我需要一份用C语言编写的MODBUS TCP服务器代码示例，它应该基于select或poll模型，同时能支持不超过8个并发连接。

在C语言中编写一个MODBUS TCP服务器，通常会使用socket编程技术。以下是一个简单的MODBUS TCP服务器示例，基于select模型，支持最多8个并发连接。请注意，这只是一个基本框架，实际应用可能需要更复杂的错误处理和协议解析：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <errno.h>
#include <sys/select.h>

#define MAX_CONNECTIONS 8

struct client {
    int socket;
    struct sockaddr_in address;
};

void handle_client(int client_socket) {
    // ...在这里添加MODBUS协议处理代码...
}

int main() {
    int server_fd, new_sock, client_index = 0;
    struct sockaddr_in server_addr, client_addr;
    socklen_t addr_len = sizeof(struct sockaddr_in);
    FD_SET(max_connections, &readfds);

    // 创建监听套接字
    server_fd = socket(AF_INET, SOCK_STREAM, 0);
    if (server_fd == -1) {
        perror("Creating server socket");
        exit(EXIT_FAILURE);
    }

    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    server_addr.sin_port = htons(502); // MODBUS TCP默认端口

    if (bind(server_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
        perror("Binding to port");
        close(server_fd);
        exit(EXIT_FAILURE);
    }

    listen(server_fd, 5); // 等待连接的最大数

    while (1) {
        // 阻塞直到有活动发生
        FD_ZERO(&readfds);
        FD_SET(server_fd, &readfds);
        for (client_index = 0; client_index < MAX_CONNECTIONS; ++client_index) {
            if (FD_ISSET(client_list[client_index].socket, &readfds)) {
                FD_SET(client_list[client_index].socket, &readfds);
            }
        }

        int num_ready = select(MAX_CONNECTIONS + 1, &readfds, NULL, NULL, NULL);
        if (num_ready <= 0) {
            perror("Error in select");
            continue;
        }

        // 处理新连接或已连接客户端的数据
        if (FD_ISSET(server_fd, &readfds)) {
            new_sock = accept(server_fd, (struct sockaddr *)&client_addr, &addr_len);
            if (new_sock == -1) {
                perror("Accepting connection");
                continue;
            }
            client_list[client_index++] = (struct client){new_sock, client_addr};
        } else {
            for (client_index = 0; client_index < MAX_CONNECTIONS; ++client_index) {
                if (FD_ISSET(client_list[client_index].socket, &readfds)) {
                    handle_client(client_list[client_index].socket);
                }
            }
        }
    }

    return 0;
}