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, except for SGC (Server Gated Cryptography). For SGC, SSL_do_handshake() may return with -1, but SSL_get_error() will yield SSL_ERROR_WANT_READ/WRITE and SSL_do_handshake() should be called again.
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.