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man pages section 3: Networking Library Functions

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Updated: July 2017



sip_stack_init - initializes SIP stack


cc [ flag ... ] file ... -lsip [ library ... ]
#include <sip.h>

int sip_stack_init(sip_stack_init_t * stack_val);


The sip_stack_init() function is used to initialize the SIP stack. The stack can be initialized by a process only once. Any shared library that is linked with a main program or another library that has already initialized the stack will encounter a failure when trying to initialize the stack.

The initialization structure is given by:

typedef struct sip_stack_init_s {
    int                      sip_version;
    uint32_t                 sip_stack_flags;
    sip_io_pointers_t       *sip_io_pointers;
    sip_ulp_pointers_t      *sip_ulp_pointers;
    sip_header_function_t   *sip_function_table;

This must be set to SIP_STACK_VERSION.


If the application wants the SIP stack to maintain dialogs, this flag must be set to SIP_STACK_DIALOGS. Otherwise, it must be set to 0. If SIP_STACK_DIALOGS is not set, the stack does not deal with dialogs at all.

Upper Layer Registrations

These include callbacks that are invoked to deliver incoming messages or error notification.

The callback functions should not create a thread and invoke a function that could recursively invoke the callback. For example, the callback function for a transition state change notification should not create a thread to send a SIP message that results in a change in the state of the transaction, which would again invoke the callback function.

The registration structure is supplied by:

typedef struct sip_ulp_pointers_s {
     void      (*sip_ulp_recv)(const sip_conn_object_t,
                     sip_msg_t, const sip_dialog_t);
     uint_t    (*sip_ulp_timeout)(void *, 
                     void (*func)(void *),
                     struct timeval *);
     boolean_t (*sip_ulp_untimeout)(uint_t);
     int       (*sip_ulp_trans_error)
                     (sip_transaction_t, int, void *);
     void      (*sip_ulp_dlg_del)(sip_dialog_t,
                     sip_msg_t, void *);
     void      (*sip_ulp_trans_state_cb)
               (sip_transaction_t, sip_msg_t,
                     int, int);
     void      (*sip_ulp_dlg_state_cb)(sip_dialog_t,
                     sip_msg_t, int, int);

This is a mandatory routine that the application registers for the stack to deliver an inbound SIP message. The SIP stack invokes the function with the connection object on which the message arrived, the SIP message, and any associated dialog.

The SIP message is freed once the function returns. If the application wishes to use the message beyond that, it has to hold a reference on the message using sip_hold_msg(). Similarly, if the application wishes to cache the dialog, it must hold a reference on the dialog using sip_hold_msg().


An application can register these two routines to implement its own routines for the stack timers. Typically, an application should allow the stack to use its own built-in timer routines. The built-in timer routines are used only by the stack and are not available to applications. If the application registers one routine, it must also register the other.

These functions must be registered for single-threaded application. Otherwise, the timer thread provided by the stack could result in invoking a registered callback function.


The application can register this routine to be notified of a transaction error. An error can occur when the transaction layer tries to send a message using a cached connection object which results in a failure. If this routine is not registered the transaction is terminated on such a failure. The final argument is for future use. It is always set to NULL.


An application can register this routine to be notified when a dialog is deleted. The dialog to be deleted is passed along with the SIP message which caused the dialog to be deleted. The final argument is for future use. It is always set to NULL.


If these callback routines are registered, the stack invokes sip_ulp_trans_state_cb when a transaction changes states and sip_ulp_dlg_state_cb when a dialog changes states.

Connection Manager Interface

The connection manager interfaces must be registered by the application to provide I/O related functionality to the stack. These interfaces act on a connection object that is defined by the application. The application registers the interfaces for the stack to work with the connection object. The connection object is application defined, but the stack requires that the first member of the connection object is a void *, used by the stack to store connection object specific information which is private to the stack.

The connection manager structure is supplied by:

typedef struct sip_io_pointers_s {
        int     (*sip_conn_send)(const sip_conn_object_t, char *, int);
        void    (*sip_hold_conn_object)(sip_conn_object_t);
        void    (*sip_rel_conn_object)(sip_conn_object_t);
        boolean_t       (*sip_conn_is_stream)(sip_conn_object_t);
        boolean_t       (*sip_conn_is_reliable)(sip_conn_object_t);
        int     (*sip_conn_remote_address)(sip_conn_object_t, struct sockaddr *,
                    socklen_t *);
        int     (*sip_conn_local_address)(sip_conn_object_t, struct sockaddr *,
                    socklen_t *);
        int     (*sip_conn_transport)(sip_conn_object_t);
        int     (*sip_conn_timer1)(sip_conn_object_t);
        int     (*sip_conn_timer2)(sip_conn_object_t);
        int     (*sip_conn_timer4)(sip_conn_object_t);
        int     (*sip_conn_timerd)(sip_conn_object_t);

This function is invoked by the stack after processing an outbound SIP message. This function is responsible for sending the SIP message to the peer. A return of 0 indicates success. The SIP message is passed to the function as a string, along with the length information and the associated connection object.


The application provides a mechanism for the stack to indicate that a connection object is in use by the stack and must not be freed. The stack uses sip_hold_conn_object to indicate that the connection object is in use and sip_rel_conn_object to indicate that it has been released. The connection object is passed as the argument to these functions. The stack expects that the application will not free the connection object if it is in use by the stack.


The stack uses this to determine whether the connection object, passed as the argument, is byte-stream oriented. Byte-stream protocols include TCP while message-based protocols include SCTP and UDP.


The stack uses this to determine whether the connection object, passed as the argument, is reliable. Reliable protocols include TCP and SCTP. Unreliable protocols include UDP.


These two interfaces are used by the stack to obtain endpoint information for a connection object. The sip_conn_local_address provides the local address/port information. The sip_conn_remote_address provides the address/port information of the peer. The caller allocates the buffer and passes its associated length along with it. On return, the length is updated to reflect the actual length.


The stack uses this to determine the transport used by the connection object, passed as the argument. The transport could be TCP, UDP, SCTP.


These four interfaces may be registered by an application to provide connection object specific timer information. If these are not registered the stack uses default values.

The interfaces provide the timer values for Timer 1 (RTT estimate - default 500 msec), Timer 2 (maximum retransmit interval for non-INVITE request and INVITE response - default 4 secs), Timer 4 (maximum duration a message will remain in the network - default 5 secs) and Timer D (wait time for response retransmit interval - default 32 secs).

Custom SIP headers

In addition to the SIP headers supported by the stack, an application can optionally provide a table of custom headers and associated parsing functions. The table is an array with an entry for each header. If the table includes headers supported by the stack, parsing functions or other application-specific table entries take precedence over libsip supported headers. The header table structure is supplied by:

typedef struct header_function_table {
     char      *header_name;
     char      *header_short_name;
     int       (*header_parse_func)
               (struct sip_header *,
                     struct sip_parsed_header **);
     boolean_t (*header_check_compliance)
               (struct sip_parsed_header *);
     boolean_t (*header_is_equal)
               (struct sip_parsed_header *,
                     struct sip_parsed_header *);
     void      (*header_free)
               (struct sip_parsed_header *);


The full name of the header. The application must ensure that he name does not conflict with existing headers. If it does, the one registered by the application takes precedence.


Compact name, if any, for the header.


The parsing function for the header. The parser will set the second argument to the resulting parsed structure. A return value of 0 indicates success.


The function that frees the parsed header


An application can optionally provide this function that will check if the header is compliant or not. The compliance for a custom header will be defined by the application.


An application can optionally provide this function to determine whether two input headers are equivalent. The equivalence criteria is defined by the application.

Return Values

On success sip_stack_init() returns 0. Otherwise, the function returns the error value.

The value of errno is not changed by these calls in the event of an error.


On failure, the sip_stack_init() function returns the following error value:


If the stack version is incorrect, or if any of the mandatory functions is missing.


See attributes(5) for descriptions of the following attributes:

Interface Stability

See Also