Updated: 2021/Apr/14


SSL_connect(3)                      OpenSSL                     SSL_connect(3)



NAME
       SSL_connect - initiate the TLS/SSL handshake with an TLS/SSL server

LIBRARY
       libcrypto, -lcrypto

SYNOPSIS
        #include <openssl/ssl.h>

        int SSL_connect(SSL *ssl);

DESCRIPTION
       SSL_connect() initiates the TLS/SSL handshake with a server. The
       communication channel must already have been set and assigned to the
       ssl by setting an underlying BIO.

NOTES
       The behaviour of SSL_connect() depends on the underlying BIO.

       If the underlying BIO is blocking, SSL_connect() will only return once
       the handshake has been finished or an error occurred.

       If the underlying BIO is nonblocking, SSL_connect() will also return
       when the underlying BIO could not satisfy the needs of SSL_connect() to
       continue the handshake, indicating the problem by the return value -1.
       In this case a call to SSL_get_error() with the return value of
       SSL_connect() will yield SSL_ERROR_WANT_READ or SSL_ERROR_WANT_WRITE.
       The calling process then must repeat the call after taking appropriate
       action to satisfy the needs of SSL_connect().  The action depends on
       the underlying BIO. When using a nonblocking socket, nothing is to be
       done, but select() can be used to check for the required condition.
       When using a buffering BIO, like a BIO pair, data must be written into
       or retrieved out of the BIO before being able to continue.

       Many systems implement Nagle's algorithm by default which means that it
       will buffer outgoing TCP data if a TCP packet has already been sent for
       which no corresponding ACK has been received yet from the peer. This
       can have performance impacts after a successful TLSv1.3 handshake or a
       successful TLSv1.2 (or below) resumption handshake, because the last
       peer to communicate in the handshake is the client. If the client is
       also the first to send application data (as is typical for many
       protocols) then this data could be buffered until an ACK has been
       received for the final handshake message.

       The TCP_NODELAY socket option is often available to disable Nagle's
       algorithm. If an application opts to disable Nagle's algorithm
       consideration should be given to turning it back on again later if
       appropriate. The helper function BIO_set_tcp_ndelay() can be used to
       turn on or off the TCP_NODELAY option.

RETURN VALUES
       The following return values can occur:

       0   The TLS/SSL handshake was not successful but was shut down
           controlled and by the specifications of the TLS/SSL protocol. Call
           SSL_get_error() with the return value ret to find out the reason.

       1   The TLS/SSL handshake was successfully completed, a TLS/SSL
           connection has been established.

       <0  The TLS/SSL handshake was not successful, because a fatal error
           occurred either at the protocol level or a connection failure
           occurred. The shutdown was not clean. It can also occur if action
           is needed to continue the operation for nonblocking BIOs. Call
           SSL_get_error() with the return value ret to find out the reason.

SEE ALSO
       SSL_get_error(3), SSL_accept(3), SSL_shutdown(3), ssl(7), bio(7),
       SSL_set_connect_state(3), SSL_do_handshake(3), SSL_CTX_new(3)

COPYRIGHT
       Copyright 2000-2020 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>.



1.1.1i                            2020-12-10                    SSL_connect(3)