title: will tcp socket recv return "resource temporarily unavailable" when recv buf already have data
body: <p>I implement TCP socket communication using epoll to monitor all the client events, only one thread process all the client in a for loop. every socket is non-blocking.</p>
  
  <p>now I just suffered a problem,  when client send data more than MTU, means several fragment packet, server always can't read all the data completely.
  like below, I read the head first, get the pdu len from the head, then read the pdu part.</p>
  
  <p>the problem is though I read the head successfully, closely following with the pdu recv(), but it always return EAGAIN several times. so my retry will break.
  Because server needs to process thousands of client event, so I think it is a great performance consumptionn to let the retry always continue which is not tolerable.</p>
  
  <p>I use tcpdump capturing the packets from client, every packet is fragment to max 1448 bytes data, but head is only 5 bytes, why I can read the head successfully, but the following data recv() operation will return EAGAIN? is it possible recv return EAGAIN when data already arrive the recv buffer?  In my opinion, I can read the first 5 bytes, so must have more data to read in the recv buffer.</p>
  
  <p>maybe related with the assemble process in tcp/ip stack.
  code is like below, every pdu recv, need 10 or more retry, maybe success.</p>
  
  <pre><code>...
  #define HDR_LEN    5
  n = epoll(epfd, events, 1000, -1)
  for(i =0; i &lt; n; i++)
  {
      uint8 pHdr[HDR_LEN] = {0};
      uint16 pdulen = 0, offset =0;
      infd = events[i].fd;
      nRead = recv(infd, pHdr, HDR_LEN);   // read the data head first
      pdulen = ntohs(*(uint16 *)(pHdr+2));   // get the pdu len from the head
      uint8 *pbuf = malloc(pdulen+HDR_LEN);
  
      memcpy(pbuf, pHdr, HDR_LEN);            // move the head to buf
      while(offset != pdulen)                 // then read the pdu data
      {
          nRead = recv(infd, pbuf+HDR_LEN+offset, pdulen-offset);
          if (nRead &lt;=0)
          {
              if (nRead == -1 &amp;&amp; errno == EAGAIN) // resource temporarily unavailable 
              {
                  if (retry &lt; 5)
                  {
                      usleep(500);
                      retry++;
                      continue;
                  }
                  else
                       break;  // already try 5 times, should always continue?
              }
              else
                  break;
          }
          else
          {
               offset += nRead;
               retry = 0;
          }
      }
      if (offset == pdulen)
          process(pbuf, pdulen+HDR_LEN); // process the complete data
      ...
  }
  ...
  </code></pre>