How can I tell when a socket is closed on the other end?

  From Andrew Gierth (andrew@erlenstar.demon.co.uk):

  AFAIK:

  If the peer calls close() or exits, without having messed with
  SO_LINGER, then our calls to read() should return 0. It is less clear
  what happens to write() calls in this case; I would expect EPIPE, not
  on the next call, but the one after.

  If the peer reboots, or sets l_onoff = 1, l_linger = 0 and then
  closes, then we should get ECONNRESET (eventually) from read(), or
  EPIPE from write().

  I should also point out that when write() returns EPIPE, it also
  raises the SIGPIPE signal - you never see the EPIPE error unless you
  handle or ignore the signal.

  If the peer remains unreachable, we should get some other error.

  I don't think that write() can legitimately return 0.  read() should
  return 0 on receipt of a FIN from the peer, and on all following
  calls.

  So yes, you must expect read() to return 0.

  As an example, suppose you are receiving a file down a TCP link; you
  might handle the return from read() like this:

  rc = read(sock,buf,sizeof(buf));
  if (rc > 0)
  {
      write(file,buf,rc);
      /* error checking on file omitted */
  }
  else if (rc == 0)
  {
      close(file);
      close(sock);
      /* file received successfully */
  }
  else /* rc < 0 */
  {
      /* close file and delete it, since data is not complete
         report error, or whatever */
  }



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