int CUDT::send(UDTSOCKET u, const char* buf, int len, int)
{
   try
   {
      CUDT* udt = s_UDTUnited.lookup(u);//這裡是通過套介面ID直接找到關聯的傳輸控制塊，而之前介紹的connect函式中，是通過套介面ID找到關聯的套介面,注意區分
      return udt->send(buf, len);//可見傳進來的第三個引數沒有用
   }
   catch (CUDTException e)
   {
      s_UDTUnited.setError(new CUDTException(e));
      return ERROR;
   }
   catch (bad_alloc&)
   {
      s_UDTUnited.setError(new CUDTException(3, 2, 0));
      return ERROR;
   }
   catch (...)
   {
      s_UDTUnited.setError(new CUDTException(-1, 0, 0));
      return ERROR;
   }
}
 

int CUDT::send(const char* data, int len)
{
   if (UDT_DGRAM == m_iSockType)//如果是UDT_DGRAM型別,則丟擲錯誤
      throw CUDTException(5, 10, 0);

   // throw an exception if not connected
   if (m_bBroken || m_bClosing)
      throw CUDTException(2, 1, 0);
   else if (!m_bConnected)
      throw CUDTException(2, 2, 0);

   if (len <= 0)
      return 0;

   CGuard sendguard(m_SendLock);

   if (m_pSndBuffer->getCurrBufSize() == 0)//若傳送快取中沒有資料
   {
      // delay the EXP timer to avoid mis-fired timeout
      uint64_t currtime;
      CTimer::rdtsc(currtime);
      m_ullLastRspTime = currtime;
   }

   if (m_iSndBufSize <= m_pSndBuffer->getCurrBufSize())//傳送快取滿了
   {//m_iSndBufSize=8192個包
      if (!m_bSynSending)
         throw CUDTException(6, 1, 0);
      else
      {
         // wait here during a blocking sending
         #ifndef WIN32
            pthread_mutex_lock(&m_SendBlockLock);
            if (m_iSndTimeOut < 0) //初始值為-1
            { 
               while (!m_bBroken && m_bConnected && !m_bClosing && (m_iSndBufSize <= m_pSndBuffer->getCurrBufSize()) && m_bPeerHealth)
                  pthread_cond_wait(&m_SendBlockCond, &m_SendBlockLock);
            }
            else
            {
               uint64_t exptime = CTimer::getTime() + m_iSndTimeOut * 1000ULL;
               timespec locktime; 
    
               locktime.tv_sec = exptime / 1000000;
               locktime.tv_nsec = (exptime % 1000000) * 1000;

               while (!m_bBroken && m_bConnected && !m_bClosing && (m_iSndBufSize <= m_pSndBuffer->getCurrBufSize()) && m_bPeerHealth && (CTimer::getTime() < exptime))
                  pthread_cond_timedwait(&m_SendBlockCond, &m_SendBlockLock, &locktime);
            }
            pthread_mutex_unlock(&m_SendBlockLock);
    ......

         // check the connection status
         if (m_bBroken || m_bClosing)
            throw CUDTException(2, 1, 0);
         else if (!m_bConnected)
            throw CUDTException(2, 2, 0);
         else if (!m_bPeerHealth)
         {
            m_bPeerHealth = true;
            throw CUDTException(7);
         }
      }
   }

   if (m_iSndBufSize <= m_pSndBuffer->getCurrBufSize())
   {
      if (m_iSndTimeOut >= 0)
         throw CUDTException(6, 3, 0); 

      return 0;
   }

   int size = (m_iSndBufSize - m_pSndBuffer->getCurrBufSize()) * m_iPayloadSize;//1500-28-16=1456
   if (size > len)
      size = len;

   // record total time used for sending
   if (0 == m_pSndBuffer->getCurrBufSize())
      m_llSndDurationCounter = CTimer::getTime();

   // insert the user buffer into the sening list
   m_pSndBuffer->addBuffer(data, size);

   // insert this socket to snd list if it is not on the list yet
   m_pSndQueue->m_pSndUList->update(this, false);//插入到m_pHeap中，signal Tick_Cond

   if (m_iSndBufSize <= m_pSndBuffer->getCurrBufSize())
   {
      // write is not available any more
      s_UDTUnited.m_EPoll.update_events(m_SocketID, m_sPollID, UDT_EPOLL_OUT, false);
   }

   return size;
}