WSASend()的评述

WSASend()的返回值~

若无错误发生且发送操作立即完成，则WSASend()函数返回0。这时，完成例程（Completion Routine）应该已经被调度，一旦调用线程处于alertable状态时就会调用它。否则，返回SOCKET_ERROR 。通过WSAGetLastError获得详细的错误代码。WSA_IO_PENDING 这个错误码（其实表示没有错误）表示重叠操作已经提交成功（就是异步IO的意思了），稍后会提示完成（这个完成可不一定是发送成功，没准出问题也不一定）。其他的错误代码都代表重叠操作没有正确开始，也不会有完成标志出现。 Error code Meaning WSAEACCES The requested address is a broadcast address, but the appropriate flag was not set. WSAECONNABORTED The virtual circuit was terminated due to a time-out or other failure. WSAECONNRESET For a stream socket, the virtual circuit was reset by the remote side. The application should close the socket as it is no longer useable. For a UDP datagram socket, this error would indicate that a previous send operation resulted in an ICMP Port Unreachable message. WSAEFAULT ThelpBuffers,lpNumberOfBytesSent,lpOverlapped,lpCompletionRoutineparameter is not totally contained in a valid part of the user address space. WSAEINTR A blocking Windows Socket 1.1 call was canceled throughWSACancelBlockingCall. WSAEINPROGRESS A blocking Windows Sockets 1.1 call is in progress, or the service provider is still processing a callback function. WSAEINVAL The socket has not been bound withbindor the socket is not created with the overlapped flag. WSAEMSGSIZE The socket is message oriented, and the message is larger than the maximum supported by the underlying transport. WSAENETDOWN The network subsystem has failed. WSAENETRESET For a stream socket, the connection has been broken due to keep-alive activity detecting a failure while the operation was in progress. For a datagram socket, this error indicates that the time to live has expired. WSAENOBUFS The Windows Sockets provider reports a buffer deadlock. WSAENOTCONN The socket is not connected. WSAENOTSOCK The descriptor is not a socket. WSAEOPNOTSUPP MSG_OOB was specified, but the socket is not stream-style such as type SOCK_STREAM, OOB data is not supported in the communication domain associated with this socket, MSG_PARTIAL is not supported, or the socket is unidirectional and supports only receive operations. WSAESHUTDOWN The socket has been shut down; it is not possible toWSASendon a socket aftershutdownhas been invoked with how set to SD_SEND or SD_BOTH. WSAEWOULDBLOCK Windows NT:Overlapped sockets: There are too many outstanding overlapped I/O requests. Nonoverlapped sockets: The socket is marked as nonblocking and the send operation cannot be completed immediately. WSANOTINITIALISED A successfulWSAStartupcall must occur before using this function. WSA_IO_PENDING An overlapped operation was successfully initiated and completion will be indicated at a later time. WSA_OPERATION_ABORTED The overlapped operation has been canceled due to the closure of the socket, the execution of the SIO_FLUSH command inWSAIoctl, or the thread that initiated the overlapped request exited before the operation completed. For more information, see the Remarks section.

和文件IO差不多，文件IO如果不会，就先文件IO吧。
简单说，就是把消息发出去，然后返回，不管实际消息是不是发出去了。然后在程序某个地方使用多种方案（如WaitForSingleObject,GetOverlappedResult，重叠IO完成例程等）来查看消息发出去了没有。

但SOCKET的重叠IO还要涉及到很多方面的，在实际编程时，不像文件IO，对程序的结构模型必须有非常高的要求，否则即使能编译通过，实际使用会出现很多问题。
譬如发消息时为什么要立刻返回，究竟在什么地方查看发消息的结果，如果消息没发出去又该如何处理等必须根据要实现的程序功能有个明确的认识。
这东西主要还是编程经验，写的程序多了，接触过了各种结构模型，才能知道各个模型的特点，并灵活使用,较难！
建议买本书看看吧！

WSASend覆盖标准的send函数，并在下面两个方面有所增强：
>它可以用于overlapped socket(重叠socket)上以进行重叠发送的操作（简单地理解为就是异步send也可以了）
>它可以一次发送多个缓冲区中的数据来进行集中写入。应该相当于unix上的writev，好处看来是避免Nagle算法。
WSASend用于在一个面向连接的socket（第一个参数s）上发出的数据。It can also be used, however, onconnectionless sockets that have a stipulated default peer address established through theconnectorWSAConnectfunction.
对于overlapped sockets来说 (通过WSASocket函数，用WSA_FLAG_OVERLAPPED标示创建），发送消息时使用的是重叠IO(overlapped I/O), 除非lpOverlapped and lpCompletionRoutine 都是NULL. 这时, 这个socket被视为非重叠的socket. 当所有的缓冲区都被发送完成了，将会执行一个动作来表示操作完成，这个动作可能是调用完成例程或者是引发一个event对象。如果操作没有立即完成, 最终的完成状态通过完成例程或者 WSAGetOverlappedResult得到
对于非重叠的sockets来说, 最后两个参数(lpOverlapped, lpCompletionRoutine) 被忽略，WSASend 和 send具有同样的语意。数据从用户缓冲区拷贝到发送缓冲区中（应该是指系统的socket堆栈）。如果socket是非阻塞的又是同时是面向流的(简单地理解为tcp), 同时发送缓冲区没有足够的大小, WSASend将只发送用户缓冲区中的部分数据。 如果同样缓存大小，而socke是阻塞的socket, WSASend将阻塞直到用户所有的数据被发送成功。
Note socket配置项SO_RCVTIMEO and SO_SNDTIMEO只能被用于阻塞的sockets。
lpBuffers这个参数是一个指针，它指向一个WSABUF结构的数组。这个数组可以是瞬态的（transient）。所谓瞬态的含义如下：如果这个操作时重叠的操作，服务提供者有责任在这个调用返回之前保存WSABUF数组。这允许用户的应用使用一个基于栈的WSABUF数组。就是说你可以定义一个局部变量，当wsasend返回后如果是重叠IO呢，你的函数局部变量已经被销毁了（例如你的函数已经返回了）。系统实际上还没发送数据呢，那你不要担心，系统会保存这个数组的副本。
对于面向消息的socket(UDP?), 不要超过下层协议的最大消息大小，这个值可以通过SO_MAX_MSG_SIZE这个socket配置项得到。如果数据太长无法原子地发送，返回WSAEMSGSIZE, 没有数据发送成功。
Windows Me/98/95:TheWSASendfunction does not support more than 16 buffers.
NoteWSASend的成功完成不代表数据已经发送成功。

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