服务器端代码实现

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <stdlib.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#define PORT 1234
#define MAXDATASIZE 100
#define BACKLOG 5
void process_cli(int connfd,struct sockaddr_in client);
main(){
	int listenfd,connfd,len,opt;
	pid_t pid;
	struct sockaddr_in server;
	struct sockaddr_in client;
	if((listenfd=socket(AF_INET,SOCK_STREAM,0))==-1){
		perror("Creating socket failed.");
		exit(1);
	}
	opt=SO_REUSEADDR;
	setsockopt(listenfd,SOL_SOCKET,SO_REUSEADDR,&opt,sizeof(opt));
	bzero(&server,sizeof(server));
	server.sin_family=AF_INET;
	server.sin_port=htons(PORT);
	server.sin_addr.s_addr=htonl(INADDR_ANY);
	if(bind(listenfd,(struct sockaddr *)&server,sizeof(server))==-1){
		perror("Bind() error");
		exit(1);
	}
	if(listen(listenfd,BACKLOG)==-1){
		perror("listen() error.\n");
		exit(1);
	}
	len=sizeof(client);
	while(1){
		if((connfd=accept(listenfd,(struct sockaddr *)&client,&len))==-1){
			perror("accept() error\n");
			exit(1);
		}
		if((pid=fork())>0){
			close(connfd);
			continue;
		}
		else if(pid==0){
			close(listenfd);
			process_cli(connfd,client);
			exit(0);
		}
		else{
			printf("fork() error\n");
			exit(0);
		}
	}
	close(listenfd);
}
void process_cli(int connfd,struct sockaddr_in client){
	int num,i;
	char recvbuf[MAXDATASIZE],sendbuf[MAXDATASIZE],cli_name[MAXDATASIZE];
	printf("You got a connection from %s,port is %d.\n.",inet_ntoa(client.sin_addr),htons(client.sin_port));
	num=recv(connfd,cli_name,MAXDATASIZE,0);
	if(num==0){
		close(connfd);
		printf("Client disconnected.\n");
		return;
	}
	cli_name[num-1]='\0';
	printf("Client's name is %s.\n",cli_name);
	while(num=recv(connfd,recvbuf,MAXDATASIZE,0)){
		recvbuf[num]='\0';
		printf("Received client(%s) message:%s\n",cli_name,recvbuf);
		i=0;
		for(i=0;i<num-1;i++){
			if((recvbuf[i]>='a'&&recvbuf[i]<='z')||(recvbuf[i]>='A'&&recvbuf[i]<='Z')){
				recvbuf[i]=recvbuf[i]+3;
				if((recvbuf[i]>'Z'&&recvbuf[i]<='Z'+3)||(recvbuf[i]>'z'))
					recvbuf[i]=recvbuf[i]-26;
			}
			sendbuf[i]=recvbuf[i];
		}
		sendbuf[num-1]='\0';
		send(connfd,sendbuf,strlen(sendbuf),0);
	}
	close(connfd);
}

客户端代码实现

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#define PORT 1234
#define MAXDATASIZE 100
void process(FILE *fp,int sockfd);
char* getMessage(char* sendline,int len,FILE* fp);
int main(int argc,char *argv[]){
	int sockfd;
	struct  hostent *he;
	struct sockaddr_in server;
	if (argc!=2){
		printf("Usage:%s<IP Address>\n",argv[0]);
		exit(1);
	}
	if((he=gethostbyname(argv[1]))==NULL){
		printf("gethostbyname() error\n");
		exit(1);
	}
	if((sockfd=socket(AF_INET,SOCK_STREAM,0))==-1){
		printf("socket() error\n");
		exit(1);
	}
	bzero(&server,sizeof(server));
	server.sin_family=AF_INET;
	server.sin_port=htons(PORT);
	server.sin_addr=*((struct in_addr *)he->h_addr);
	if(connect(sockfd,(struct sockaddr *)&server,sizeof(server))==-1){
		printf("connect() error\n");
		exit(1);
	}
	process(stdin,sockfd);
	close(sockfd);
}
void process(FILE *fp,int sockfd){
	char sendline[MAXDATASIZE],recvline[MAXDATASIZE];
	int num;
	printf("Connected to server.\n");
	printf("Input client's name:");
	if(fgets(sendline,MAXDATASIZE,fp)==NULL){
		printf("\nExit.\n");
		return;
	}
	send(sockfd,sendline,strlen(sendline),0);
	while(getMessage(sendline,MAXDATASIZE,fp)!=NULL){
		send(sockfd,sendline,strlen(sendline),0);
		if((num=recv(sockfd,recvline,MAXDATASIZE,0))==0){
			printf("Server terminated.\n");
			return;
		}
		recvline[num]='\0';
		printf("Server Message:%s\n",recvline);
	}
	printf("\nExit.\n");
}
char* getMessage(char* sendline,int len,FILE* fp){
	printf("Input string to server:");
	return(fgets(sendline,MAXDATASIZE,fp));
}	

运行示例