SHCreateStreamOnFileEx 大于 2**32 字节的文件

Question

我正在使用 SHCreateStreamOnFileEx 获取文件的 IStream，但是当搜索指针的新位置为 2 时，它的 Read() 方法在超大文件上似乎表现不正常** 32 字节或更远的文件。

ISequentialStream::Read's documentation says:

This method adjusts the seek pointer by the actual number of bytes read.

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，这并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，符合预期。
Seek(2 ** 32 - 1, SEEK_SET, &pos)、Read(buf, 1, &bytesRead)、Seek(0, MOVE_CUR, &pos) → bytesRead == 1，但 pos == (2 ** 32 - 1) + 4096 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，我的应用程序无法运行！

我是不是“拿错了”？是否需要设置一些标志才能使 class 正常运行？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

这确实是 windows 错误。在几个 windows 版本上测试，包括最新的 SHCore.DLL 版本 10.0.14393.0 x64。重现的简单方法：

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* ); 并且可以确认此函数不是设计用于处理超过 4GB 大小的文件

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

(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 (100000FFF + (1 - 1000)) == 100000000 但

了解 SetFilePointer

If lpDistanceToMoveHigh is NULL and the new file position does not fit in a 32-bit value, the function fails and returns INVALID_SET_FILE_POINTER.

结果 SetFilePointer 失败（return INVALID_SET_FILE_POINTER）但 CFileStream 甚至不检查这个。然后它调用 SetFilePointerEx(hFile, 0, &newpos, FILE_CURRENT) 和 return 给你 newpos 仍然 100000FFF

SHCreateStreamOnFileEx 大于 2**32 字节的文件

SHCreateStreamOnFileEx on files larger than 2**32 bytes

c++

com

winapi