winsock服务器同时发送和接收

Question

我是 WinSock 的新手，我正在尝试一些东西。我有相互通信的客户端和服务器程序。如果客户端键入某些内容，服务器只会将其回显。我希望它们同时接收和发送，所以我将客户端设置为非阻塞模式，它工作正常。但是当我尝试将服务器置于非阻塞状态时，它崩溃说 recv() == SOCKET_ERROR.

那么问题来了：为什么客户端可以非阻塞工作，而服务端却不行？我该如何解决？

TCP_SERVER:

#include <iostream>
#include <WS2tcpip.h>
#include <Windows.h>

#pragma comment (lib,"ws2_32.lib")

using namespace std;


string receive(SOCKET clientSocket, char* buf)
{
   ZeroMemory(buf, 4096);
   int bytesReceived = recv(clientSocket, buf, 4096, 0);
   string bufStr = buf;
   cout << "bytes received: " << bytesReceived << endl;
   if (bytesReceived == SOCKET_ERROR)
   {
      cerr << "error met recv() in de reciev() functie" << endl;
      exit(EXIT_FAILURE);
   }

   if (bytesReceived == 0)
   {
      cout << "client disconnected" << endl;
      exit(EXIT_FAILURE);
   }

   return bufStr;
}


 void main()
 {
  //initialize winsock
  WSADATA wsData;
  WORD ver = MAKEWORD(2, 2);

  int wsOk = WSAStartup(ver, &wsData);
  if (wsOk != 0) {
      cerr << "can't initialize winsock ABORT";
      return;
  }


  //create socket
  SOCKET listening = socket(AF_INET, SOCK_STREAM, 0);
  if (listening == INVALID_SOCKET) {
     cerr << "cant create socket ABORT" << std::endl;
  }
  //bind IP adress and port to socket
  sockaddr_in hint;
  hint.sin_family = AF_INET;
  hint.sin_port = htons(54000);
  hint.sin_addr.S_un.S_addr = INADDR_ANY; //could also inet_pton


  bind(listening, (sockaddr*)&hint, sizeof(hint));

  //tell winsock the socket is for listening
  listen(listening, SOMAXCONN);

  //wait for connection
  sockaddr_in client;
  int clientSize = sizeof(client);

  SOCKET clientSocket = accept(listening, (sockaddr*)&client, &clientSize);
  if (clientSocket == INVALID_SOCKET) {
     cerr << "somthing went wrong with client socket accept ABORT";
     exit(EXIT_FAILURE);
  }

  char host[NI_MAXHOST];      //client remote name
  char service[NI_MAXSERV];   //service (i.e port) the client is connected on

  ZeroMemory(host, NI_MAXHOST);
  ZeroMemory(service, NI_MAXSERV);

  if (getnameinfo((sockaddr*)&client, sizeof(client), host, NI_MAXHOST, service, NI_MAXSERV, 0) == 0) 
  {
     cout << host << " connected on port " << service << endl;

 }
 else {
    inet_ntop(AF_INET, &client.sin_addr, host, NI_MAXHOST);
    cout << host << " connected on port " << ntohs(client.sin_port) << endl;
  }

//close listening socket
closesocket(listening);


//non blocking socket leads to error
u_long mode = 1;  // 1 to enable non-blocking socket
ioctlsocket(clientSocket, FIONBIO, &mode);
//non blocking socket


//while loop: accept and echo message to client
char buf[4096];
string inputTxt;

while (true)
{
    inputTxt = receive(clientSocket,buf);
    send(clientSocket, buf, inputTxt.size() + 1, 0);

}

  closesocket(clientSocket);
  WSACleanup();

}

TCP_CLIENT:

#include <iostream>
#include <WS2tcpip.h>
#include <Windows.h>

#pragma comment (lib,"ws2_32.lib")

using namespace std;


string receive(SOCKET clientSocket, char* buf)
{
   ZeroMemory(buf, 4096);
   int bytesReceived = recv(clientSocket, buf, 4096, 0);
   string bufStr = buf;
   cout << "bytes received: " << bytesReceived << endl;
   if (bytesReceived == SOCKET_ERROR)
   {
      cerr << "error met recv() in de reciev() functie" << endl;
      exit(EXIT_FAILURE);
   }

   if (bytesReceived == 0)
   {
      cout << "client disconnected" << endl;
      exit(EXIT_FAILURE);
   }

   return bufStr;
}

void main()
{
//initialize winsock
WSADATA wsData;
WORD ver = MAKEWORD(2, 2);

int wsOk = WSAStartup(ver, &wsData);
if (wsOk != 0) {
    cerr << "can't initialize winsock ABORT";
    return;
}


//create socket
SOCKET listening = socket(AF_INET, SOCK_STREAM, 0);
if (listening == INVALID_SOCKET) {
    cerr << "cant create socket ABORT" << std::endl;
}
//bind IP adress and port to socket
sockaddr_in hint;
hint.sin_family = AF_INET;
hint.sin_port = htons(54000);
hint.sin_addr.S_un.S_addr = INADDR_ANY; //could also inet_pton


bind(listening, (sockaddr*)&hint, sizeof(hint));

//tell winsock the socket is for listening
listen(listening, SOMAXCONN);

//wait for connection
sockaddr_in client;
int clientSize = sizeof(client);

SOCKET clientSocket = accept(listening, (sockaddr*)&client, &clientSize);
if (clientSocket == INVALID_SOCKET) {
    cerr << "somthing went wrong with client socket accept ABORT";
    exit(EXIT_FAILURE);
}

char host[NI_MAXHOST];      //client remote name
char service[NI_MAXSERV];   //service (i.e port) the client is connected on

ZeroMemory(host, NI_MAXHOST);
ZeroMemory(service, NI_MAXSERV);

if (getnameinfo((sockaddr*)&client, sizeof(client), host, NI_MAXHOST, service, NI_MAXSERV, 0) == 0) {
    cout << host << " connected on port " << service << endl;

}
else {
    inet_ntop(AF_INET, &client.sin_addr, host, NI_MAXHOST);
    cout << host << " connected on port " << ntohs(client.sin_port) << endl;
}

//close listening socket
closesocket(listening);

/*
//non blocking socket leads to error
u_long mode = 1;  // 1 to enable non-blocking socket
ioctlsocket(clientSocket, FIONBIO, &mode);
//non blocking socket
*/

//while loop: accept and echo message to client
char buf[4096];
string inputTxt;

while (true)
{
    inputTxt = receive(clientSocket,buf);
    send(clientSocket, buf, inputTxt.size() + 1, 0);

}

closesocket(clientSocket);
WSACleanup();

}

Answer 1

您没有处理 send()/recv() 由于 WSAEWOULDBLOCK 错误而失败的情况，这是 NOT 致命的错误。这只是说明此时没有工作要做，请稍后再试。

对于recv()，这意味着没有可从套接字的接收缓冲区中读取的字节。当有字节可供读取时，套接字将处于可读状态，或者对等方已执行 graceful 断开连接。

对于send()，这意味着对等方的接收缓冲区已满，无法接收新字节，直到对等方读取一些字节以清除缓冲区space。任何未发送的字节都必须稍后再次传递给 send()。当新字节可以发送到对端时，套接字将处于 writable 状态，当对端缓冲区已满时，不会处于 writable 状态.

当您的服务器接受一个客户端并尝试从它 receive() 时，recv() 将继续失败 WSAEWOULDBLOCK 直到客户端实际发送一些东西。

因此，您需要正确处理 WSAEWOULDBLOCK 并根据需要重试。或者更好，使用select()（或WSAAsyncSelect()，或WSAEventSelect()，或Overlapped I/O）来检测套接字的实际状态以了解何时 send()/recv() 可以安全调用而不会导致 WSAEWOULDBLOCK 错误。

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