SSL_connect.3ssl man page
SSL_connect — initiate the TLS/SSL handshake with an 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 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 non-blocking 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.
The following return values can occur:
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.
The TLS/SSL handshake was successfully completed, a TLS/SSL connection has been established.
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 of action is need to continue the operation for non-blocking BIOs. Call SSL_get_error() with the return value ret to find out the reason.
SSL_get_error(3), SSL_accept(3), SSL_shutdown(3), ssl(3), bio(3), SSL_set_connect_state(3), SSL_do_handshake(3), SSL_CTX_new(3)