/**
 * @ingroup libosip2 The GNU oSIP stack
 * @defgroup howto0_initialize How-To initialize libosip2.

When using osip, your first task is to initialize
the parser and the state machine. This must be done
prior to any use of libosip2.

<PRE>
	#include <sys/time.h>
	#include <osip2/osip.h>

	int i;
	osip_t *osip;
	i=osip_init(&osip);
	if (i!=0)
	  return -1;
</PRE>

In case you want to use the state machines (transaction management),
the second step will be to set a few callbacks which will inform
you of any change in the state of a SIP transaction.

Most of the following callbacks are optional. The following ones are
usefull.

<PRE>
	// callback called when a SIP message must be sent.
	osip_set_cb_send_message(osip, &cb_udp_snd_message);
	
	// callback called when a SIP transaction is TERMINATED.
	osip_set_kill_transaction_callback(osip ,OSIP_ICT_KILL_TRANSACTION,
				 &cb_ict_kill_transaction);
	osip_set_kill_transaction_callback(osip ,OSIP_NIST_KILL_TRANSACTION,
				 &cb_ist_kill_transaction);
	osip_set_kill_transaction_callback(osip ,OSIP_NICT_KILL_TRANSACTION,
				 &cb_nict_kill_transaction);
	osip_set_kill_transaction_callback(osip ,OSIP_NIST_KILL_TRANSACTION,
				 &cb_nist_kill_transaction);

	// callback called when the callback to send message have failed.
	osip_set_transport_error_callback(osip ,OSIP_ICT_TRANSPORT_ERROR,
				    &cb_transport_error);
	osip_set_transport_error_callback(osip ,OSIP_IST_TRANSPORT_ERROR,
				    &cb_transport_error);
	osip_set_transport_error_callback(osip ,OSIP_NICT_TRANSPORT_ERROR,
				    &cb_transport_error);
	osip_set_transport_error_callback(osip ,OSIP_NIST_TRANSPORT_ERROR,
				    &cb_transport_error);
	
	// callback called when a received answer has been accepted by the transaction.
	osip_set_message_callback(osip ,OSIP_ICT_STATUS_1XX_RECEIVED, &cb_rcv1xx);
	osip_set_message_callback(osip ,OSIP_ICT_STATUS_2XX_RECEIVED, &cb_rcv2xx);
	osip_set_message_callback(osip ,OSIP_ICT_STATUS_3XX_RECEIVED, &cb_rcv3456xx);
	osip_set_message_callback(osip ,OSIP_ICT_STATUS_4XX_RECEIVED, &cb_rcv3456xx);
	osip_set_message_callback(osip ,OSIP_ICT_STATUS_5XX_RECEIVED, &cb_rcv3456xx);
	osip_set_message_callback(osip ,OSIP_ICT_STATUS_6XX_RECEIVED, &cb_rcv3456xx);
	
	// callback called when a received answer has been accepted by the transaction.
	osip_set_message_callback(osip ,OSIP_NICT_STATUS_1XX_RECEIVED, &cb_rcv1xx);
	osip_set_message_callback(osip ,OSIP_NICT_STATUS_2XX_RECEIVED, &cb_rcv2xx);
	osip_set_message_callback(osip ,OSIP_NICT_STATUS_3XX_RECEIVED, &cb_rcv3456xx);
	osip_set_message_callback(osip ,OSIP_NICT_STATUS_4XX_RECEIVED, &cb_rcv3456xx);
	osip_set_message_callback(osip ,OSIP_NICT_STATUS_5XX_RECEIVED, &cb_rcv3456xx);
	osip_set_message_callback(osip ,OSIP_NICT_STATUS_6XX_RECEIVED, &cb_rcv3456xx);
	    
	// callback called when a received request has been accepted by the transaction.
	osip_set_message_callback(osip ,OSIP_IST_INVITE_RECEIVED,     &cb_rcvreq);
	osip_set_message_callback(osip ,OSIP_IST_ACK_RECEIVED,        &cb_rcvreq);
	osip_set_message_callback(osip ,OSIP_NIST_REGISTER_RECEIVED,  &cb_rcvreq);
	osip_set_message_callback(osip ,OSIP_NIST_BYE_RECEIVED,       &cb_rcvreq);
	osip_set_message_callback(osip ,OSIP_NIST_CANCEL_RECEIVED,    &cb_rcvreq);
	osip_set_message_callback(osip ,OSIP_NIST_INFO_RECEIVED,      &cb_rcvreq);
	osip_set_message_callback(osip ,OSIP_NIST_OPTIONS_RECEIVED,   &cb_rcvreq);
	osip_set_message_callback(osip ,OSIP_NIST_SUBSCRIBE_RECEIVED, &cb_rcvreq);
	osip_set_message_callback(osip ,OSIP_NIST_NOTIFY_RECEIVED,    &cb_rcvreq);
	osip_set_message_callback(osip ,OSIP_NIST_UNKNOWN_REQUEST_RECEIVED, &cb_rcvreq);

	// other callbacks exists... They are optionnal.
</PRE>

*/