The FD_WRITE network event is handled slightly differently. An FD_WRITE network event is recorded when a socket is first connected with connect/WSAConnect or accepted with accept/WSAAccept, and then after a send fails with WSAEWOULDBLOCK and buffer space becomes available. Therefore, an application can assume that sends are possible starting from the first FD_WRITE network event setting and lasting until a send returns WSAEWOULDBLOCK. After such a failure the application will find out that sends are again possible when an FD_WRITE network event is recorded and the associated event object is set
①client 通过connect（WSAConnect）首次和server建立连接时，在client端会触发FD_WRITE事件
②server通过accept（WSAAccept）接受client连接请求时，在server端会触发FD_WRITE事件
③send（WSASend）/sendto（WSASendTo）发送失败返回WSAEWOULDBLOCK，并且当缓冲区有可用空间时，则会触发FD_WRITE事件
①②其实是同一种情况，在第一次建立连接时，C/S端都会触发一个FD_WRITE事件。
主要是③这种情况：send出去的数据其实都先存在winsock的发送缓冲区中，然后才发送出去，如果缓冲区满了，那么再调用send（WSASend,sendto,WSASendTo）的话，就会返回一个 WSAEWOULDBLOCK的错误码，接下来随着发送缓冲区中的数据被发送出去，缓冲区中出现可用空间时，一个 FD_WRITE 事件才会被触发，这里比较容易混淆的是 FD_WRITE 触发的前提是 缓冲区要先被充满然后随着数据的发送又出现可用空间，而不是缓冲区中有可用空间