a.sin_family=af_inet;
a.sin_addr.s_addr=0;
a.sin_port=htons(5050);

from.sin_family=af_inet;
from.sin_addr.s_addr=inaddr_broadcast;
from.sin_port=htons(5050);
int fromlength=sizeof(sockaddr);
//用udp初始化套接字
s=socket(af_inet,sock_dgram,0);
//设置该套接字为广播类型，
setsockopt(s,sol_socket,so_broadcast,(char far *)&optval,sizeof(optval));
//把该套接字绑定在一个具体的地址上
bind(s,(sockaddr *)&a,sizeof(sockaddr_in));
char buf[256];
while(1)
{ //从广播地址接收消息，注意用来绑定的地址和接收消息的地址是不一样的
recvfrom(s,buf,256,0,(struct sockaddr far *)&from,(int far *)&fromlength);
sleep(2000);
printf("%s\n",buf);
zeromemory(buf,256);
}
.....