| 词条 | OVERLAPPED |
| 释义 | 结构类型声明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; OVERLAPPED结构的作用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. |
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