SSL_do_handshake.3ssl man page
SSL_do_handshake — perform a TLS/SSL handshake
#include <openssl/ssl.h> int SSL_do_handshake(SSL *ssl);
SSL_do_handshake() will wait for a SSL/TLS handshake to take place. If the connection is in client mode, the handshake will be started. The handshake routines may have to be explicitly set in advance using either SSL_set_connect_state(3) or SSL_set_accept_state(3).
The behaviour of SSL_do_handshake() depends on the underlying BIO.
If the underlying BIO is blocking, SSL_do_handshake() will only return once the handshake has been finished or an error occurred.
If the underlying BIO is non-blocking, SSL_do_handshake() will also return when the underlying BIO could not satisfy the needs of SSL_do_handshake() to continue the handshake. In this case a call to SSL_get_error() with the return value of SSL_do_handshake() 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_do_handshake(). 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_connect(3), SSL_accept(3), ssl(3), bio(3), SSL_set_connect_state(3)