Question

我使用SHCreateStreamOnFileEx为文件获取了IStream，但当搜索指针的新位置为2时，其Read()方法似乎在极大文件上行为异常32个字节或更远的文件。

ISequentialStream::Read's documentation says：

此方法通过读取的实际字节数来调整搜索指针。

这与我所知道的所有平台上的read(2)和fread(3)行为相同。

但是对于这些流，这不是我在某些情况下看到的实际行为：

Seek(2 ** 32 - 2, SEEK_SET, &pos)，Read(buf, 1, &bytesRead)，Seek(0, MOVE_CUR, &pos)→bytesRead == 1和pos == 2 ** 32 - 1。
Seek(2 ** 32 - 1, SEEK_SET, &pos)，Read(buf, 1, &bytesRead)，Seek(0, MOVE_CUR, &pos)→bytesRead == 1，但pos == (2 ** 32 - 1) + 4096，这是不正确的。这意味着任何后续读取（没有另一个Seek来修复光标位置）读取错误的数据，我的应用程序不起作用！

我“抱错了”？是否需要设置一些标志才能使此类行为正常？或者这是Shlwapi.dll中的错误？

下面的代码为我重现了这个问题。（设置OFFSET = WORKS以查看成功案例。）

#include "stdafx.h"

static const int64_t TWO_THIRTY_TWO = 4294967296LL;
static const int64_t WORKS = TWO_THIRTY_TWO - 2LL;
static const int64_t FAILS = TWO_THIRTY_TWO - 1LL;
static const int64_t OFFSET = FAILS;

static void checkPosition(CComPtr< IStream > fileStream, ULONGLONG expectedPosition)
{
    LARGE_INTEGER move;
    ULARGE_INTEGER newPosition;

    move.QuadPart = 0;
    HRESULT hr = fileStream->Seek(move, SEEK_CUR, &newPosition);
    ASSERT(SUCCEEDED(hr));
    ULONGLONG error = newPosition.QuadPart - expectedPosition;
    ASSERT(error == 0);
}

int main()
{
    const wchar_t *path = /* path to a file larger than 2**32 bytes */ L"C:\\users\\wjt\\Desktop\\eos-eos3.1-amd64-amd64.170216-122002.base.img";
    CComPtr< IStream > fileStream;

    HRESULT hr;
    hr = SHCreateStreamOnFileEx(path, STGM_READ, FILE_ATTRIBUTE_NORMAL, FALSE, NULL, &fileStream);
    ASSERT(SUCCEEDED(hr));

    LARGE_INTEGER move;
    ULARGE_INTEGER newPosition;

    // Advance
    move.QuadPart = OFFSET;
    hr = fileStream->Seek(move, SEEK_SET, &newPosition);
    ASSERT(SUCCEEDED(hr));
    ASSERT(newPosition.QuadPart == OFFSET);

    // Check position
    checkPosition(fileStream, OFFSET);

    // Read
    char buf[1];
    ULONG bytesRead = 0;
    hr = fileStream->Read(buf, 1, &bytesRead);
    ASSERT(SUCCEEDED(hr));
    ASSERT(bytesRead == 1);

    // Check position: this assertion fails if the Read() call moves the cursor
    // across the 2**32 byte boundary
    checkPosition(fileStream, OFFSET + 1);

    return 0;
}

Answer 1

这是真正的Windows bug。在几个Windows版本上测试，包括最新的SHCore.DLL版本10.0.14393.0 x64。简单的重现方式：

void BugDemo(PCWSTR path)
{
    // FILE_FLAG_DELETE_ON_CLOSE !
    HANDLE hFile = CreateFile(path, FILE_GENERIC_WRITE, FILE_SHARE_READ|FILE_SHARE_DELETE, 0, 
        CREATE_NEW, FILE_ATTRIBUTE_TEMPORARY|FILE_FLAG_DELETE_ON_CLOSE, 0);

    if (hFile != INVALID_HANDLE_VALUE)
    {
        ULONG dwBytesRet;
        // i not want really take disk space
        if (DeviceIoControl(hFile, FSCTL_SET_SPARSE, NULL, 0, NULL, 0, &dwBytesRet, NULL))
        {
            static FILE_END_OF_FILE_INFO eof = { 0, 2 };// 8GB
            if (SetFileInformationByHandle(hFile, FileEndOfFileInfo, &eof, sizeof(eof)))
            {
                IStream* pstm;
                if (!SHCreateStreamOnFileEx(path, STGM_READ|STGM_SHARE_DENY_NONE, 0,FALSE, NULL, &pstm))
                {
                    LARGE_INTEGER pos = { 0xffffffff };
                    ULARGE_INTEGER newpos;
                    if (!pstm->Seek(pos, STREAM_SEEK_SET, &newpos) && !pstm->Read(&newpos, 1, &dwBytesRet))
                    {
                        pos.QuadPart = 0;
                        if (!pstm->Seek(pos, STREAM_SEEK_CUR, &newpos))
                        {
                            DbgPrint("newpos={%I64x}\n", newpos.QuadPart);//newpos={100000fff}
                        }
                    }
                    pstm->Release();
                }
            }
        }

        // close and delete
        CloseHandle(hFile);
    }
}

void BugDemo()
{
    WCHAR path[MAX_PATH];
    if (ULONG len = GetTempPath(RTL_NUMBER_OF(path), path))
    {
        if (len + 16 < MAX_PATH)
        {
            FILETIME ft;
            GetSystemTimeAsFileTime(&ft);
            swprintf(path + len, L"%08x%08x", ~ft.dwLowDateTime, ft.dwHighDateTime);
            BugDemo(path);
        }
    }
}

我在调试器下跟踪virtual long CFileStream::Seek(LARGE_INTEGER, ULONG, ULARGE_INTEGER* );并且可以确认此功能不适用于超过4GB大小的文件

如果更确切地说，为什么100000FFF偏移 - CFileStream使用内部缓冲区来读取1000字节大小。当你要求从FFFFFFFF偏移量读取1个字节时 - 它实际上将1000个字节读取到缓冲区，文件偏移量变为100000FFF。然后，当您拨打Seek(0, STREAM_SEEK_CUR, &newpos) - CFileStream致电SetFilePointer(hFile, 1-1000, 0/*lpDistanceToMoveHigh*/, FILE_CURRENT)

时

（1这是缓冲区中的内部位置，因为我们读取1个字节减去缓冲区大小1000）。如果不考虑溢出可以是(100000FFF + (1 - 1000)) == 100000000但

阅读SetFilePointer

如果 lpDistanceToMoveHigh NULL 且新文件位置不合适在32位值中，函数失败并返回的 INVALID_SET_FILE_POINTER

结果SetFilePointer失败（返回INVALID_SET_FILE_POINTER），但CFileStream甚至没有检查。然后它会调用SetFilePointerEx(hFile, 0, &newpos, FILE_CURRENT)并返回到仍然100000FFF
的newpos

对大于2 ** 32字节的文件进行SHCreateStreamOnFileEx

1 个答案: