为什么 WriteFile 不推进此代码中后续写入的文件偏移量？

Question

我创建了一个重叠的 WriteFile 方案，其中一个线程填充环形缓冲区并推进头指针并调用 WriteFile，另一个线程监视来自 OVERLAPPED 写入的事件以推进尾指针。逻辑按预期运行，但文件大小没有增加，它保持不变，只是从位置 0 开始覆盖。我通过将增量值写入写入文件的内存进行测试，索引数据增加，但是它一直在有效地写入 fseek(0).

这是代码。

首先，我将文件打开为 GENERIC_WRITE 和 FILE_FLAG_OVERLAPPED，然后我创建了 8 个事件，每个事件对应我要编写的每一页。每页为 256KBytes。我最初只使用一个事件，但我想验证事件数不是问题。

void
FileWriter::open(string fn)
{
    cout << "Opening file " << fn << endl;
    m_file_handle = CreateFileA(
        fn.c_str(),
        GENERIC_WRITE,
        0,
        NULL,
        CREATE_ALWAYS,
        FILE_FLAG_OVERLAPPED,
        NULL);
    if (m_file_handle == INVALID_HANDLE_VALUE)
    {
        throw runtime_error("Unable to create FileWriter file handle");
    }
    for (size_t i = 0; i < MAX_OVERLAPPED_WRITES; ++i)
    {
        m_events[i] = CreateEvent(NULL, TRUE, FALSE, NULL);
    }
}

现在，在另一个线程中，只要缓冲区填满（8 x 256K 页），我就会调用此 queue_page 函数。 m_ov 是一个包含 8 个 OVERLAPPED 结构的数组。在此调用之前，m_pages[][] 充当环形缓冲区，并且 m_pages[m_head][] 前进，向下发送 m_pages[old page][] 写入磁盘。

void
FileWriter::queue_page(unsigned page, unsigned len)
{
    ZeroMemory(&m_ov[page], sizeof(OVERLAPPED));
    m_ov[page].hEvent = m_events[page];
    if (!WriteFile(
        m_file_handle,
        (LPCVOID)m_pages[page],
        len * sizeof(float),
        NULL,
        (LPOVERLAPPED)&m_ov[page]))
    {
        DWORD err = GetLastError();
        if (err != ERROR_IO_PENDING)
        {
            cout << "GetLastError() = " << err << endl;
            throw runtime_error("Failed to write overlapped");
        }
    }
}

异步写入和移动循环缓冲区尾指针的线程很简单：

void
FileWriter::wait(DWORD msec)
{
    DWORD ret = WaitForMultipleObjects(MAX_OVERLAPPED_WRITES, m_events, FALSE, msec);
    if (ret < MAXIMUM_WAIT_OBJECTS)
    {
        unsigned page =  ret - WAIT_OBJECT_0;
        if (page < MAX_OVERLAPPED_WRITES) {
            ResetEvent(m_events[page]);
        }
        cout << "Page write completed " << ret << " page=" << page << endl;
        m_tail = (m_tail + 1) & 0x7;
    }
}

经检查，环形缓冲区工作正常，没有溢出，但输出文件始终为 256KBytes。

不确定如何调试正在发生的事情，或者我错过了什么。

Answer 1

来自@RbMm的回答，字节访问文件要求调用者在重叠结构中设置Offset/OffsetHigh，根据这个：https://docs.microsoft.com/en-us/windows/win32/api/minwinbase/ns-minwinbase-overlapped

修复看起来像这样：

void
FileWriter::queue_page(unsigned page, unsigned len)
{
    ZeroMemory(&m_ov[page], sizeof(OVERLAPPED));
    m_ov[page].hEvent = m_events[page];
    m_ov[page].OffsetHigh = (m_file_offset >> 32) & 0xFFFF'FFFF;
    m_ov[page].Offset = m_file_offset & 0xFFFF'FFFF;
    if (!WriteFile(
        m_file_handle,
        (LPCVOID)m_pages[page],
        len * sizeof(float),
        NULL,
        (LPOVERLAPPED)&m_ov[page]))
    {
        DWORD err = GetLastError();
        if (err != ERROR_IO_PENDING)
        {
            cout << "GetLastError() = " << err << endl;
            throw runtime_error("Failed to write overlapped");
        }
    }
    m_file_offset += len * sizeof(float);
}

为什么 WriteFile 不推进此代码中后续写入的文件偏移量？

Why is WriteFile not advancing the file offset for subsequent writes in this code?

c++

windows

win32-process

overlapped-io