OVERLAPPED

OVERLAPPED结构类型声明如下：

　　typedef struct _OVERLAPPED {

　　ULONG_PTR Internal; //操作系统保留，指出一个和系统相关的状态

　　ULONG_PTR InternalHigh; //指出发送或接收的数据长度

　　union {

　　struct {

　　DWORD Offset; //文件传送的开始位置

　　DWORD OffsetHigh; //文件传送的字节偏移量的高位字

　　};

　　PVOID Pointer; //指针，指向文件传送位置

　　};

　　HANDLE hEvent; //指定一个I/O操作完成后触发的事件

　　} OVERLAPPED, *LPOVERLAPPED

　　I/O设备处理必然让主程序停下来干等I/O的完成,解决这个问题，可以使用OVERLAPPED。

　　OVERLAPPED I/O是WIN32的一项技术， 你可以要求操作系统为你传送数据，并且在传送完毕时通知你。这项技术使你的程序在I/O进行过程中仍然能够继续处理事务。事实上，操作系统内部正是以线程来I/O完成OVERLAPPED I/O。你可以获得线程的所有利益，而不需付出什么痛苦的代价。也就是说，OVERLAPPED主要是设置异步I/O操作，异步I/O操作是指应用程序可以在后台读或者写数据，而在前台做其他事情。

　　Allen denver在他的《Serial Communication in Win32》中是这样解释OVERLAPPED I/O的：我个人认为还是比较准确的

　　Overlapped I/O is not as straightforward as nonoverlapped I/O, but allows more flexibility and efficiency. A port open for overlapped operations allows multiple threads to do I/O operations at the same time and perform other work while the operations are pending. Furthermore, the behavior of overlapped operations allows a single thread to issue many different requests and do work in the background while the operations are pending.