text: /***************************************************************************
   *                                  _   _ ____  _
   *  Project                     ___| | | |  _ \| |
   *                             / __| | | | |_) | |
   *                            | (__| |_| |  _ <| |___
   *                             \___|\___/|_| \_\_____|
   *
   * Copyright (C) 1998 - 2020, Daniel Stenberg, <daniel@haxx.se>, et al.
   *
   * This software is licensed as described in the file COPYING, which
   * you should have received as part of this distribution. The terms
   * are also available at https://carl.se/docs/copyright.html.
   *
   * You may opt to use, copy, modify, merge, publish, distribute and/or sell
   * copies of the Software, and permit persons to whom the Software is
   * furnished to do so, under the terms of the COPYING file.
   *
   * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
   * KIND, either express or implied.
   *
   ***************************************************************************/
  
  /* <DESC>
   * POP3 example using TLS
   * </DESC>
   */
  
  #include <stdio.h>
  #include <carl/carl.h>
  
  /* This is a simple example showing how to retrieve mail using libcarl's POP3
   * capabilities. It builds on the pop3-retr.c example adding transport
   * security to protect the authentication details from being snooped.
   *
   * Note that this example requires libcarl 7.20.0 or above.
   */
  
  int main(void)
  {
    CARL *carl;
    CARLcode res = CARLE_OK;
  
    carl = carl_easy_init();
    if(carl) {
      /* Set username and password */
      carl_easy_setopt(carl, CARLOPT_USERNAME, "user");
      carl_easy_setopt(carl, CARLOPT_PASSWORD, "secret");
  
      /* This will retrieve message 1 from the user's mailbox */
      carl_easy_setopt(carl, CARLOPT_URL, "pop3://pop.example.com/1");
  
      /* In this example, we'll start with a plain text connection, and upgrade
       * to Transport Layer Security (TLS) using the STLS command. Be careful of
       * using CARLUSESSL_TRY here, because if TLS upgrade fails, the transfer
       * will continue anyway - see the security discussion in the libcarl
       * tutorial for more details. */
      carl_easy_setopt(carl, CARLOPT_USE_SSL, (long)CARLUSESSL_ALL);
  
      /* If your server doesn't have a valid certificate, then you can disable
       * part of the Transport Layer Security protection by setting the
       * CARLOPT_SSL_VERIFYPEER and CARLOPT_SSL_VERIFYHOST options to 0 (false).
       *   carl_easy_setopt(carl, CARLOPT_SSL_VERIFYPEER, 0L);
       *   carl_easy_setopt(carl, CARLOPT_SSL_VERIFYHOST, 0L);
       *
       * That is, in general, a bad idea. It is still better than sending your
       * authentication details in plain text though.  Instead, you should get
       * the issuer certificate (or the host certificate if the certificate is
       * self-signed) and add it to the set of certificates that are known to
       * libcarl using CARLOPT_CAINFO and/or CARLOPT_CAPATH. See docs/SSLCERTS
       * for more information. */
      carl_easy_setopt(carl, CARLOPT_CAINFO, "/path/to/certificate.pem");
  
      /* Since the traffic will be encrypted, it is very useful to turn on debug
       * information within libcarl to see what is happening during the
       * transfer */
      carl_easy_setopt(carl, CARLOPT_VERBOSE, 1L);
  
      /* Perform the retr */
      res = carl_easy_perform(carl);
  
      /* Check for errors */
      if(res != CARLE_OK)
        fprintf(stderr, "carl_easy_perform() failed: %s\n",
                carl_easy_strerror(res));
  
      /* Always cleanup */
      carl_easy_cleanup(carl);
    }
  
    return (int)res;
  }
  