= Message, Buffers, and Event =

SIP message (request or response) is represented by [http://www.pjsip.org/pjsip/docs/html/structpjsip__msg.htm pjsip_msg] structure, but application would rarely see this structure passed directly in the API. For incoming and outgoing messages, the message would be contained in a RX or TX data buffer, [http://www.pjsip.org/pjsip/docs/html/structpjsip__rx__data.htm pjsip_rx_data] and [http://www.pjsip.org/pjsip/docs/html/structpjsip__tx__data.htm pjsip_tx_data] respectively. When passing SIP messages among modules, PJSIP also normally does not pass the buffers directly, but rather encapsulate it as [http://www.pjsip.org/pjsip/docs/html/structpjsip__event.htm pjsip_event] structure. 

The [http://www.pjsip.org/pjsip/docs/html/structpjsip__event.htm pjsip_event] is PJSIP's way to represent any known SIP event types, such as:
 - arrival of SIP message
 - transmission of SIP message
 - transaction timeout event
 - transport error
 - and so on

For detailed information about SIP messaging and RX/TX buffer design in PJSIP, please read the PJSIP Developer's Guide PDF in [http://www.pjsip.org/docs.htm PJSIP Documentation] page.

----

= pjsip_event =

pjsip_event structure mainly consists of two main member:
 - {{{pjsip_event.type}}} shows the [http://www.pjsip.org/pjsip/docs/html/group__PJSIP__EVENT.htm#g887dd71c4680ebf9f85ec69fd87b8399 event type]
 - {{{pjsip_event.body}}} contains the data describing the event. This member is a C union, and which data to be extracted from the union depends on the event type.

== pjsip_event Structure ==

{{{
typedef enum pjsip_event_id_e
{
    PJSIP_EVENT_UNKNOWN,         /** Unidentified event. */
    PJSIP_EVENT_TIMER,           /** Timer event, normally only used internally in transaction. */
    PJSIP_EVENT_TX_MSG,          /** Message transmission event. */
    PJSIP_EVENT_RX_MSG,          /** Message received event. */
    PJSIP_EVENT_TRANSPORT_ERROR, /** Transport error event. */
    PJSIP_EVENT_TSX_STATE,       /** Transaction state changed event. */
    PJSIP_EVENT_USER             /** Indicates that the event was triggered by user action. */
} pjsip_event_id_e;


struct pjsip_event
{
    /** This is necessary so that we can put events as a list. */
    PJ_DECL_LIST_MEMBER(struct pjsip_event);

    /** The event type */
    pjsip_event_id_e type;

    /** The event body as union, which fields depends on the event type. */
    union {
        /** Timer event. */
        struct {
            pj_timer_entry *entry;      /**< The timer entry.           */
        } timer;

        /** Transaction state has changed event. */
        struct {
            union {
                pjsip_rx_data   *rdata; /**< The incoming message.      */
                pjsip_tx_data   *tdata; /**< The outgoing message.      */
                pj_timer_entry  *timer; /**< The timer.                 */
                pj_status_t      status;/**< Transport error status.    */
                void            *data;  /**< Generic data.              */
            } src;
            pjsip_transaction   *tsx;   /**< The transaction.           */
	    int			 prev_state; /**< Previous state.	*/
            pjsip_event_id_e     type;  /**< Type of event source:      */
        } tsx_state;

        /** Message transmission event. */
        struct {
            pjsip_tx_data       *tdata; /**< The transmit data buffer.  */
        } tx_msg;

        /** Transmission error event. */
        struct {
            pjsip_tx_data       *tdata; /**< The transmit data.         */
            pjsip_transaction   *tsx;   /**< The transaction.           */
        } tx_error;

        /** Message arrival event. */
        struct {
            pjsip_rx_data       *rdata; /**< The receive data buffer.   */
        } rx_msg;

        /** User event. */
        struct {
            void                *user1; /**< User data 1.               */
            void                *user2; /**< User data 2.               */
            void                *user3; /**< User data 3.               */
            void                *user4; /**< User data 4.               */
        } user;

    } body;
};
}}}


== Events ==

This section describes various event types and what relevant data can be extracted from the event.

=== Timer Event ===

||Type: || PJSIP_EVENT_TIMER ||
||Data: || event.body.'''timer''' contains the timer entry ||
||Description: || The timer event is emitted when a scheduled timer has elapsed. It is only used internally in transaction layer, and it will never reach application 
||

=== RX Message Event ===

||Type: || PJSIP_EVENT_RX_MSG ||
||Data: || event.body.'''rx_msg''' contains [http://www.pjsip.org/pjsip/docs/html/structpjsip__rx__data.htm pjsip_rx_data] ||
||Description: || The RX message event is emitted by the transport layer upon arrival of a packet and after the packet is parsed successfully. The transport layer then hand over this event to pjsip_endpoint which will distribute this event to all registered modules. An application (which is a module in PJSIP) only receives this event for SIP messages that are '''outside''' any transactions. For SIP messages belonging to a transaction, the transaction layer would have filtered this event (thus it will not reach dialog or application layer), and the event will be forwarded to upper layer as ''Transaction State Progression'' event instead (see below).
||

=== TX Message Event ===

||Type: || PJSIP_EVENT_TX_MSG ||
||Data: || event.body.'''tx_msg''' contains [http://www.pjsip.org/pjsip/docs/html/structpjsip__tx__data.htm pjsip_tx_data] ||
||Description: || This event represents transmission of outgoing message.
||

=== Transmission Error Event ===

||Type: || PJSIP_EVENT_TRANSPORT_ERROR ||
||Data: || event.body.'''tx_error''' contains the pjsip_tx_data that failed to be sent and optionally the transaction that sent the message ||
||Description: || This event is only used internally in PJSIP, and it will never reach application.
||

=== Transaction State Progression Event ===

||Type: || PJSIP_EVENT_TSX_STATE ||
||Data: || event.body.'''tsx_state''' contains description about the state progression (see below) ||
||Description: || This event is emitted by SIP transaction to notify the transaction user that the transaction state has changed. If the transaction user is a dialog, the dialog will also notify the dialog usages (such as invite session or presence subscription), and the dialog usage may further notify the application about the state change. ||

This event is the most common event seen by application. Various callbacks in PJSIP that emit this event:
 - In PJSUA-API: ''on_call_state()'' and ''on_call_tsx_state()'' callbacks
 - In invite session layer: ''on_state_changed()'', ''on_new_session()'', and ''on_tsx_state_changed()''
 - In SIMPLE/presence layer: ''on_evsub_state()'' and ''on_tsx_state()'' callbacks

The event.body.'''tsx_state''' contains these fields:
 - '''tsx''': the transaction which state has changed
 - '''prev_state''': the previous transaction state
 - '''type''': the type of event that caused the transaction state to change (arrival of message, timeout event, etc.)
 - '''src''': a union containing further data associated with the event that caused the transaction state change. 

Application can inspect the original event that caused the transaction state to change by inspecting the '''type''' and '''src''' fields as described above. The relationship between '''type''' and '''src''' fields are as follows.

Incoming message::
 If transaction changed state because of incoming message:
   - ''type'': PJSIP_EVENT_RX_MSG
   - ''src'': ''src.rdata'' contains [http://www.pjsip.org/pjsip/docs/html/structpjsip__rx__data.htm pjsip_rx_data]

Outgoing transmission::
 If transaction changed state because of outgoing transmission:
   - ''type'': PJSIP_EVENT_TX_MSG
   - ''src'': ''src.tdata'' contains [http://www.pjsip.org/pjsip/docs/html/structpjsip__tx__data.htm pjsip_tx_data]

Timer event::
 If transaction changed state because of (timeout) timer event:
   - ''type'': PJSIP_EVENT_TIMER
   - ''src'': ''src.timer'' contains the timer entry

Transport error::
 If transaction changed state because of transport error:
   - ''type'': PJSIP_EVENT_TRANSPORT_ERROR
   - ''src'': ''src.status'' contains the transport error status


=== User Event ===

||Type: || PJSIP_EVENT_USER ||
||Data: || event.body.'''user''' contains user data ||
||Description: || Currently this is not used ||


----

= Getting the SIP Message from pjsip_event =

Before we can get the SIP message, first we need to get the [http://www.pjsip.org/pjsip/docs/html/structpjsip__rx__data.htm pjsip_rx_data] or [http://www.pjsip.org/pjsip/docs/html/structpjsip__tx__data.htm pjsip_tx_data], depending on whether the event is related to receipt or transmission of SIP message.

== Retrieving SIP Message from an Incoming Message ==

First we need to extract [http://www.pjsip.org/pjsip/docs/html/structpjsip__rx__data.htm pjsip_rx_data] from the event:

{{{
static pjsip_rx_data *get_rx_data(pjsip_event *e)
{
   if (e->type == PJSIP_EVENT_RX_MSG)
      return e->body.rx_msg.rdata;

   if (e->type == PJSIP_EVENT_TSX_STATE && e->body.tsx_state.type == PJSIP_EVENT_RX_MSG)
      return e->body.tsx_state.src.rdata;
   
   // There's no rdata on this eevnt
   return NULL;
}
}}}

Once we get the [http://www.pjsip.org/pjsip/docs/html/structpjsip__rx__data.htm pjsip_rx_data], we can extract the SIP message:

{{{
   pjsip_msg *msg = rdata->msg_info.msg;
}}}


== Retrieving SIP Message from an Outgoing Message ==

First we need to extract [http://www.pjsip.org/pjsip/docs/html/structpjsip__tx__data.htm pjsip_tx_data] from the arbitrary event:

{{{
static pjsip_rx_data *get_tx_data(pjsip_event *e)
{
   if (e->type == PJSIP_EVENT_TX_MSG)
      return e->body.tx_msg.tdata;

   if (e->type == PJSIP_EVENT_TSX_STATE && e->body.tsx_state.type == PJSIP_EVENT_TX_MSG)
      return e->body.tsx_state.src.tdata;
   
   // There's no tdata on this eevnt
   return NULL;
}
}}}

Once we get the [http://www.pjsip.org/pjsip/docs/html/structpjsip__tx__data.htm pjsip_tx_data], we can extract the SIP message:

{{{
   pjsip_msg *msg = tdata->msg;
}}}

----

= Manipulating the SIP Message =

Below are few snippets on how to manipulate SIP messages. Please see [http://www.pjsip.org/pjsip/docs/html/group__PJSIP__MSG.htm SIP Messaging Elements] in [http://www.pjsip.org/pjsip/docs/html/index.htm PJSIP documentation] for the complete documentation.

== Processing Request Line ==

{{{
  if (msg->type == PJSIP_REQUEST_MSG) {
     pjsip_method *m = &msg->line.req.method;
     pjsip_uri *uri = msg->line.req.uri;
  }
}}}

== Processing Status Line ==

{{{
  if (msg->type == PJSIP_RESPONSE_MSG) {
     int code = msg->line.status.code;
     pj_str_t reason = msg->line.status.reason;
  }
}}}

== Finding a Particular Header ==

To find header by its type:

{{{
  pjsip_contact_hdr *h = (pjsip_contact_hdr*) pjsip_msg_find_hdr(msg, PJSIP_H_CONTACT, NULL);
}}}

or to find header by its name:

{{{
  const pj_str_t hname = pj_str("Contact");
  pjsip_contact_hdr *h = (pjsip_contact_hdr*) pjsip_msg_find_hdr_by_name(msg, &hname, NULL);
}}}

== Finding Multiple Headers ==

To find all Contact headers:

{{{
  pjsip_hdr *h;
  
  h = msg->hdr.next;
  while (h != &msg->hdr) {
     if (h->type == PJSIP_H_CONTACT) {
        // h is Contact hdr, do something with it ..
        pjsip_contact_hdr *hc = (pjsip_contact_hdr*)h;
        ...
     }
     h = h->next;
  }
}}}

or

{{{
  pjsip_hdr *h;

  h = pjsip_msg_find_hdr(msg, PJSIP_H_CONTACT, NULL);
  while (h != NULL) {
    // h is Contact hdr, do something with it ..
    pjsip_contact_hdr *hc = (pjsip_contact_hdr*)h;
    ...

    // find next Contact header after h, only if h is not the last header
    if (h->next == &msg->hdr)
       break;
    h = pjsip_msg_find_hdr(msg, PJSIP_H_CONTACT, h->next);
  }
}}}


== Adding a Header ==

{{{
  pjsip_hdr *hdr = .. your header ..

  // To add hdr as the last header in msg:
  pjsip_msg_add_hdr(msg, hdr);
  
  // To add hdr as the first header in msg
  //   pjsip_msg_insert_first_hdr(msg, hdr)

  // To add hdr after some header (for example after Content-Type)
  //   pjsip_hdr *hctype = pjsip_msg_find_hdr(msg, PJSIP_H_CONTENT_TYPE, NULL);
  //   if (hctype)
  //        pj_list_insert_after(hctype, hdr);

}}}