用setsockopt()来控制recv()与send()的超时
在send(),recv()过程中有时由于网络状况等原因，收发不能预期进行,而设置收发超时控制：
在Linux下需要注意的是时间的控制结构是struct timeval而并不是某一整型数，以下是来自于网上一篇文章中的摘录，它是这样写的：
int nNetTimeout=1000;//1秒，
//设置发送超时
setsockopt(socket，SOL_SOCKET,SO_SNDTIMEO，(char *)&nNetTimeout,sizeof(int));
//设置接收超时
setsockopt(socket，SOL_SOCKET,SO_RCVTIMEO，(char *)&nNetTimeout,sizeof(int));
int nNetTimeout=1000;//1秒，
//设置发送超时
setsockopt(socket，SOL_SOCKET,SO_SNDTIMEO，(char *)&nNetTimeout,sizeof(int));
//设置接收超时
setsockopt(socket，SOL_SOCKET,SO_RCVTIMEO，(char *)&nNetTimeout,sizeof(int));
这样做在Linux环境下是不会产生效果的，须如下定义：struct timeval timeout = {3,0};
//设置发送超时
setsockopt(socket，SOL_SOCKET,SO_SNDTIMEO，(char *)&timeout,sizeof(struct timeval));
//设置发送超时
setsockopt(socket，SOL_SOCKET,SO_SNDTIMEO，(char *)&timeout,sizeof(struct timeval));
//设置接收超时
setsockopt(socket，SOL_SOCKET,SO_RCVTIMEO，(char *)&timeout,sizeof(struct timeval));
setsockopt(socket，SOL_SOCKET,SO_RCVTIMEO，(char *)&timeout,sizeof(struct timeval));
有两点注意就是：
1）recv ()的第四个参数需为MSG_WAITALL，在阻塞模式下不等到指定数目的数据不会返回，除非超时时间到。还要注意的是只要设置了接收超时，在没有MSG_WAITALL时也是有效的。说到底超时就是不让你的程序老在那儿等，到一定时间进行一次返回而已。
2）即使等待超时时间值未到，但对方已经关闭了socket， 则此时recv()会立即返回，并收到多少数据返回多少数据。
今天测试了一下，从网上转载的这篇文章，结果确实如此。但是，有几个错误，需要指出一下。在windows下，不需要设置recv()第四个参数为MSG_WAITALL，windows下也没有这个参数，只需要指定为0即可。同时，原文中-----
"
在阻塞模式下不等到指定数目的数据不会返回，除非超时时间到"
----原文中这一句是有问题的。当recv接收到数据时，不论数据多少，都会立即返回，而不会等到指定数据的数据。
阻塞模式下可以用setsockopt设置SO_RCVTIMEO
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.
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.