以下程式均未做錯誤處理，等有時間一定完善

客戶端

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

int main(int argc,char **argv)
{
	int sockfd;
	int n;
	struct sockaddr_in servaddr,cliaddr;
	char sendbuf[1000];
	char recvbuf[1000];

	if(argc != 2)
	{
		printf("usage:client <ip address>\n");
		exit(-1);
	}

	sockfd = socket(AF_INET,SOCK_STREAM,0);
	bzero(&servaddr,sizeof(servaddr));

	servaddr.sin_family = AF_INET;
	servaddr.sin_addr.s_addr = inet_addr(argv[1]);
	servaddr.sin_port = htons(3200);

	connect(sockfd,(struct sockaddr*)&servaddr,sizeof(servaddr));

	while(fgets(sendbuf,1000,stdin) != NULL)
	{
		sendto(sockfd,sendbuf,strlen(sendbuf),0,(struct sockaddr *)&servaddr,sizeof(servaddr));
		n = recvfrom(sockfd,recvbuf,1000,0,NULL,NULL);//(struct sockaddr*)&cliaddr,
		recvbuf[n] = '\0';
		fputs(recvbuf,stdout);
	}

	close(sockfd);

    return 0;
}
 

伺服器

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

void strlwr(char *str)
{
	char *p;

	p = str;

	while(*p != '\0')
	{
		if((*p >= 'a') && (*p <= 'z'))
		{
			*p -= 32;
		}

		p++;
	}
}

int main()
{
	pid_t pid;
	int listenfd,connfd;
	int n;
	struct sockaddr_in servaddr,cliaddr;
	socklen_t clilen;

	char mesg[1000];

	listenfd = socket(AF_INET,SOCK_STREAM,0);
	bzero(&servaddr,sizeof(servaddr));
	servaddr.sin_family = AF_INET;
	servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
	servaddr.sin_port = htons(3200);

	int opt = 1;
	setsockopt(listenfd,SOL_SOCKET,SO_REUSEADDR,&opt,sizeof(opt));
	
	bind(listenfd,(struct sockaddr *)&servaddr,sizeof(servaddr));
	listen(listenfd,1024);
	
	for(;;)
	{
		clilen = sizeof(cliaddr);
		connfd = accept(listenfd,(struct sockaddr*)&cliaddr,&clilen);

		if((pid = fork()) == 0)
		{
			close(listenfd);
			
			for(;;)
			{
				n = recvfrom(connfd,mesg,1000,0,(struct sockaddr*)&cliaddr,&clilen);

				if(n == 0)
				{
					printf("client closed!\n");
					exit(-1);
				}

				printf("********************\n");
				mesg[n] = '\0';
				printf("%s\n",mesg);
				printf("*********************\n");
		
				strlwr(mesg);

				sendto(connfd,mesg,n,0,(struct sockaddr *)&cliaddr,sizeof(cliaddr));
			}

			close(connfd);
		}
	}
	close(listenfd);

    return 0;
}