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 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：
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;
}
struct timeval timeout = {3,0};
setsockopt(socketFileDescriptor, SOL_SOCKET, SO_RCVTIMEO, (char *)&timeout, sizeof(struct timeval));
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");
}UNP 16.1
With 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.
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;
}
>> 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
>> send success
>> recvfrom fail--errno = 35
>> send success
>> recvfrom fail--errno = 35
会发现recvfrom方法返回失败的概率很大，而且errno返回EWOULDBLOCK错误，这是因为在非阻塞的情况下有可能因为服务器阻塞导致客户端这边的UDP套接字的接收缓冲区为空， 因此为了解决这个问题我们在recvfrom方法之前添加如下代码：
fd_set read_fdset;
FD_ZERO(&read_fdset);
FD_SET(socketFileDescriptor, &read_fdset);
int ret;
do
{
ret = select(socketFileDescriptor + 1, &read_fdset, NULL, NULL, NULL);
}
while (ret < 0 && errno == EINTR);
if (ret == 0)
{
printf("time out\n");
return -1;
}>> send success
>> server said:01&c0:5e:79:fc:0e:0c&192.168.0.1&iShare-R1B011D2199
>> send success
>> server said:01&c0:5e:79:fc:0e:0c&192.168.0.1&iShare-R1B011D2199
>> send success
>> server said:01&c0:5e:79:fc:0e:0c&192.168.0.1&iShare-R1B011D2199
>> send success
>> server said:01&c0:5e:79:fc:0e:0c&192.168.0.1&iShare-R1B011D2199
>> send success
>> server said:01&c0:5e:79:fc:0e:0c&192.168.0.1&iShare-R1B011D2199
参考：
《UNIX Network ProgrammingVolume 1, Third Edition: TheSockets Networking API》