#include <stdio.h>#include <sys/types.h>#include <sys/socket.h>#include <sys/select.h>#include <netdb.h>int main(int argc,char *argv[]){	int srv_sock;	socklen_t clt_len;	struct sockaddr_in srv_addr;	struct sockaddr_in clt_addr;	struct timeval wait_time;	int port;	int ret;	fd_set read_fds;	int num;		char recv_buf[1024];	if(argc!=2){		printf("Usage: %s port_name\n",argv[0]);		return 1;	}		port=atoi(argv[1]);	srv_sock=socket(PF_INET,SOCK_DGRAM,0);	if(srv_sock<0){		perror("cannot create socket");		return 1;	}	memset(&srv_addr,0,sizeof(srv_addr));	srv_addr.sin_family=AF_INET;	srv_addr.sin_addr.s_addr=htonl(INADDR_ANY);	srv_addr.sin_port=htons(port);	ret=bind(srv_sock,(struct sockaddr*)&srv_addr,sizeof(srv_addr));	if(ret<0){		perror("cannot bind the socket");		return 1;	}	while(1){		wait_time.tv_sec=3;		wait_time.tv_usec=0;		FD_ZERO(&read_fds);		FD_SET(srv_sock,&read_fds);				num=select(srv_sock+1,&read_fds,NULL,NULL,&wait_time);		if(num<0){			perror("select fail");			continue;		}				if(FD_ISSET(srv_sock,&read_fds)){			int n=recvfrom(srv_sock,recv_buf,sizeof(recv_buf),0,(struct sockaddr*)&clt_addr,&clt_len);			if(n<0){				perror("cannot receive client message");				close(srv_sock);				return 1;			}			printf("server receive: %s\n",recv_buf);			memset(recv_buf,0,sizeof(recv_buf));		}		printf("waiting....\n");	}}