int main(void){

fd_set rfds;

struct timeval tv;

int retval;

FD_ZERO(&rfds);//清空文件描述符

FD_SET(0,&rfds);//设置文件描述符

tv.tv_sec = 3;

tv.tv_usec = 0;

while(1){

retval = select(1,&rfds,NULL,NULL,&tv); //第一个参数是最大fd+1

if(retval == -1)

perror("select err\n");

else if(retval){

printf("data is  ready\n");

if(FD_ISSET(0,&rfds)){//文件描述符是否发生变化

printf("0fd is ready\n");

exit(0);

}

}else

printf("No data\n");

}

return 0;

}