网络编程Socket之UDP(三)超时设置和非阻塞
    前面遗留的两个问题：
1.一个已连接UDP套接字能且仅能与一个对端交换数据报，那么客户端发送广播的时候如何防止recvfrom方法阻塞；
2.服务端忙的时候，已连接的UDP套接字也会被阻塞。
方法一：设置超时
UNP 14.2
There are three ways to place a timeout on an I/O operation involving a socket:
1.Callalarm, which generates theSIGALRMsignal
when the specified time hasexpired. This involves signal handling, which can differ from oneimplementation to the next, and it may interfere with other existing calls toalarmin
the process.2.Block waiting for I/O inselect, which has a time limit built-in, instead网络编程之delphi
ofblocking in a call toread
orwrite.
3.Use the newerSO_RCVTIMEO
andSO_SNDTIMEO
socket options. The problemwith this approach is that not all implementations support these two socket options.
第一种方法我们可以用ios提供的定时器来实现；
第二种方法在recvfrom方法调用前添加代码如下，这里是设置3秒超时：
[cpp] view plain copy <span >        fd_set read_fdset;          struct timeval timeout;                    FD_ZERO(&read_fdset);          FD_SET(socketFileDescriptor,
&read_fdset);                    timeout.tv_sec = 3;          timeout.tv_usec = 0;                    int ret;          do          {              ret = select(socketFileDescriptor + 1, &read_fdset, NULL, NULL, &timeout);          }          while (ret < 0 && errno == EINTR);                    if (ret == 0)          {              printf("time out\n");              return -1;          }  </span>  第三种方法代码如下：
[cpp] view plain copy <span >    struct timeval timeout = {3,0};      setsockopt(socketFileDescriptor, SOL_SOCKET, SO_RCVTIMEO, (char *)&timeout, sizeof(struct timeval));</span>  设置3秒超时，如果3秒以后无法读到数据，recvfrom方法返回-1，而且errno返回EWOULDBLOCK错误
[cpp] view plain copy <span >    n = recvfrom(sockfd, recvline, MAXLINE, 0, NULL, NULL);      if (n < 0) {          if (errno == EWOULDBLOCK) {              printf("time out\n");              return -1;                        } else              err_sys("recvfrom error");      }</span>  方法二：使用非阻塞式IO
UNP 16.1With a nonblocking socket, if the input operation cannot be satisfied (at leastone byte of data for a TCP socket or a complete datagram for a UDP socket),we see an
immediate return with an error of EWOULDBLOCK. 首先设置非阻塞套接字：
[cpp] view plain copy <span >    int ret;      int flags = fcntl(socketFileDescriptor, F_GETFL);      if (flags == -1)      {          printf("fcntl error");          return -1;      }            flags |= O_NONBLOCK;      ret = fcntl(socketFileDescriptor, F_SETFL, flags);      if (ret == -1)      {          printf("fcntl error");          return -1;      }</span>  然后运行，打印信息如下：>> send success
  >> recvfrom fail--errno = 35
>> send success
  >> recvfrom fail--errno = 35
>> send success
  >> recvfrom fail--errno = 35
>> send success
  >> recvfrom fail--errno = 35
>> send success
  >> server said:01&c0:5e:79:fc:0e:0c&192.168.0.1&iShare-R1B011D2199
>> send success
  >> recvfrom fail--errno = 35

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