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SSL_CONNECT(3) Library Functions Manual SSL_CONNECT(3)

SSL_connect
initiate the TLS/SSL handshake with a TLS/SSL server

#include <openssl/ssl.h>
int
SSL_connect(SSL *ssl);

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.
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 non-blocking, SSL_connect() will also return when the underlying BIO could not satisfy the needs of SSL_connect() to continue the handshake, indicating the problem with the return value −1. In this case a call to SSL_get_error(3) 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 non-blocking socket, nothing is to be done, but select(2) 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.

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(3) with the return value ret to find out the reason.
 
 
1
The TLS/SSL handshake was successfully completed, and a TLS/SSL connection has been established.
 
 
<0
The TLS/SSL handshake was not successful, because either a fatal error occurred 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 non-blocking BIOs. Call SSL_get_error(3) with the return value ret to find out the reason.

BIO_new(3), ssl(3), SSL_accept(3), SSL_CTX_new(3), SSL_do_handshake(3), SSL_get_error(3), SSL_set_connect_state(3), SSL_shutdown(3)

SSL_connect() appeared in SSLeay 0.4 or earlier and has been available since OpenBSD 2.4.
March 27, 2018 OpenBSD-current