使用socket option-SO_RCVTIMEO为recvfrom设置超时

使用recvfrom来接受数据时，如果没有数据回来，会一直阻塞下去。

如果不想一直阻塞，可以通过设置socket option的方式来对 recvfrom 设置超时机制。

SO_RCVTIMEO
      Sets  the  timeout  value  that specifies the maximum amount of time an input function waits until it completes. It accepts a timeval structure with the number of seconds and microseconds specifying the limit on how long to wait for an input operation to  complete. If a receive operation has blocked for this much time without receiving additional data, it shall return with apartial count or errno set to [EAGAIN] or [EWOULDBLOCK] if no data is received. The default for this option  is  zero,  which  indicates  that  a receive operation shall not time out. This option takes a timeval structure. Note that not all implementa- tions allow this option to be set

struct timeval tv;
int ret;
tv.tv_sec = 10;
tv.tv_usec = 0;
if(setsockopt(s, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv))<0){
printf("socket option 
SO_RCVTIMEO not support\n");
return;
}
if ((
ret = recvfrom(s, buf, sizeof buf, 0, NULL,NULL)) < 0)
{
   	if(ret == EWOULDBLOCK || ret== EAGAIN )
printf("recvfrom timeout\n");
else
printf("recvfrom err:%d\n",ret);
}