怎么触发读事件的？

因为写io是我们主动操作的，那么读是怎么进行操作的呢？这是一个被动的状态

首先我们了解一个结构体。golang中所有的网络事件和文件读写都用fd进行标识(位于internal包下)。

// FD is a file descriptor. The net and os packages use this type as a
// field of a larger type representing a network connection or OS file.
type FD struct {
	// Lock sysfd and serialize access to Read and Write methods.
	fdmu fdMutex

	// System file descriptor. Immutable until Close.
	Sysfd int

	// I/O poller.
	pd pollDesc

	// Writev cache.
	iovecs *[]syscall.Iovec

	// Semaphore signaled when file is closed.
	csema uint32

	// Non-zero if this file has been set to blocking mode.
	isBlocking uint32

	// Whether this is a streaming descriptor, as opposed to a
	// packet-based descriptor like a UDP socket. Immutable.
	IsStream bool

	// Whether a zero byte read indicates EOF. This is false for a
	// message based socket connection.
	ZeroReadIsEOF bool

	// Whether this is a file rather than a network socket.
	isFile bool
}

我们看到，fd中关联的pollDesc,通过pollDesc调用了runtime包内部的实现的各种平台的io事件。

当我们进行read操作时（下面是代码截取）

for {
	n, err := ignoringEINTRIO(syscall.Read, fd.Sysfd, p)
	if err != nil {
		n = 0
		if err == syscall.EAGAIN && fd.pd.pollable() {
			if err = fd.pd.waitRead(fd.isFile); err == nil {
				continue
			}
		}
	}
	err = fd.eofError(n, err)
	return n, err
	}

会阻塞调用waiteRead方法，方法内部主要就是调用的runtime_pollWait。

func poll_runtime_pollWait(pd *pollDesc, mode int) int {
	errcode := netpollcheckerr(pd, int32(mode))
	if errcode != pollNoError {
		return errcode
	}
	// As for now only Solaris, illumos, and AIX use level-triggered IO.
	if GOOS == "solaris" || GOOS == "illumos" || GOOS == "aix" {
		netpollarm(pd, mode)
	}
	for !netpollblock(pd, int32(mode), false) {
		errcode = netpollcheckerr(pd, int32(mode))
		if errcode != pollNoError {
			return errcode
		}
		// Can happen if timeout has fired and unblocked us,
		// but before we had a chance to run, timeout has been reset.
		// Pretend it has not happened and retry.
	}
	return pollNoError
}

这里主要是由netpollblock控制,netpollblock方法我们上面就说过，当io还未就绪的时候，直接释放当前的执行权，否则就是已经课读写的io事件，直接进行读取操作即可。