在Linux环境下使用C举例实现一个局域网络聊天工具的编写，要求完成上线通知、下线通知、发送消息显示、单文件接收、单文件发送、显示在线用户名单等基本功能；利用UDP传送聊天信息，使用TCP传送文件数据。

实现一个局域网聊天工具需要涉及到网络编程、多线程编程、文件传输等知识。下面是一个简单的示例代码，仅供参考。

注：以下代码中使用了第三方库 cJSON，需要先进行安装。

1. 客户端代码 client.c：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <fcntl.h>
#include <cjson/cJSON.h>

#define MAX_MSG_LEN 1024
#define MAX_USERNAME_LEN 32
#define MAX_FILENAME_LEN 64

// 消息类型
enum MessageType {
    LOGIN,
    LOGOUT,
    CHAT,
    FILE_REQUEST,
    FILE_ACCEPT,
    FILE_REJECT,
    FILE_DATA,
    FILE_FINISH
};

struct Message {
    enum MessageType type;
    char username[MAX_USERNAME_LEN];
    char data[MAX_MSG_LEN];
    int data_len;
};

struct FileRequest {
    char filename[MAX_FILENAME_LEN];
    int file_size;
};

// 服务器地址和端口号
char *server_ip = "127.0.0.1";
int server_port = 8888;

// 用户名
char username[MAX_USERNAME_LEN];

// UDP套接字和TCP套接字
int udp_sockfd;
int tcp_sockfd;

// 服务器地址
struct sockaddr_in server_addr;

// 线程处理函数：接收聊天消息
void *recv_chat_msg(void *arg) {
    while (1) {
        char buf[MAX_MSG_LEN];
        struct sockaddr_in peer_addr;
        socklen_t peer_addr_len = sizeof(peer_addr);
        int len = recvfrom(udp_sockfd, buf, MAX_MSG_LEN - 1, 0, (struct sockaddr *)&peer_addr, &peer_addr_len);
        if (len > 0) {
            buf[len] = '\0';
            cJSON *root = cJSON_Parse(buf);
            if (root != NULL) {
                cJSON *type = cJSON_GetObjectItem(root, "type");
                if (type != NULL) {
                    if (strcmp(type->valuestring, "chat") == 0) {
                        cJSON *username = cJSON_GetObjectItem(root, "username");
                        cJSON *data = cJSON_GetObjectItem(root, "data");
                        if (username != NULL && data != NULL) {
                            printf("%s: %s\n", username->valuestring, data->valuestring);
                        }
                    } else if (strcmp(type->valuestring, "file_request") == 0) {
                        cJSON *filename = cJSON_GetObjectItem(root, "filename");
                        cJSON *file_size = cJSON_GetObjectItem(root, "file_size");
                        if (filename != NULL && file_size != NULL) {
                            printf("收到文件传输请求：%s（%d字节），是否接受？（输入Y或N）\n", filename->valuestring, file_size->valueint);
                        }
                    } else if (strcmp(type->valuestring, "file_accept") == 0) {
                        cJSON *filename = cJSON_GetObjectItem(root, "filename");
                        cJSON *port = cJSON_GetObjectItem(root, "port");
                        if (filename != NULL && port != NULL) {
                            struct sockaddr_in file_addr;
                            file_addr.sin_family = AF_INET;
                            file_addr.sin_addr.s_addr = htonl(INADDR_ANY);
                            file_addr.sin_port = htons(port->valueint);
                            if (bind(tcp_sockfd, (struct sockaddr *)&file_addr, sizeof(file_addr)) == 0) {
                                printf("正在发送文件%s...\n", filename->valuestring);

                                // 打开文件
                                int file_fd = open(filename->valuestring, O_RDONLY);
                                if (file_fd < 0) {
                                    perror("打开文件失败");
                                    continue;
                                }

                                // 发送文件数据
                                char file_buf[MAX_MSG_LEN];
                                int read_len;
                                while ((read_len = read(file_fd, file_buf, MAX_MSG_LEN)) > 0) {
                                    int send_len = send(tcp_sockfd, file_buf, read_len, 0);
                                    if (send_len != read_len) {
                                        perror("发送文件数据失败");
                                        close(file_fd);
                                        close(tcp_sockfd);
                                        return NULL;
                                    }
                                }

                                // 关闭文件
                                close(file_fd);

                                // 发送文件传输结束消息
                                cJSON *root = cJSON_CreateObject();
                                cJSON_AddStringToObject(root, "type", "file_finish");
                                cJSON_AddStringToObject(root, "filename", filename->valuestring);
                                cJSON_AddStringToObject(root, "username", username);
                                char *json_str = cJSON_Print(root);
                                int send_len = sendto(udp_sockfd, json_str, strlen(json_str), 0, (struct sockaddr *)&server_addr, sizeof(server_addr));
                                if (send_len != strlen(json_str)) {
                                    perror("发送文件传输结束消息失败");
                                }
                                free(json_str);
                                cJSON_Delete(root);

                                printf("文件%s发送完成\n", filename->valuestring);
                            } else {
                                perror("绑定TCP套接字失败");
                            }
                        }
                    } else if (strcmp(type->valuestring, "file_reject") == 0) {
                        cJSON *filename = cJSON_GetObjectItem(root, "filename");
                        if (filename != NULL) {
                            printf("文件%s传输被拒绝\n", filename->valuestring);
                        }
                    } else if (strcmp(type->valuestring, "file_data") == 0) {
                        cJSON *filename = cJSON_GetObjectItem(root, "filename");
                        cJSON *data = cJSON_GetObjectItem(root, "data");
                        if (filename != NULL && data != NULL) {
                            int write_len = write(tcp_sockfd, data->valuestring, data->valueint);
                            if (write_len != data->valueint) {
                                perror("写文件数据失败");
                            }
                        }
                    } else if (strcmp(type->valuestring, "file_finish") == 0) {
                        cJSON *filename = cJSON_GetObjectItem(root, "filename");
                        cJSON *username = cJSON_GetObjectItem(root, "username");
                        if (filename != NULL && username != NULL) {
                            printf("文件%s接收完成\n", filename->valuestring);
                        }
                    }
                }
                cJSON_Delete(root);
            }
        }
    }
}

// 线程处理函数：接收文件数据
void *recv_file_data(void *arg) {
    int file_fd = *(int *)arg;
    char buf[MAX_MSG_LEN];
    int len;
    while ((len = recv(tcp_sockfd, buf, MAX_MSG_LEN, 0)) > 0) {
        int write_len = write(file_fd, buf, len);
        if (write_len != len) {
            perror("写文件数据失败");
            break;
        }
    }
    close(file_fd);
    close(tcp_sockfd);
}

// 发送聊天消息
void send_chat_msg(char *msg) {
    cJSON *root = cJSON_CreateObject();
    cJSON_AddStringToObject(root, "type", "chat");
    cJSON_AddStringToObject(root, "username", username);
    cJSON_AddStringToObject(root, "data", msg);
    char *json_str = cJSON_Print(root);
    int send_len = sendto(udp_sockfd, json_str, strlen(json_str), 0, (struct sockaddr *)&server_addr, sizeof(server_addr));
    if (send_len != strlen(json_str)) {
        perror("发送聊天消息失败");
    }
    free(json_str);
    cJSON_Delete(root);
}

// 发送文件传输请求
void send_file_request(char *filename, int file_size) {
    cJSON *root = cJSON_CreateObject();
    cJSON_AddStringToObject(root, "type", "file_request");
    cJSON_AddStringToObject(root, "username", username);
    cJSON_AddStringToObject(root, "filename", filename);
    cJSON_AddNumberToObject(root, "file_size", file_size);
    char *json_str = cJSON_Print(root);
    int send_len = sendto(udp_sockfd, json_str, strlen(json_str), 0, (struct sockaddr *)&server_addr, sizeof(server_addr));
    if (send_len != strlen(json_str)) {
        perror("发送文件传输请求失败");
    }
    free(json_str);
    cJSON_Delete(root);
}

// 发送文件传输接受消息
void send_file_accept(char *filename, int port) {
    cJSON *root = cJSON_CreateObject();
    cJSON_AddStringToObject(root, "type", "file_accept");
    cJSON_AddStringToObject(root, "username", username);
    cJSON_AddStringToObject(root, "filename", filename);
    cJSON_AddNumberToObject(root, "port", port);
    char *json_str = cJSON_Print(root);
    int send_len = sendto(udp_sockfd, json_str, strlen(json_str), 0, (struct sockaddr *)&server_addr, sizeof(server_addr));
    if (send_len != strlen(json_str)) {
        perror("发送文件传输接受消息失败");
    }
    free(json_str);
    cJSON_Delete(root);
}

// 发送文件传输拒绝消息
void send_file_reject(char *filename) {
    cJSON *root = cJSON_CreateObject();
    cJSON_AddStringToObject(root, "type", "file_reject");
    cJSON_AddStringToObject(root, "username", username);
    cJSON_AddStringToObject(root, "filename", filename);
    char *json_str = cJSON_Print(root);
    int send_len = sendto(udp_sockfd, json_str, strlen(json_str), 0, (struct sockaddr *)&server_addr, sizeof(server_addr));
    if (send_len != strlen(json_str)) {
        perror("发送文件传输拒绝消息失败");
    }
    free(json_str);
    cJSON_Delete(root);
}

// 发送文件传输数据
void send_file_data(char *filename, char *data, int data_len) {
    cJSON *root = cJSON_CreateObject();
    cJSON_AddStringToObject(root, "type", "file_data");
    cJSON_AddStringToObject(root, "username", username);
    cJSON_AddStringToObject(root, "filename", filename);
    cJSON_AddStringToObject(root, "data", data);
    cJSON_AddNumberToObject(root, "data_len", data_len);
    char *json_str = cJSON_Print(root);
    int send_len = sendto(udp_sockfd, json_str, strlen(json_str), 0, (struct sockaddr *)&server_addr, sizeof(server_addr));
    if (send_len != strlen(json_str)) {
        perror("发送文件传输数据失败");
    }
    free(json_str);
    cJSON_Delete(root);
}

// 发送文件传输结束消息
void send_file_finish(char *filename) {
    cJSON *root = cJSON_CreateObject();
    cJSON_AddStringToObject(root, "type", "file_finish");
    cJSON_AddStringToObject(root, "username", username);
    cJSON_AddStringToObject(root, "filename", filename);
    char *json_str = cJSON_Print(root);
    int send_len = sendto(udp_sockfd, json_str, strlen(json_str), 0, (struct sockaddr *)&server_addr, sizeof(server_addr));
    if (send_len != strlen(json_str)) {
        perror("发送文件传输结束消息失败");
    }
    free(json_str);
    cJSON_Delete(root);
}

int main(int argc, char *argv[]) {
    // 创建UDP套接字
    udp_sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (udp_sockfd < 0) {
        perror("创建UDP套接字失败");
        exit(1);
    }

    // 创建TCP套接字
    tcp_sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (tcp_sockfd < 0) {
        perror("创建TCP套接字失败");
        exit(1);
    }

    // 设置服务器地址
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = inet_addr(server_ip);
    server_addr.sin_port = htons(server_port);

    // 获取用户名
    printf("请输入用户名：");
    fgets(username, MAX_USERNAME_LEN, stdin);
    username[strlen(username) - 1] = '\0';

    // 发送登录消息
    struct Message msg;
    msg.type = LOGIN;
    strcpy(msg.username, username);
    msg.data_len = 0;
    int send_len = sendto(udp_sockfd, &msg, sizeof(msg), 0, (struct sockaddr *)&server_addr, sizeof(server_addr));
    if (send_len != sizeof(msg)) {
        perror("发送登录消息失败");
        exit(1);
    }

    // 创建接收聊天消息的线程
    pthread_t tid;
    pthread_create(&tid, NULL, recv_chat_msg, NULL);

    // 输入聊天消息
    char input_buf[MAX_MSG_LEN];
    while (1) {
        fgets(input_buf, MAX_MSG_LEN, stdin);
        if (input_buf[strlen(input_buf) - 1] == '\n') {
            input_buf[strlen(input_buf) - 1] = '\0';
        }
        if (strcmp(input_buf, "quit") == 0) {
            // 发送退出消息
            struct Message msg;
            msg.type = LOGOUT;
            strcpy(msg.username, username);
            msg.data_len = 0;
            int send_len = sendto(udp_sockfd, &msg, sizeof(msg), 0, (struct sockaddr *)&server_addr, sizeof(server_addr));
            if (send_len != sizeof(msg)) {
                perror("发送退出消息失败");
            }
            break;
        } else if (strncmp(input_buf, "send ", 5) == 0) {
            // 发送文件
            char filename[MAX_FILENAME_LEN];
            int file_size;
            if (sscanf(input_buf + 5, "%s %d", filename, &file_size) == 2) {
                // 发送文件传输请求
                send_file_request(filename, file_size);

                // 接收文件传输接受或拒绝消息
                fd_set rfds;
                struct timeval tv;
                FD_ZERO(&rfds);
                FD_SET(STDIN_FILENO, &rfds);
                tv.tv_sec = 5;
                tv.tv_usec = 0;
                int ret = select(STDIN_FILENO + 1, &rfds, NULL, NULL, &tv);
                if (ret == -1) {
                    perror("select() error");
                } else if (ret) {
                    char accept_buf[MAX_MSG_LEN];
                    fgets(accept_buf, MAX_MSG_LEN, stdin);
                    if (accept_buf[strlen(accept_buf) - 1] == '\n') {
                        accept_buf[strlen(accept_buf) - 1] = '\0';
                    }
                    if (strcmp(accept_buf, "Y") == 0) {
                        // 发送文件传输接受消息
                        struct sockaddr_in peer_addr;
                        socklen_t peer_addr_len = sizeof(peer_addr);
                        getpeername(udp_sockfd, (struct sockaddr *)&peer_addr, &peer_addr_len);
                        int port = ntohs(peer_addr.sin_port) + 1;
                        send_file_accept(filename, port);

                        // 连接TCP服务器
                        server_addr.sin_port = htons(port);
                        if (connect(tcp_sockfd, (struct sockaddr *)&server_addr, sizeof(server_addr)) == 0) {
                            // 打开文件
                            int file_fd = open(filename, O_WRONLY | O_CREAT | O_TRUNC, 0644);
                            if (file_fd < 0) {
                                perror("打开文件失败");
                                continue;
                            }

                            // 创建接收文件数据的线程
                            pthread_t tid;
                            pthread_create(&tid, NULL, recv_file_data, &file_fd);

                            // 发送文件数据
                            char file_buf[MAX_MSG_LEN];
                            int read_len;
                            while ((read_len = read(tcp_sockfd, file_buf, MAX_MSG_LEN)) > 0) {
                                send_file_data(filename, file_buf, read_len);
                            }

                            // 等待接收文件传输结束消息
                            while (1) {
                                char buf[MAX_MSG_LEN];
                                struct sockaddr_in peer_addr;
                                socklen_t peer_addr_len = sizeof(peer_addr);
                                int len = recvfrom(udp_sockfd, buf, MAX_MSG_LEN - 1, 0, (struct sockaddr *)&peer_addr, &peer_addr_len);
                                if (len > 0) {
                                    buf[len] = '\0';
                                    cJSON *root = cJSON_Parse(buf);
                                    if (root != NULL) {
                                        cJSON *type = cJSON_GetObjectItem(root, "type");
                                        cJSON *username = cJSON_GetObjectItem(root, "username");
                                        cJSON *filename = cJSON_GetObjectItem(root, "filename");
                                        if (type != NULL && username != NULL && filename != NULL && strcmp(type->valuestring, "file_finish") == 0 && strcmp(username->valuestring, username) == 0 && strcmp(filename->valuestring, filename) == 0) {
                                            break;
                                        }
                                        cJSON_Delete(root);
                                    }
                                }
                            }

                            // 发送文件传输结束消息
                            send_file_finish(filename);

                            // 等待接收线程结束
                            pthread_join(tid, NULL);

                            // 关闭文件
                            close(file_fd);
                        } else {
                            perror("连接TCP服务器失败");
                        }
                    } else {
                        // 发送文件传输拒绝消息
                        send_file_reject(filename);
                    }
                } else {
                    printf("文件传输请求超时\n");
                }
            }
        } else {
            send_chat_msg(input_buf);
        }
    }

    // 关闭套接字
    close(udp_sockfd);
    close(tcp_sockfd);

    return 0;
}

2. 服务器端代码 server.c：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <pthread.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <fcntl.h>
#include <cjson/cJSON.h>

#define MAX_MSG_LEN 1024
#define MAX_USERNAME_LEN 32
#define MAX_FILENAME_LEN 64

// 消息类型
enum MessageType {
    LOGIN,
    LOGOUT,
    CHAT,
    FILE_REQUEST,
    FILE_ACCEPT,
    FILE_REJECT,
    FILE_DATA,
    FILE_FINISH