简单的，使用套接字实UDP通信（循环服务器）

/*udp_server.c*/

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

#define N 100

int main()
{
int sockfd;
struct sockaddr_in servaddr;
struct sockaddr_in cliaddr;
socklen_t clilen;
char msg
;
int ret;
int i;
int n;
int opt = 1;

sockfd = socket(AF_INET,SOCK_DGRAM,0);
if(sockfd == -1)
{
printf("socket errpor!\n");
exit(-1);
}

ret = setsockopt(sockfd,SOL_SOCKET,SO_REUSEADDR,&opt,sizeof(opt));
if(ret == -1)
{
printf("setsockopt error!\n");
exit(-1);
}

bzero(&servaddr,sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
servaddr.sin_port = htons(8000);

ret = bind(sockfd,(struct sockaddr *)&servaddr,sizeof(servaddr));
if(ret == -1)
{
printf("bind error!\n");
exit(-1);
}

clilen = sizeof(cliaddr);

while(1)
{
n = recvfrom(sockfd,msg,N,0,(struct sockaddr*)&cliaddr,&clilen);
if(n == 0)
{
printf("client is closed!\n");
break;
}
msg
= '\0';
printf("recv:%s",msg);
for(i = 0; i < n; i++)
{
if(msg[i] >= 'a' && msg[i] <= 'z')
{
msg[i] -= 32;
}
}
printf("send:%s",msg);
sendto(sockfd,msg,n,0,(struct sockaddr*)&cliaddr,sizeof(cliaddr));
}

close(sockfd);

return 0;
}

/*udp_client.c*/

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

#define N 100

int main(int argc,char **argv)
{
int sockfd;
struct sockaddr_in servaddr;
char sendbuf
;
char recvbuf
;
int n;
int ret;

if(argc != 2)
{
printf("usage:server <ip address>\n");
exit(-1);
}
sockfd = socket(AF_INET,SOCK_DGRAM,0);
if(sockfd == -1)
{
printf("socket error!\n");
exit(-1);
}

bzero(&servaddr,sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = inet_addr(argv[1]);
servaddr.sin_port = htons(8000);

while(fgets(sendbuf,N,stdin) != 0)
{
sendto(sockfd,sendbuf,strlen(sendbuf),0,(struct sockaddr *)&servaddr,sizeof(servaddr));
n = recvfrom(sockfd,recvbuf,N,0,NULL,NULL);
recvbuf
= '\0';
printf("recv:%s",recvbuf);
}

close(sockfd);
return 0;
}