SSL_get_fd

 #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(3) , bio(3)