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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 ssoocckkeett BBIIOO is automatically created to interface between the ssssll and ffdd. The BIO and hence the SSL engine inherit the behaviour of ffdd. If ffdd is non-blocking, the ssssll will also have non-blocking behaviour.
If there was already a BIO connected to ssssll, _B_I_O___f_r_e_e_(_) will be called (for both the reading and writing side, if different).
_S_S_L___s_e_t___r_f_d_(_) and _S_S_L___s_e_t___w_f_d_(_) perform the respective action, but only for the read channel or the write channel, which can be set independently.