C++ Winsock服务器只与单个客户端通信，而应该与每个客户端通信。

我正在用GUI编写一个聊天程序。我想写一个可以接受许多客户机的服务器。每个客户端都可以成功连接。但在发送和接收数据方面存在一个奇怪的问题。我使用select和一个线程来同时处理多个套接字。如果客户机向服务器发送一些数据，它将接收数据并将其发送回该客户机。客户机是在没有预测的情况下编写的。但是服务器不会将它进一步发送到其他客户端，就像每个客户端都有自己与服务器的私人对话一样。这是我的密码：

// this is rewritten from the Beej's tutorial with a little and insignificant changes
/* in the thread */
fd_set mainfd;
fd_set readfd;
// sin-size, newfd, maxfd - int
while(TRUE)
{
    readfd = mainfd;
    if(select(maxfd+1, &readfd, NULL, NULL, NULL) == -1)
    {
        MessageBoxA(NULL, "Error while trying to accept incoming connections (select)", "Error", 16);
        itoa(GetLastError(), buf, 10);
        MessageBoxA(NULL, buf, buf, 0);
        break;
    }
    for(int i = 0; i <= maxfd; i++)
    {
        char* psr;
        char srMsg[256];
        if(FD_ISSET(i, &readfd))
        {
            if(i == mainSocket)
            {
                sin_size = sizeof(their_addr);
                newfd = accept(mainSocket, (struct sockaddr*)&their_addr, &sin_size);
                if(newfd == SOCKET_ERROR)
                {
                    AddTextToEdit(hStaticChat, "* Error: couldn't accept incoming connection.", TRUE);
                }
                else
                {
                    FD_SET(newfd, &mainfd);
                    if(newfd > maxfd)
                    {
                        maxfd = newfd;
                    }

                }
            }
            else
            {
                len = recv(i, srMsg, 256, 0);
                if(len == 0 || len == SOCKET_ERROR)
                {
                    AddTextToEdit(hStaticChat, "* Client has disconnected", TRUE);
                    close(i);
                    FD_CLR(i, &mainfd);
                }
                else
                {
                        AddTextToEdit(hStaticChat, srMsg, TRUE);
                        for(int j = 0; j <= maxfd; j++)
                        {
                          if(FD_ISSET(j, &readfd))
                          {
                                  send(j, srMsg, len, 0);
                          }
                        }

                }
             }
        }
     }
}

---

您只将数据发送给fd位于readfd中的客户机，也就是说，只发送给刚刚与您通信的客户机。尝试测试FD_ISSETj，改为mainfd。

此代码在WinSock下无效。Windows不像其他平台那样使用整数文件描述符处理套接字。套接字使用实际的内核对象来表示，所以不能将循环计数器用作套接字句柄等。还有API差异closesocket代替close，maxfd被select忽略，FD_XXX expect套接字句柄代替int，等等

在Windows上，您需要使用类似以下内容：

fd_set mainfd;
SOCKET newfd;
int sin_size;
...

while(TRUE)
{
    fd_set readfd = mainfd;
    if (select(0, &readfd, NULL, NULL, NULL) == SOCKET_ERROR)
    {
        itoa(WSAGetLastError(), buf, 10);
        MessageBoxA(NULL, "Error while trying to accept incoming connections (select)", "Error", 16);
        MessageBoxA(NULL, buf, buf, 0);
        break;
    }

    for(int i = 0; i < readfd.fd_count; i++)
    {
        if (readfd.fd_array[i] == mainSocket)
        {
            sin_size = sizeof(their_addr);
            newfd = accept(mainSocket, (struct sockaddr*)&their_addr, &sin_size);
            if (newfd == INVALID_SOCKET)
            {
                AddTextToEdit(hStaticChat, "* Error: couldn't accept incoming connection.", TRUE);
            }
            else
            {
                // Note that fd_set can only hold FD_SETSIZE (64) sockets as a time!
                FD_SET(newfd, &mainfd);
            }
        }
        else
        {
            char srMsg[257];

            len = recv(readfd.fd_array[i], srMsg, 256, 0);
            if (len < 1)
            {
                if (len == 0)
                    AddTextToEdit(hStaticChat, "* Client has disconnected", TRUE);
                else
                    AddTextToEdit(hStaticChat, "* Error: couldn't read from a client connection.", TRUE);

                closesocket(readfd.fd_array[i]);
                FD_CLR(readfd.fd_array[i], &mainfd);
            }
            else
            {
                srMsg[len] = 0;
                AddTextToEdit(hStaticChat, srMsg, TRUE);

                for (int j = 0; j < mainfd.fd_count; j++)
                {
                    if (mainfd.fd_array[i] != mainSocket)
                        send(mainfd.fd_array[j], srMsg, len, 0);
                }
            }
        }
     }
}