int libssh2_session_startup(LIBSSH2_SESSION *session, int socket);
socket - Connected socket descriptor. Typically a TCP connection though the protocol allows for any reliable transport and the library will attempt to use any berkeley socket.
Begin transport layer protocol negotiation with the connected host.
LIBSSH2_ERROR_BANNER_SEND - Unable to send banner to remote host.
LIBSSH2_ERROR_KEX_FAILURE - >Encryption key exchange with the remote host failed.
LIBSSH2_ERROR_SOCKET_SEND - Unable to send data on socket.
LIBSSH2_ERROR_SOCKET_DISCONNECT - The socket was disconnected.
LIBSSH2_ERROR_PROTO - An invalid SSH protocol response was received on the socket.
LIBSSH2_ERROR_EAGAIN - Marked for non-blocking I/O but the call would block.