/** * @ingroup libeXosip2 The eXtented eXosip stack * @defgroup howto2_registration How-To send or update registrations.eXosip2 offers a flexible API to help you to register one or several identities.<H2>Initiate a registration</H2>To start a registration, you need to build a default REGISTER requestbby providing several mandatory headers<PRE>  osip_message_t *reg = NULL;  int id;  int i;  eXosip_lock ();  id = eXosip_register_build_initial_register (identity, registrar, NULL,                                               1800, &reg);  if (id < 0)    {      eXosip_unlock ();      return -1;    }  osip_message_set_supported (reg, "100rel");  osip_message_set_supported(reg, "path");  i = eXosip_register_send_register (id, reg);  eXosip_unlock ();  return i;</PRE>The returned element of eXosip_register_build_initial_register is theregistration identifier that you can use to update your registration.In future events about this registration, you'll see that registrationidentifier when applicable.<H2>Update a registration</H2>You just need to reuse the registration identifier:<PRE>  int i;  eXosip_lock ();  i = eXosip_register_build_register (id, 1800, &reg);  if (i < 0)    {      eXosip_unlock ();      return -1;    }  eXosip_register_send_register (id, reg);  eXosip_unlock ();</PRE><b>Note</b>: The above code also shows that the stack is sometimes able tobuild and send a default SIP messages with only one API call<H2>Closing the registration</H2>A softphone should delete its registration on the SIP server when terminating.To do so, you have to send a REGISTER request with the expires header set tovalue "0".The same code as for updating a registration is used with 0 instead of 1800for the expiration delay.*/