SSL_CONNECT(3SSL) OpenSSL SSL_CONNECT(3SSL)
NAME
SSL_connect - initiate the TLS/SSL handshake with an TLS/SSL server
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. ssl MUST NOT be NULL.
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 Apache License 2.0 (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>.
3.5.7 2026-06-09 SSL_CONNECT(3SSL)
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