IO Completion Port与文件IO

尽管现在网络上关于IO Completion Port的文章多如牛毛，但绝大多数是以socket为例的。而socket I/O比File I/O要复杂得多，导致相应的例子也比较复杂。下面是一个使用IO Completion Port实现File I/O的程序。为了简单，没有错误处理。

#include <iostream>
#include <string.h>
#include "windows.h"

using namespace std;

// 文件操作次数
#define NR_FILE_OP_TIMES    4
// 每次IO的数据量
#define BUFFER_SIZE         (1024 * 1024)

static DWORD WINAPI ThreadFunc(LPVOID param);

int main()
{
    HANDLE hFile = CreateFile("test.dat",
                              GENERIC_READ | GENERIC_WRITE,
                              FILE_SHARE_READ,
                              0,
                              CREATE_ALWAYS,
                              FILE_FLAG_OVERLAPPED,
                              0);

    // 初始化数据缓冲区
    char* ppBuf[NR_FILE_OP_TIMES];
    for(int i = 0; i < NR_FILE_OP_TIMES; i++)
    {
        ppBuf[i] = new char[BUFFER_SIZE];
        memset(ppBuf[i], i, BUFFER_SIZE);
    }

    // 初始化overlapped结构
    OVERLAPPED overlapped[NR_FILE_OP_TIMES];
    memset(overlapped, 0, sizeof(OVERLAPPED) * NR_FILE_OP_TIMES);

    // 创建completion port
    HANDLE hPort = CreateIoCompletionPort(hFile, 0, 0, 0);

    // 创建Threads
    HANDLE hThread[NR_FILE_OP_TIMES];
    for(int i = 0; i < NR_FILE_OP_TIMES; i++)
    {
        // Create Thread Pool
        hThread[i] = CreateThread(0, 0, ThreadFunc, &hPort, 0, 0);
    }

    // 启动File I/O
    DWORD bytesWritten;
    HANDLE hEvent[NR_FILE_OP_TIMES];
    for(int i = 0; i < NR_FILE_OP_TIMES; i++)
    {
        overlapped[i].Offset = BUFFER_SIZE * i;
        hEvent[i] = CreateEvent(0, TRUE, FALSE, 0);
        overlapped[i].hEvent = hEvent[i];
        WriteFile(hFile, ppBuf[i], BUFFER_SIZE, &bytesWritten, &(overlapped[i]));
    }

    // 等待I/O完成
    WaitForMultipleObjects(4, hEvent, TRUE, INFINITE);

    // Cleanup
    for(int i = 0; i < NR_FILE_OP_TIMES; i++)
    {
        delete ppBuf[i];
        CloseHandle(hThread[i]);
    }
    CloseHandle(hPort);
    CloseHandle(hFile);
    return 0;
}

static DWORD WINAPI ThreadFunc(LPVOID param)
{
    HANDLE hPort = *((HANDLE *)param);

    OVERLAPPED* pOverlapped;
    DWORD bytesTransferred;
    DWORD key;

    GetQueuedCompletionStatus(hPort,
                              &bytesTransferred,
                              &key,
                              &pOverlapped,
                              INFINITE);

    // 打印出I/O相关信息，设置Event，然后这个Thread就退出了。
    cout << "Bytes transferred from " << pOverlapped->Offset << ", length " << bytesTransferred << endl;

    SetEvent(pOverlapped->hEvent);

    return 0;
}

下面是运行结果：

从运行结果看，写入顺序是1 2 3 4，完成顺序是1 2 4 3。