使用 WINAPI ReadFile 从文件异步读取行

Question

我需要同时从文件中读取多行，即异步。文件中的行大小相同。

例如，我需要读取文件的第二行和第四行来分隔变量或数组。

我更习惯于 c# 的 async/await，所有这些 OVERLAPPED 东西对我来说有点难以理解。

使用msdn的例子，我实现了这个，它只是从文件中读取数据作为一个字符串（这甚至是异步读取吗？）：

BOOL ReadFromFileAsync(PCTSTR path)
{
    HANDLE hFile = CreateFile(path, GENERIC_READ, FILE_SHARE_READ, NULL, OPEN_EXISTING,
        FILE_FLAG_OVERLAPPED | FILE_FLAG_NO_BUFFERING, NULL);

    if (hFile == INVALID_HANDLE_VALUE)
    {
        _tprintf_s(TEXT("INVALID_HANDLE_VALUE\n"));
        return FALSE;
    }

    BOOL bResult;
    BYTE bReadBuf[2048];

    OVERLAPPED oRead = { 0 };
    oRead.hEvent = CreateEvent(NULL, FALSE, FALSE, NULL);
    bResult = ReadFile(hFile, bReadBuf, _countof(bReadBuf), NULL, &oRead);

    if (!bResult && GetLastError() != ERROR_IO_PENDING)
    {
        _tprintf_s(TEXT("ERROR io pending"));
        CloseHandle(hFile);
        return FALSE;
    }

    // perform some stuff asynchronously
    _tprintf_s(TEXT("HEY\n"));

    HANDLE hEvents[2];
    hEvents[0] = oRead.hEvent;
    hEvents[1] = oRead.hEvent;
    DWORD dwWaitRes = WaitForMultipleObjects(_countof(hEvents), hEvents, FALSE, INFINITE);

    switch (dwWaitRes - WAIT_OBJECT_0)
    {
        case 0: // reading finished
            _tprintf_s(TEXT("String that was read from file: "));

            for (int i = 0; i < oRead.InternalHigh; ++i)
                _tprintf_s(TEXT("%c"), bReadBuf[i]);

            _tprintf_s(TEXT("\n"));
            break;

        default:
            _tprintf_s(TEXT("Nooo"));
    }

    CloseHandle(hFile);

    return TRUE;
}

你能帮我异步读取文件中的两行吗？

我应该使用 SetFilePointer 来移动这些行吗？

Answer 1

当您打开带有 FILE_FLAG_OVERLAPPED 标志的文件，然后使用带有 ReadFile() 的 OVERLAPPED 结构时，请使用 OVERLAPPED.Offset 和 OVERLAPPED.OffsetHigh 字段来指定读取开始的字节偏移量。此外，如果您同时运行它们，则必须为每个 ReadFile() 使用单独的 OVERLAPPED 实例。这在 documentation:

中有明确说明

If hFile is opened with FILE_FLAG_OVERLAPPED, the lpOverlapped parameter must point to a valid and unique OVERLAPPED structure, otherwise the function can incorrectly report that the read operation is complete.

For an hFile that supports byte offsets, if you use this parameter you must specify a byte offset at which to start reading from the file or device. This offset is specified by setting the Offset and OffsetHigh members of the OVERLAPPED structure. For an hFile that does not support byte offsets, Offset and OffsetHigh are ignored.

由于你的行长度相同，你可以很容易地计算出第二行和第四行的偏移量，然后为这些偏移量发出两个异步 ReadFile() 调用，然后等待两个操作完成需要。

附带说明一下，你真的不应该使用 FILE_FLAG_NO_BUFFERING 除非你真的知道你在做什么：

There are strict requirements for successfully working with files opened with CreateFile using the FILE_FLAG_NO_BUFFERING flag, for details see File Buffering.

尝试更像这样的东西：

#include <vector>

BOOL ReadFromFileAsync(PCTSTR path)
{
    BOOL bResult = FALSE;

    HANDLE hFile = CreateFile(path, GENERIC_READ, FILE_SHARE_READ, NULL, OPEN_EXISTING, FILE_FLAG_OVERLAPPED /*| FILE_FLAG_NO_BUFFERING*/, NULL);    
    if (hFile == INVALID_HANDLE_VALUE)
    {
        _tprintf_s(TEXT("Error opening file: %s\n"), path);
        return FALSE;
    }

    DWORD dwLineSize = ...; // size of each line, in bytes
    std::vector<BYTE> bSecondLineBuf(dwLineSize);
    std::vector<BYTE> bFourthLineBuf(dwLineSize);

    OVERLAPPED oReadSecondLine = { 0 };
    OVERLAPPED oReadFourthLine = { 0 };

    oReadSecondLine.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
    if (!oReadSecondLine.hEvent)
    {
        _tprintf_s(TEXT("Error creating I/O event for reading second line\n"));
        goto done;
    }
    oReadSecondLine.Offset = ...; // offset of second line

    oReadFourthLine.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
    if (!oReadFourthLine.hEvent)
    {
        _tprintf_s(TEXT("Error creating I/O event for reading fourth line\n"));
        goto done;
    }
    oReadFourthLine.Offset = ...; // offset of fourth line

    if (!ReadFile(hFile, &bSecondLineBuf[0], dwLineSize, NULL, &oReadSecondLine))
    {
        if (GetLastError() != ERROR_IO_PENDING)
        {
            _tprintf_s(TEXT("Error starting I/O to read second line\n"));
            goto done;
        }
    }

    if (!ReadFile(hFile, &bFourthLineBuf[0], dwLineSize, NULL, &oReadFourthLine))
    {
        if (GetLastError() != ERROR_IO_PENDING)
        {
            _tprintf_s(TEXT("Error starting I/O to read fourth line\n"));
            CancelIo(hFile);
            goto done;
        }
    }

    // perform some stuff asynchronously
    _tprintf_s(TEXT("HEY\n"));

    HANDLE hEvents[2];
    hEvents[0] = oReadSecondLine.hEvent;
    hEvents[1] = oReadFourthLine.hEvent;

    DWORD dwWaitRes = WaitForMultipleObjects(_countof(hEvents), hEvents, TRUE, INFINITE);
    if (dwWaitRes == WAIT_FAILED)
    {
        _tprintf_s(TEXT("Error waiting for I/O to finish\n"));
        CancelIo(hFile);
        goto done;
    }    

    _tprintf_s(TEXT("Strings that were read from file: "));

    for (int i = 0; i < oReadSecondLine.InternalHigh; ++i)
        _tprintf_s(TEXT("%c"), (TCHAR) &bSecondLineBuf[i]);    
    _tprintf_s(TEXT("\n"));

    for (int i = 0; i < oReadFourthLine.InternalHigh; ++i)
        _tprintf_s(TEXT("%c"), (TCHAR) &bFourthLineBuf[i]);    
    _tprintf_s(TEXT("\n"));

done:
    if (oReadSecondLine.hEvent) CloseHandle(oReadSecondLine.hEvent);
    if (oReadFourthLine.hEvent) CloseHandle(oReadFourthLine.hEvent);
    CloseHandle(hFile);

    return bResult;
}

或者：

#include <vector>

BOOL ReadFromFileAsync(PCTSTR path)
{
    BOOL bResult = FALSE;

    HANDLE hFile = CreateFile(path, GENERIC_READ, FILE_SHARE_READ, NULL, OPEN_EXISTING, FILE_FLAG_OVERLAPPED /*| FILE_FLAG_NO_BUFFERING*/, NULL);    
    if (hFile == INVALID_HANDLE_VALUE)
    {
        _tprintf_s(TEXT("Error opening file: %s\n"), path);
        return FALSE;
    }

    DWORD dwLineSize = ...; // size of each line, in bytes
    std::vector<BYTE> bSecondLineBuf(dwLineSize);
    std::vector<BYTE> bFourthLineBuf(dwLineSize);

    OVERLAPPED oReadSecondLine = { 0 };
    OVERLAPPED oReadFourthLine = { 0 };

    oReadSecondLine.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
    if (!oReadSecondLine.hEvent)
    {
        _tprintf_s(TEXT("Error creating I/O event for reading second line\n"));
        goto done;
    }
    oReadSecondLine.Offset = ...; // offset of second line

    oReadFourthLine.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
    if (!oReadFourthLine.hEvent)
    {
        _tprintf_s(TEXT("Error creating I/O event for reading fourth line\n"));
        goto done;
    }
    oReadFourthLine.Offset = ...; // offset of fourth line

    if (!ReadFile(hFile, &bSecondLineBuf[0], dwLineSize, NULL, &oReadSecondLine))
    {
        if (GetLastError() != ERROR_IO_PENDING)
        {
            _tprintf_s(TEXT("Error starting I/O to read second line\n"));
            goto done;
        }
    }

    if (!ReadFile(hFile, &bFourthLineBuf[0], dwLineSize, NULL, &oReadFourthLine))
    {
        if (GetLastError() != ERROR_IO_PENDING)
        {
            _tprintf_s(TEXT("Error starting I/O to read fourth line\n"));
            CancelIo(hFile);
            goto done;
        }
    }

    // perform some stuff asynchronously
    _tprintf_s(TEXT("HEY\n"));

    HANDLE hEvents[2];
    hEvents[0] = oReadSecondLine.hEvent;
    hEvents[1] = oReadFourthLine.hEvent;

    OVERLAPPED* pOverlappeds[2];
    pOverlappeds[0] = &oReadSecondLine;
    pOverlappeds[1] = &oReadFourthLine;

    BYTE* pBufs[2];
    pBufs[0] = &bSecondLineBuf[0];
    pBufs[1] = &bFourthLineBuf[0];

    DWORD dwNumReads = _countof(hEvents);

    do
    {
        DWORD dwWaitRes = WaitForMultipleObjects(dwNumReads, hEvents, FALSE, INFINITE);
        if (dwWaitRes == WAIT_FAILED)
        {
            _tprintf_s(TEXT("Error waiting for I/O to finish\n"));
            CancelIo(hFile);
            goto done;
        }    

        if ((dwWaitRes >= WAIT_OBJECT_0) && (dwWaitRes < (WAIT_OBJECT_0+dwNumReads)))
        {
            DWORD dwIndex = dwWaitRes - WAIT_OBJECT_0;

            _tprintf_s(TEXT("String that was read from file: "));

            for (int i = 0; i < pOverlappeds[dwIndex]->InternalHigh; ++i)
                _tprintf_s(TEXT("%c"), (TCHAR) pBufs[dwIndex][i]);    
            _tprintf_s(TEXT("\n"));

           --dwNumReads;
           if (dwNumReads == 0)
               break;

           if (dwIndex == 0)
           {
               hEvents[0] = hEvents[1];
               pOverlappeds[0] = pOverlappeds[1];
               pBufs[0] = pBufs[1];
           }
        }
    }
    while (true);

done:
    if (oReadSecondLine.hEvent) CloseHandle(oReadSecondLine.hEvent);
    if (oReadFourthLine.hEvent) CloseHandle(oReadFourthLine.hEvent);
    CloseHandle(hFile);

    return bResult;
}

使用 WINAPI ReadFile 从文件异步读取行

Read lines from file async using WINAPI ReadFile

c++

multithreading

winapi

createfile

overlapped-io