基于Socket实现UDP编程（4H）

UDP（User Datagram Protocol）是一种无连接的传输层协议，它不保证数据包传输的可靠性和顺序，但是具有高效和低延迟的特点。在UDP编程中，需要使用Socket接口来实现UDP数据包的发送和接收。下面是基于Socket实现UDP编程的步骤：

1. 创建Socket

使用socket()函数创建一个Socket，指定协议族为AF_INET（IPv4）或AF_INET6（IPv6），协议类型为SOCK_DGRAM（UDP）。

int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
if (sockfd < 0) {
    perror("socket creation failed");
    exit(EXIT_FAILURE);
}

2. 绑定Socket

使用bind()函数将Socket与本地IP地址和端口号绑定，这样就可以接收到发送到该IP地址和端口号的UDP数据包。

struct sockaddr_in server_addr;
memset(&server_addr, 0, sizeof(server_addr));
server_addr.sin_family = AF_INET;
server_addr.sin_addr.s_addr = INADDR_ANY;
server_addr.sin_port = htons(PORT);

if (bind(sockfd, (struct sockaddr*)&server_addr, sizeof(server_addr)) < 0) {
    perror("bind failed");
    exit(EXIT_FAILURE);
}

3. 发送数据包

使用sendto()函数向指定的IP地址和端口号发送UDP数据包。

struct sockaddr_in client_addr;
memset(&client_addr, 0, sizeof(client_addr));
client_addr.sin_family = AF_INET;
client_addr.sin_addr.s_addr = inet_addr("127.0.0.1");
client_addr.sin_port = htons(PORT);

char buffer[MAXLINE];
strcpy(buffer, "Hello, World!");

if (sendto(sockfd, buffer, strlen(buffer), 0, (struct sockaddr*)&client_addr, sizeof(client_addr)) < 0) {
    perror("sendto failed");
    exit(EXIT_FAILURE);
}

4. 接收数据包

使用recvfrom()函数从Socket接收UDP数据包，并存储发送方的IP地址和端口号。

struct sockaddr_in client_addr;
memset(&client_addr, 0, sizeof(client_addr));
socklen_t len = sizeof(client_addr);

char buffer[MAXLINE];
int n = recvfrom(sockfd, buffer, MAXLINE, 0, (struct sockaddr*)&client_addr, &len);
if (n < 0) {
    perror("recvfrom failed");
    exit(EXIT_FAILURE);
}

buffer[n] = '\0';
printf("Received message: %s\n", buffer);
printf("Client address: %s:%d\n", inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));

5. 关闭Socket

使用close()函数关闭Socket。

close(sockfd);

完整的UDP编程示例代码如下：

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

#define PORT 8080
#define MAXLINE 1024

int main() {
    int sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd < 0) {
        perror("socket creation failed");
        exit(EXIT_FAILURE);
    }

    struct sockaddr_in server_addr;
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = INADDR_ANY;
    server_addr.sin_port = htons(PORT);

    if (bind(sockfd, (struct sockaddr*)&server_addr, sizeof(server_addr)) < 0) {
        perror("bind failed");
        exit(EXIT_FAILURE);
    }

    struct sockaddr_in client_addr;
    memset(&client_addr, 0, sizeof(client_addr));
    client_addr.sin_family = AF_INET;
    client_addr.sin_addr.s_addr = inet_addr("127.0.0.1");
    client_addr.sin_port = htons(PORT);

    char buffer[MAXLINE];
    strcpy(buffer, "Hello, World!");

    if (sendto(sockfd, buffer, strlen(buffer), 0, (struct sockaddr*)&client_addr, sizeof(client_addr)) < 0) {
        perror("sendto failed");
        exit(EXIT_FAILURE);
    }

    memset(&client_addr, 0, sizeof(client_addr));
    socklen_t len = sizeof(client_addr);

    int n = recvfrom(sockfd, buffer, MAXLINE, 0, (struct sockaddr*)&client_addr, &len);
    if (n < 0) {
        perror("recvfrom failed");
        exit(EXIT_FAILURE);
    }

    buffer[n] = '\0';
    printf("Received message: %s\n", buffer);
    printf("Client address: %s:%d\n", inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));

    close(sockfd);

    return 0;
}