当 TCP keep-alive 断开连接时，我会得到什么套接字错误? [英] What socket error do I get when TCP keep-alive breaks the connection?

问题描述

我有一组保持活动状态的 TCP 套接字(间隔 1 分钟)，由 控制select(2) 循环(选择读取).

I have a set of TCP sockets with keep-alive (interval 1 min), controlled by a select(2) loop (selecting for read).

• 如果集合中的一个套接字发生了保持活动超时，select(2) 是否会返回错误?
• read(2) 会返回哪个错误?

• Will select(2) return an error if keep-alive timeout has happened for one of the sockets in the set?
• Which error will read(2) return?

推荐答案

• select() 如果它选择的套接字之一发出错误信号，则它本身不会返回错误.[实际上，API 无法以这种方式指示每个套接字的错误，因为在一次 select() 调用期间，两个不同的套接字可能会分别获得一个未决错误.select() 会返回哪一个?]
• 在 select() 循环的每次迭代之后，您改为使用 FD_ISSET 宏在每个标记为可读的套接字上尝试 read().
• 每当套接字设置了未决错误时，它的读取事件(和写入事件)都会发出信号，并且 select() 返回，允许您获取由于保持而导致的超时错误-马上活了.请注意，选择标记套接字进行读取并不表示有数据要读取，仅表示读取尝试不会阻塞.如果套接字有待检索的未决错误，则读取不会阻塞.read(2) 和 write(2) 在尝试处理任何数据之前首先检索套接字上的任何未决错误.
  • select() itself does not return an error if an error is signalled for one of the sockets it is selecting for. [Indeed, the API can't indicate per-socket errors this way, because two different sockets could each acquire a pending error during a single call of select(). Which one would select() return?]
  • After each iteration of the select() loop, you instead use the FD_ISSET macro to attempt a read() on each socket marked readable.
  • Any time a socket has a pending error set, its read event (and write event) are signalled, and select() returns, allowing you to pick up timed-out errors due to keep-alive immediately. Note that select marking a socket for read does not indicate that there is data to read, only that an attempt to read will not block. If the socket has a pending error to retrieve, reading will not block. Both read(2) and write(2) first retrieve any pending error on the socket before even attempting to handle any data.

    当使用 O_NONBLOCK 清除对输入函数的调用不会阻塞时，无论该函数是否成功传输数据，都应认为描述符已准备好读取.(该函数可能返回数据、文件结束指示或错误，而不是表明它被阻塞的错误，在这些情况下，描述符应被视为准备好读取.)[POSIX:select()]

  • 最后返回了什么错误?至关重要的是，这取决于保活失败的方式.如果另一端完全消失，您将获得 ETIMEDOUT.如果发生数据包传送错误，您将改为通过(因此，如果保持活动数据包收到 ICMP 错误回复，例如主机无法访问"，您将获得 EHOSTUNREACH 传送).[有关这些情况的更多详细信息，请参阅 Stevens，Unix 网络编程，第 1 卷".]

  A descriptor shall be considered ready for reading when a call to an input function with O_NONBLOCK clear would not block, whether or not the function would transfer data successfully. (The function might return data, an end-of-file indication, or an error other than one indicating that it is blocked, and in each of these cases the descriptor shall be considered ready for reading.) [POSIX:select()]

  Finally, what error is returned? Crucially, it depends on how the keepalive failed. You'll get ETIMEDOUT if the other end vanishes totally. If a packet delivery error occurs, you'll get that through instead (so if the keep-alive packet gets an ICMP error reply, like "host unreachable", you'll have EHOSTUNREACH delivered). [For more details on these cases, see Stevens, "Unix Network Programming, vol 1".]

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