OVERLAPPED

OVERLAPPED結構類型聲明如下：

typedef struct _OVERLAPPED {

ULONG_PTR Internal; //操作系統保留，指出一個和系統相關的狀態

ULONG_PTR InternalHigh; //指出發送或接收的數據長度

union {

struct {

DWORD Offset; //文件傳送的開始位置

DWORD OffsetHigh; //文件傳送的位元組偏移量的高位字

};

PVOID Pointer; //指針，指向文件傳送位置

};

HANDLE hEvent; //指定一個I/O操作完成後觸發的事件

} OVERLAPPED, *LPOVERLAPPED;

I/O設備處理必然讓主程序停下來乾等I/O的完成，解決這個問題，可以使用OVERLAPPED。

OVERLAPPED I/O是WIN32的一項技術，你可以要求操作系統為你傳送數據，並且在傳送完畢時通知你。這項技術使你的程序在I/O進行過程中仍然能夠繼續處理事務。事實上，操作系統內部正是以線程來I/O完成OVERLAPPED I/O。你可以獲得線程的所有利益，而不需付出什麼痛苦的代價。也就是說，OVERLAPPED主要是設置非同步I/O操作，非同步I/O操作是指應用程序可以在後台讀或者寫數據，而在前台做其他事情。

Allen denver在他的《Serial Communication in Win32》中是這樣解釋OVERLAPPED I/O的：我個人認為還是比較準確的

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.