQL</productname> version 15, passing NULL as
       the <literal>conn</literal> argument always resulted in NULL.
       Client programs needing to differentiate between the newer and older
       implementations of this case may check the
       <literal>LIBPQ_HAS_SSL_LIBRARY_DETECTION</literal> feature macro.)
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQsslAttributeNames">
     <term><function>PQsslAttributeNames</function><indexterm><primary>PQsslAttributeNames</primary></indexterm></term>
     <listitem>
      <para>
       Returns an array of SSL attribute names that can be used
       in <function>PQsslAttribute()</function>.
       The array is terminated by a NULL pointer.
<synopsis>
const char * const * PQsslAttributeNames(const PGconn *conn);
</synopsis>
      </para>

      <para>
       If <literal>conn</literal> is NULL, the attributes available for the
       default SSL library are returned, or an empty list
       if <application>libpq</application> was compiled without any SSL
       support.  If <literal>conn</literal> is not NULL, the attributes
       available for the SSL library in use for the connection are returned,
       or an empty list if the connection is not encrypted.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQsslStruct">
     <term><function>PQsslStruct</function><indexterm><primary>PQsslStruct</primary></indexterm></term>
     <listitem>
      <para>
       Returns a pointer to an SSL-implementation-specific object describing
       the connection.  Returns NULL if the connection is not encrypted
       or the requested type of object is not available from the connection's
       SSL implementation.
<synopsis>
void *PQsslStruct(const PGconn *conn, const char *struct_name);
</synopsis>
      </para>
      <para>
       The struct(s) available depend on the SSL implementation in use.
       For <productname>OpenSSL</productname>, there is one struct,
       available under the name <literal>OpenSSL</literal>,
       and it returns a pointer to
       <productname>OpenSSL</productname>'s <literal>SSL</literal> struct.
       To use this function, code along the following lines could be used:
<programlisting><![CDATA[
#include <libpq-fe.h>
#include <openssl/ssl.h>

...

    SSL *ssl;

    dbconn = PQconnectdb(...);
    ...

    ssl = PQsslStruct(dbconn, "OpenSSL");
    if (ssl)
    {
        /* use OpenSSL functions to access ssl */
    }
]]></programlisting>
      </para>
      <para>
       This structure can be used to verify encryption levels, check server
       certificates, and more. Refer to the <productname>OpenSSL</productname>
       documentation for information about this structure.
      </para>
     </listitem>
    </varlistentry>

    <varlistentry id="libpq-PQgetssl">
     <term><function>PQgetssl</function><indexterm><primary>PQgetssl</primary></indexterm></term>