#include <openssl/ssl.h> int SSL_set_fd(SSL *ssl, int fd); int SSL_set_rfd(SSL *ssl, int fd); int SSL_set_wfd(SSL *ssl, int fd);
When performing the operation, a socket BIO is automatically created to interface between the ssl and fd. The BIO and hence the SSL engine inherit the behaviour of fd. If fd is non-blocking, the ssl will also have non-blocking behaviour.
If there was already a BIO connected to ssl, BIO_free() will be called (for both the reading and writing side, if different).
SSL_set_rfd() and SSL_set_wfd() perform the respective action, but only for the read channel or the write channel, which can be set independently.