《Linux C 实现 UDP 客户端与服务器》

1 简介

本文演示在 Linux 下用 C 语言实现基于 UDP 的 client/server：client 发送数据，server 回显后返回。UDP 属于无连接传输，编程上比 TCP 简单，只需 socket + bind + recvfrom/sendto。

2 服务器端

2.1 server.c

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

#define PORT 1234          /* 监听端口 */
#define MAXDATASIZE 100

int main()
{
    int sockfd;
    struct sockaddr_in server;
    struct sockaddr_in client;
    socklen_t addrlen;
    int num;
    char buf[MAXDATASIZE];

    /* 创建 UDP socket */
    if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) == -1) {
        perror("failed to create socket.");
        return 1;
    }

    bzero(&server, sizeof(server));
    server.sin_family = AF_INET;
    server.sin_port = htons(PORT);
    server.sin_addr.s_addr = htonl(INADDR_ANY);   /* 绑定所有本机地址 */

    if (bind(sockfd, (struct sockaddr *)&server, sizeof(server)) == -1) {
        perror("failed to bind address.");
        return 1;
    }

    addrlen = sizeof(client);
    while (1) {
        /* 阻塞接收，client 参数带回发端地址与端口 */
        num = recvfrom(sockfd, buf, MAXDATASIZE, 0,
                       (struct sockaddr *)&client, &addrlen);
        if (num < 0) {
            perror("failed to receive msg from client.");
            return 1;
        }

        buf[num] = '\0';
        printf("got a message [%s] from client.\n"
               "It's ip is %s, port is %d.\n",
               buf, inet_ntoa(client.sin_addr), ntohs(client.sin_port));

        /* 回显给发送方 */
        sendto(sockfd, buf, sizeof(buf), 0,
               (struct sockaddr *)&client, addrlen);
        if (!strncmp(buf, "bye", 3))
            break;
    }

    close(sockfd);
    return 0;
}

2.2 编译运行

gcc server.c -o udpserver
./udpserver

3 客户端

3.1 client.c

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

#define PORT 1234          /* 服务器监听端口 */
#define MAXDATASIZE 100

int main(int argc, char *argv[])
{
    int sockfd;
    struct sockaddr_in server;
    socklen_t addrlen;
    int num;
    char buf[MAXDATASIZE];

    if (argc < 3) {
        fprintf(stderr, "usage: %s <server-ip> <message>\n", argv[0]);
        return 1;
    }

    /* 创建 UDP socket */
    if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) == -1) {
        perror("failed to create socket.");
        return 1;
    }

    bzero(&server, sizeof(server));
    server.sin_family = AF_INET;
    server.sin_port = htons(PORT);
    if (inet_pton(AF_INET, argv[1], &server.sin_addr) <= 0) {
        perror("invalid server ip");
        return 1;
    }

    strncpy(buf, argv[2], sizeof(buf) - 1);
    addrlen = sizeof(server);

    /* 发送数据（无需 connect） */
    if (sendto(sockfd, buf, strlen(buf), 0,
               (struct sockaddr *)&server, addrlen) < 0) {
        perror("failed to send.");
        return 1;
    }

    /* 阻塞等待服务器回显 */
    num = recvfrom(sockfd, buf, MAXDATASIZE, 0,
                   NULL, NULL);
    if (num < 0) {
        perror("failed to receive.");
        return 1;
    }
    buf[num] = '\0';
    printf("echo from server: [%s]\n", buf);

    close(sockfd);
    return 0;
}

3.2 编译运行

gcc client.c -o client
./client 127.0.0.1 hello

4 说明

服务端收到 bye 前缀消息后退出循环并关闭 socket；
UDP 发送没有连接概念，sendto 中必须带上对端地址；
recvfrom 的 src/fromlen 参数可填 NULL 表示不关心来源。

5 参考文档

https://blog.csdn.net/yueguanghaidao/article/details/7055985

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