五、用户数据报传输（UDP）

1、UDP常用的发送和接收函数

int recvfrom(int sockfd,void *buf,int len,unsigned int flags,struct sockaddr *from,int *fromlen);
int sendto(int sockfd,const void *msg,int len,unsigned int flags,struct sockaddr *to,int tolen);

sockfd，buf，len的意义和read，write中的参数是一样的，分别表示套接字描述符，发送或接收的缓冲区及大小。

recvfrom负责从sockfd接收数据，如果from不是NULL，那么在from里面存储了信息来源的情况，如果对信息的来源不感兴趣，可以将from和fromlen设置为NULL。

sendto负责向to发送消息，此时在to里面存储了信息接收方的详细资料。

2、实例演示

服务端程序

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <netdb.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/types.h>
#include <arpa/inet.h>
#define SERVER_PORT     8888
#define MAX_MSG_SIZE    1024

void udps_respon(int sockfd)
{
    struct sockaddr_in addr;
    int n;
    socklen_t addrlen;
    char msg[MAX_MSG_SIZE];

    while (1)
    {       /* 从网络上读,写到网络上面去   */
        memset(msg, 0, sizeof(msg));
        addrlen = sizeof(struct sockaddr);
        n = recvfrom(sockfd, msg, MAX_MSG_SIZE, 0,
            (struct sockaddr*)&addr, &addrlen);
        /* 显示服务端已经收到了信息  */
        fprintf(stdout, "I have received %s", msg);
        sendto(sockfd, msg, n, 0, (struct sockaddr*)&addr, addrlen);
    }
}

int main(void)
{
    int sockfd;
    struct sockaddr_in addr;

    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd<0)
    {
        fprintf(stderr, "Socket Error:%s\n", strerror(errno));
        exit(1);
    }
    bzero(&addr, sizeof(struct sockaddr_in));
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = htonl(INADDR_ANY);
    addr.sin_port = htons(SERVER_PORT);
    if (bind(sockfd, (struct sockaddr *)&addr, sizeof(struct sockaddr_in))<0)
    {
        fprintf(stderr, "Bind Error:%s\n", strerror(errno));
        exit(1);
    }
    udps_respon(sockfd);
    close(sockfd);
}

客户端程序

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <unistd.h>
#include <netdb.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <sys/types.h>
#include <arpa/inet.h>
#define MAX_BUF_SIZE    1024

void udpc_requ(int sockfd, const struct sockaddr_in *addr, socklen_t len)
{
    char buffer[MAX_BUF_SIZE];
    int n;
    while (fgets(buffer, MAX_BUF_SIZE, stdin))
    {        /*   从键盘读入,写到服务端   */
        sendto(sockfd, buffer, strlen(buffer), 0, addr, len);
        bzero(buffer, MAX_BUF_SIZE);

        /*   从网络上读,写到屏幕上    */
        memset(buffer, 0, sizeof(buffer));
        n = recvfrom(sockfd, buffer, MAX_BUF_SIZE, 0, NULL, NULL);
        if (n <= 0)
        {
            fprintf(stderr, "Recv Error %s\n", strerror(errno));
            return;
        }
        buffer[n] = 0;
        fprintf(stderr, "get %s", buffer);
    }
}


int main(int argc, char **argv)
{
    int sockfd, port;
    struct sockaddr_in addr;

    if (argc != 3)
    {
        fprintf(stderr, "Usage:%s server_ip server_port\n", argv[0]);
        exit(1);
    }

    if ((port = atoi(argv[2]))<0)
    {
        fprintf(stderr, "Usage:%s server_ip server_port\n", argv[0]);
        exit(1);
    }

    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd<0)
    {
        fprintf(stderr, "Socket  Error:%s\n", strerror(errno));
        exit(1);
    }
    /*      填充服务端的资料      */
    bzero(&addr, sizeof(struct sockaddr_in));
    addr.sin_family = AF_INET;
    addr.sin_port = htons(port);
    if (inet_aton(argv[1], &addr.sin_addr)<0)
    {
        fprintf(stderr, "Ip error:%s\n", strerror(errno));
        exit(1);
    }
    if (connect(sockfd, (struct sockaddr *)&addr, sizeof(struct sockaddr_in)) == -1)
    {
        fprintf(stderr, "connect error %s\n", strerror(errno));
        exit(1);
    }
    udpc_requ(sockfd, &addr, sizeof(struct sockaddr_in));
    close(sockfd);
}