SIP经典例子
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编译方法：
    gcc xxx.c -o xxx -leXosip2
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/*******************UAS**********************************************

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#include <eXosip2/eXosip.h>
#include <osip2/osip_mt.h>
#include <stdio.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <sys/types.h>
int
main (int argc, char *argv[])
{
eXosip_event_t *je = NULL;
osip_message_t *ack = NULL;
osip_message_t *invite = NULL;
osip_message_t *answer = NULL;
sdp_message_t *remote_sdp = NULL;
int call_id, dialog_id;
int i,j;
int id;
char *sour_call = "sip:133@192.168.0.133";
char *dest_call = "sip:140@192.168.0.140:5060";
char command;
char tmp[4096];
char localip[128];
int pos = 0;
//初始化sip
i = eXosip_init ();
if (i != 0)
    {
      printf ("Can't initialize eXosip!\n");
      return -1;
    }
else
    {
      printf ("eXosip_init successfully!\n");
    }
i = eXosip_listen_addr (IPPROTO_UDP, NULL, 5060, AF_INET, 0);
if (i != 0)
    {
      eXosip_quit ();
      fprintf (stderr, "eXosip_listen_addr error!\nCouldn't initialize transport layer!\n");
    }
for(;;)
    {
      //侦听是否有消息到来
      je = eXosip_event_wait (0,50);
      //协议栈带有此语句,具体作用未知
      eXosip_lock ();
      eXosip_default_action (je);
      eXosip_automatic_refresh ();
      eXosip_unlock ();
      if (je == NULL)//没有接收到消息
    continue;
      // printf ("the cid is %s, did is %s\n", je->did, je->cid);
      switch (je->type)
    {
    case EXOSIP_MESSAGE_NEW://新的消息到来
      printf (" EXOSIP_MESSAGE_NEW!\n");
      if (MSG_IS_MESSAGE (je->request))//如果接受到的消息类型是MESSAGE
        {
      {
        osip_body_t *body;
        osip_message_get_body (je->request, 0, &body);
        printf ("I get the msg is: %s\n", body->body);
        //printf ("the cid is %s, did is %s\n", je->did, je->cid);
      }
      //按照规则，需要回复200 OK信息
      eXosip_message_build_answer (je->tid, 200,&answer);
      eXosip_message_send_answer (je->tid, 200,answer);
        }
      break;
    case EXOSIP_CALL_INVITE:
      //得到接收到消息的具体信息
            printf ("Received a INVITE msg from %s:%s, UserName is %s, password is %s\n",je->request->req_uri->host,
          je->request->req_uri->port, je->request->req_uri->username, je->request->req_uri->password);
      //得到消息体,认为该消息就是SDP格式.
      remote_sdp = eXosip_get_remote_sdp (je->did);
      call_id = je->cid;
      dialog_id = je->did;
     
      eXosip_lock ();
      eXosip_call_send_answer (je->tid, 180, NULL);
      i = eXosip_call_build_answer (je->tid, 200, &answer);
      if (i != 0)
        {
          printf ("This request msg is invalid!Cann't response!\n");
          eXosip_call_send_answer (je->tid, 400, NULL);
        }
      else
        {
          snprintf (tmp, 4096,
              "v=0\r\n"
              "o=anonymous 0 0 IN IP4 0.0.0.0\r\n"
              "t=1 10\r\n"
switch语句具体例子
              "a=username:rainfish\r\n"
              "a=password:123\r\n");
         
          //设置回复的SDP消息体,下一步计划分析消息体
          //没有分析消息体，直接回复原来的消息，这一块做的不好。
          osip_message_set_body (answer, tmp, strlen(tmp));
          osip_message_set_content_type (answer, "application/sdp");
         
          eXosip_call_send_answer (je->tid, 200, answer);
          printf ("send 200 over!\n");
        }
      eXosip_unlock ();
     
      //显示出在sdp消息体中的 attribute 的内容,里面计划存放我们的信息
      printf ("the INFO is :\n");
      while (!osip_list_eol (remote_sdp->a_attributes, pos))
        {
          sdp_attribute_t *at;
         
          at = (sdp_attribute_t *) osip_list_get (remote_sdp->a_attributes, pos);
          printf ("%s : %s\n", at->a_att_field, at->a_att_value);//这里解释了为什么在SDP消息体中属性a里面存放必须是两列
         
          pos ++;
        }
      break;
    case EXOSIP_CALL_ACK:
      printf ("ACK recieved!\n");
      // printf ("the cid is %s, did is %s\n", je->did, je->cid);
      break;
    case EXOSIP_CALL_CLOSED:
      printf ("the remote hold the session!\n");
      // eXosip_call_build_ack(dialog_id, &ack);
      //eXosip_call_send_ack(dialog_id, ack);
      i = eXosip_call_build_answer (je->tid, 200, &answer);
      if (i != 0)
        {
          printf ("This request msg is invalid!Cann't response!\n");
          eXosip_call_send_answer (je->tid, 400, NULL);
       
          }
      else
        {
          eXosip_call_send_answer (je->tid, 200, answer);
          printf ("bye send 200 over!\n");
        }       
      break;
    case EXOSIP_CALL_MESSAGE_NEW://至于该类型和EXOSIP_MESSAGE_NEW的区别，源代码这么解释的
    /*
    /* request related events within calls (except INVITE) */
          EXOSIP_CALL_MESSAGE_NEW,            /**< announce new incoming request. */
    /* response received for request outside calls */
            EXOSIP_MESSAGE_NEW,            /**< announce new incoming request. */
            我也不是很明白，理解是： EXOSIP_CALL_MESSAGE_NEW是一个呼叫中的新的消息到来，比如ring trying都算，所以在接受到后必须判断
            该消息类型，EXOSIP_MESSAGE_NEW而是表示不是呼叫内的消息到来。
            该解释有不妥地方，仅供参考。
    */
      printf(" EXOSIP_CALL_MESSAGE_NEW\n");
      if (MSG_IS_INFO(je->request)//如果传输的是INFO方法
      {
                  eXosip_lock ();
              i = eXosip_call_build_answer (je->tid, 200, &answer);
              if (i == 0)
                {
                  eXosip_call_send_answer (je->tid, 200, answer);
                }
              eXosip_unlock ();
          {
        osip_body_t *body;
        osip_message_get_body (je->request, 0, &body);
        printf ("the body is %s\n", body->body);
          }
      }
      break;
    default:
      printf ("Could not parse the msg!\n");
    }
    }
}
/*******************UAC*****************************************************
本文可以任意转载，但必须保留出处
作者：rainfish
网址：blog.csdn/bat603/
测试环境：eXosip3.0.1/redhat AS 4
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