\begin{verbatim}

#include <stdio.h>
#include <stdlib.h>
#include <gnutls/gnutls.h>
#include <gnutls/x509.h>

/* This callback should be associated with a session by calling
 * gnutls_certificate_client_set_select_function( session, cert_callback),
 * before a handshake.
 */

static int cert_callback(gnutls_session session,
			 const gnutls_datum * client_certs, int client_certs_num,
			 const gnutls_datum * req_ca_rdn, int nreqs)
{
   char issuer_dn[256];
   int i, ret;
   size_t len;

   /* Print the server's trusted CAs
    */
   if (nreqs > 0)
      printf("- Server's trusted authorities:\n");
   else
      printf("- Server did not send us any trusted authorities names.\n");

   /* print the names (if any) */
   for (i = 0; i < nreqs; i++) {
      len = sizeof(issuer_dn);
      ret = gnutls_x509_rdn_get(&req_ca_rdn[i], issuer_dn, &len);
      if (ret >= 0) {
         printf("   [%d]: ", i);
         printf("%s\n", issuer_dn);
      }
   }

   /* Select a certificate from the client_certs and return its
    * index.
    */

   return -1;

}

\end{verbatim}