用C语言与Winsock完成一对多Socket客户端和用户端程序，消息发送与接收不会阻塞，同时支持多个用户端

实现一对多通信需要使用TCP/IP协议，Winsock库提供了相应的API函数来实现网络通信。以下是一个简单的基于Winsock的TCP/IP客户端和服务器端程序的示例代码，可以实现多个客户端连接到服务器并发送消息，同时服务器可以向所有客户端广播消息。

服务器端代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <winsock2.h>

#pragma comment(lib,"ws2_32.lib") //加载 ws2_32.dll

#define PORT 8888   //服务器监听端口号

int main()
{
    WSADATA wsaData;
    SOCKET serverSocket, clientSocket;
    SOCKADDR_IN serverAddr, clientAddr;
    int addrLen = sizeof(SOCKADDR_IN);
    char recvBuf[1024];
    int recvLen;
    char sendBuf[1024];
    int sendLen;
    int i;
    int clientCount = 0;
    SOCKET clientSockets[10];

    //初始化Winsock库
    if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0)
    {
        printf("WSAStartup failed!\n");
        return 1;
    }

    //创建套接字
    serverSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (serverSocket == INVALID_SOCKET)
    {
        printf("socket failed!\n");
        WSACleanup();
        return 1;
    }

    //绑定套接字到本地地址和端口
    memset(&serverAddr, 0, sizeof(serverAddr));
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_addr.s_addr = htonl(INADDR_ANY);
    serverAddr.sin_port = htons(PORT);
    if (bind(serverSocket, (SOCKADDR*)&serverAddr, sizeof(serverAddr)) == SOCKET_ERROR)
    {
        printf("bind failed!\n");
        closesocket(serverSocket);
        WSACleanup();
        return 1;
    }

    //监听连接请求
    if (listen(serverSocket, SOMAXCONN) == SOCKET_ERROR)
    {
        printf("listen failed!\n");
        closesocket(serverSocket);
        WSACleanup();
        return 1;
    }

    printf("Server started, waiting for clients...\n");

    while (1)
    {
        //接受连接请求
        clientSocket = accept(serverSocket, (SOCKADDR*)&clientAddr, &addrLen);
        if (clientSocket == INVALID_SOCKET)
        {
            printf("accept failed!\n");
            continue;
        }

        //将客户端套接字加入数组
        clientSockets[clientCount] = clientSocket;
        clientCount++;

        //打印客户端地址和端口号
        printf("Client %s:%d connected.\n", inet_ntoa(clientAddr.sin_addr), ntohs(clientAddr.sin_port));

        //向所有客户端广播有新客户端连接
        sprintf(sendBuf, "Client %s:%d connected.", inet_ntoa(clientAddr.sin_addr), ntohs(clientAddr.sin_port));
        for (i = 0; i < clientCount - 1; i++)
        {
            sendLen = send(clientSockets[i], sendBuf, strlen(sendBuf) + 1, 0);
        }
    }

    //关闭所有客户端套接字
    for (i = 0; i < clientCount; i++)
    {
        closesocket(clientSockets[i]);
    }

    //关闭服务器套接字
    closesocket(serverSocket);

    //清理Winsock库
    WSACleanup();

    return 0;
}

客户端代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <winsock2.h>

#pragma comment(lib,"ws2_32.lib") //加载 ws2_32.dll

#define SERVER_IP "127.0.0.1"   //服务器IP地址
#define PORT 8888   //服务器监听端口号

int main()
{
    WSADATA wsaData;
    SOCKET clientSocket;
    SOCKADDR_IN serverAddr;
    char recvBuf[1024];
    int recvLen;
    char sendBuf[1024];
    int sendLen;

    //初始化Winsock库
    if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0)
    {
        printf("WSAStartup failed!\n");
        return 1;
    }

    //创建套接字
    clientSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (clientSocket == INVALID_SOCKET)
    {
        printf("socket failed!\n");
        WSACleanup();
        return 1;
    }

    //连接到服务器
    memset(&serverAddr, 0, sizeof(serverAddr));
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_addr.s_addr = inet_addr(SERVER_IP);
    serverAddr.sin_port = htons(PORT);
    if (connect(clientSocket, (SOCKADDR*)&serverAddr, sizeof(serverAddr)) == SOCKET_ERROR)
    {
        printf("connect failed!\n");
        closesocket(clientSocket);
        WSACleanup();
        return 1;
    }

    //向服务器发送消息
    sprintf(sendBuf, "Hello from client %d.", GetCurrentProcessId());
    sendLen = send(clientSocket, sendBuf, strlen(sendBuf) + 1, 0);

    //接收服务器消息
    recvLen = recv(clientSocket, recvBuf, sizeof(recvBuf), 0);
    printf("Received from server: %s\n", recvBuf);

    //关闭套接字
    closesocket(clientSocket);

    //清理Winsock库
    WSACleanup();

    return 0;
}

以上代码只是基础示例，还需要进行错误处理和优化。同时如果要支持多个客户端，可以使用线程或者IOCP模型来处理多个客户端的连接和消息传输。