SSL_set_bio.3ssl man page
SSL_set_bio — connect the SSL object with a BIO
#include <openssl/ssl.h> void SSL_set_bio(SSL *ssl, BIO *rbio, BIO *wbio);
SSL_set_bio() connects the BIOs rbio and wbio for the read and write operations of the TLS/SSL (encrypted) side of ssl.
The SSL engine inherits the behaviour of rbio and wbio, respectively. If a BIO 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_bio() cannot fail.
SSL_get_rbio(3), SSL_connect(3), SSL_accept(3), SSL_shutdown(3), ssl(3), bio(3)