项目需求：
1.如果有用户登录，其他用户可以收到这个人的登录信息
2.如果有人发送信息，其他用户可以收到这个人的群聊信息
3.如果有人下线，其他用户可以收到这个人的下线信息
4.服务器可以发送系统信息

服务器代码：

#include <myhead.h>
typedef struct group
{
	char type;
	char name[20];
	char text[128];
}gp_t;

typedef struct Node
{
	int PORT;
	struct Node* next;
}*Linklist;

Linklist create_node()
{
	Linklist s=(Linklist)malloc(sizeof(struct Node));
	if(NULL == s)
		return NULL;
	s->PORT =0;
	s->next =NULL;
	return s;
}

Linklist insert_rear(Linklist head,int element)
{
	Linklist s=create_node();
	s->PORT=element;

	if(NULL == head)
	{
		head = s;
		return head;
	}
	Linklist p = head;
	while(p->next != NULL)
	{
		p=p->next;
	}
	p->next = s;
	return head;
}



int lenth(Linklist head)
{
	if(head == NULL)
		return 0;
	int count=0;
	Linklist p=head;
	while(p!=NULL)
	{
		count++;
		p=p->next;
	}
	free(p);
	p=NULL;
	return count;
}

int find_element(Linklist head,int element)
{
	Linklist p=head;
	for(int i=0;i<lenth(head);i++)
	{
		if(p->PORT == element)
			return i;
		p=p->next;
	}
}

Linklist del_head(Linklist head)
{
	if(head->next == NULL)
	{
		free(head);head=NULL;
		return head;
	}
	Linklist del=head->next;
	head->PORT=del->PORT;
	head->next=del->next;
	free(del);del=NULL;
	return head;
}

Linklist del_rear(Linklist head)
{
	if(head->next == NULL)
	{
		free(head);
		head = NULL;
		return head;
	}
	Linklist del=head;
	while(del->next->next!=NULL)
	{
		del=del->next;
	}
	free(del->next);
	del->next=NULL;
	return head;
}

Linklist del_pos(Linklist head,int pos)
{
	if(pos == lenth(head)-1)
	{
		head = del_rear(head);
		return head;
	}
	else if(pos == 0)
	{
		head = del_head(head);
		return head;
	}
	else
	{
		Linklist p=head;
		for(int i=0;i<pos-1;i++)
		{
			p=p->next;
		}
		Linklist r=p->next;
		p->next=r->next;
		free(r);
		r=NULL;
		return head;
	}
}

Linklist del(Linklist head,int element)
{
	if(head ==NULL)
		return head;
	int pos = find_element(head,element);
	head = del_pos(head,pos);
	return head;

}

int main(int argc, const char *argv[])
{
	if(argc != 3)
	{
		printf("请输入服务器IP和端口号!\n");
		return -1;
	}

	int sfd = socket(AF_INET,SOCK_DGRAM,0);
	if(sfd == -1)
	{
		perror("socket error");
		return -1;
	}
	
	struct sockaddr_in sin;
	sin.sin_family = AF_INET;
	sin.sin_port = htons(atoi(argv[2]));
	sin.sin_addr.s_addr = inet_addr(argv[1]);

	if(bind(sfd,(struct sockaddr*)&sin,sizeof(sin))==-1)
	{
		perror("bind error");
		return -1;
	}
	printf("bind success\n");

	struct sockaddr_in cin;
	cin.sin_family = AF_INET;
	socklen_t socklen = sizeof(cin);

	Linklist Usr_PORT=NULL;
	gp_t usr;
	char buf[149] = "";
	char rbuf[130] = "";
	struct pollfd fds[2];
	fds[0].fd = 0;
	fds[0].events = POLLIN;
	fds[1].fd = sfd;
	fds[1].events = POLLIN;
	int res = 0;  //接收select的返回值

	while(1)
	{
		res = poll(fds,2,-1);
		if(res == -1)
		{
			perror("poll error");
			return -1;
		}
		else if(res == 0)
		{
			printf("time out");
			return -1;
		}
	
		bzero(buf,sizeof(buf));

		if(fds[0].revents == POLLIN)
		{
			strcpy(buf,"system:");
			fgets(buf+7,sizeof(buf)-7,stdin);
			buf[strlen(buf)-1] = '\0';
			Linklist p = Usr_PORT;
			while(p!= NULL)
			{
				cin.sin_port = htons(p->PORT);
				sendto(sfd,buf,sizeof(buf),0,(struct sockaddr*)&cin,sizeof(cin));
				p=p->next;
			}
		}

		if(fds[1].revents == POLLIN)
		{
			
			recvfrom(sfd,&usr,sizeof(usr),0,(struct sockaddr*)&cin,&socklen);
			if(usr.type == 'L')
			{
				Usr_PORT = insert_rear(Usr_PORT,ntohs(cin.sin_port));
			
				printf("[%s:%d]已经上线\n",usr.name,ntohs(cin.sin_port));
				sprintf(buf,"-----%s已经上线-----",usr.name);
				printf("buf = %s\n",buf);
				Linklist p = Usr_PORT;
				while(p->next!= NULL)
				{
					cin.sin_port = htons(p->PORT);

					sendto(sfd,buf,sizeof(buf),0,(struct sockaddr*)&cin,sizeof(cin));
					p=p->next;
				}
			}
			else if(usr.type == 'C')
			{
				sprintf(buf,"%s:%s",usr.name,usr.text);

				Linklist p = Usr_PORT;
				int NONE=ntohs(cin.sin_port);
				while(p!= NULL)
				{
					if(NONE!=p->PORT)
					{
						cin.sin_port = htons(p->PORT);
						sendto(sfd,buf,sizeof(buf),0,(struct sockaddr*)&(cin),sizeof(cin));
					}
					p=p->next;
				}
			}
			else if(usr.type == 'Q')
			{
				sprintf(buf,"-----%s已经下线-----",usr.name);
				printf("[%s:%d]已经离线\n",usr.name,ntohs(cin.sin_port));
				Usr_PORT = del(Usr_PORT,ntohs(cin.sin_port));
				Linklist p = Usr_PORT;
				while(p!= NULL)
				{
					cin.sin_port = htons(p->PORT);
					sendto(sfd,buf,sizeof(buf),0,(struct sockaddr*)&cin,sizeof(cin));
					p=p->next;
				}
			}
		}
	}
	close(sfd);
	return 0;
}

客户端代码：

#include <myhead.h>

typedef struct
{
	char type;
	char name[20];
	char text[128];
}gp_t;

int main(int argc, const char *argv[])
{
	if(argc != 3)
	{
		printf("请输入服务器IP和端口号!\n");
		return -1;
	}

	int cfd = socket(AF_INET,SOCK_DGRAM,0);
	if(cfd == -1)
	{
		perror("socket error");
		return -1;
	}

	gp_t GP;
	char name[20] = "";
	printf("请输入用户名>>");
	scanf("%s",GP.name);
	getchar();

	struct sockaddr_in sin;
	sin.sin_family = AF_INET;
	sin.sin_port = htons(atoi(argv[2]));
	sin.sin_addr.s_addr = inet_addr(argv[1]);

	char buf[130] = "";
	char rbuf[128] = "";

	bzero(buf,sizeof(buf));
	GP.type = 'L';
	sendto(cfd,&GP,sizeof(GP),0,(struct sockaddr*)&sin,sizeof(sin));

	struct pollfd fds[2];
	fds[0].fd = 0;
	fds[0].events = POLLIN;

	fds[1].fd = cfd;
	fds[1].events = POLLIN;
	int res = 0;

	while(1)
	{
		res = poll(fds,2,-1);
		if(res == -1)
		{
			perror("poll error");
			return -1;
		}
		else if(res == 0)
		{
			printf("time out\n");
			return -1;
		}

		bzero(buf,sizeof(buf));
		bzero(rbuf,sizeof(rbuf));

		if(fds[1].revents == POLLIN)
		{
			recvfrom(cfd,rbuf,sizeof(rbuf),0,NULL,NULL);
			printf("%s\n",rbuf);
		}

		if(fds[0].revents == POLLIN)
		{
			fgets(GP.text,sizeof(GP.text),stdin);
			GP.text[strlen(GP.text)-1]='\0';

			if(strcmp(GP.text,"quit")==0)
			{
				GP.type = 'Q';
				sendto(cfd,&GP,sizeof(GP),0,(struct sockaddr*)&sin,sizeof(sin));
				goto A;
			}
			GP.type = 'C';
			sendto(cfd,&GP,sizeof(GP),0,(struct sockaddr*)&sin,sizeof(sin));
		}
	}
A:
	close(cfd);
	return 0;
}