SSL_get_fd, SSL_get_rfd, SSL_get_wfd - get file descriptor linked to an SSL
  object
 #include <openssl/ssl.h>
 int SSL_get_fd(const SSL *ssl);
 int SSL_get_rfd(const SSL *ssl);
 int SSL_get_wfd(const SSL *ssl);
SSL_get_fd() returns the file descriptor which is linked to ssl.
  SSL_get_rfd() and SSL_get_wfd() return the file descriptors for
  the read or the write channel, which can be different. If the read and the
  write channel are different, SSL_get_fd() will return the file
  descriptor of the read channel.
The following return values can occur:
  - -1
- The operation failed, because the underlying BIO is not of the correct
      type (suitable for file descriptors).
- >=0
- The file descriptor linked to ssl.
SSL_set_fd(3), ssl(7) , bio(7)
Copyright 2000-2016 The OpenSSL Project Authors. All Rights Reserved.Licensed under the OpenSSL license (the "License"). You
    may not use this file except in compliance with the License. You can obtain
    a copy in the file LICENSE in the source distribution or at
    <https://www.openssl.org/source/license.html>.