Error while reading disk - c++

Question

I'm creating file deletion tool, so working with raw disk access. Have made some functions to read data.
A bit sorry because posting so much code, but not sure where is the real problem.
struct Extent
{
LONGLONG ClustersCount;
LARGE_INTEGER Lcn; //lcn - logical cluster number - the offset of a cluster from some arbitary point within volume
Extent() : ClustersCount(), Lcn()
{}
Extent(LONGLONG clustersCount, LARGE_INTEGER lcn) : ClustersCount(clustersCount), Lcn(lcn)
{}
};
typedef std::vector<Extent> ExtentsVector;
bool GetFileExtentPoints(const std::wstring& filePath, ExtentsVector& output)
{
output.clear();
DWORD err = ERROR_SUCCESS;
HANDLE file = CreateFile(filePath.c_str(), FILE_READ_ATTRIBUTES, (FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE), NULL, OPEN_EXISTING, 0, 0);
if (file != INVALID_HANDLE_VALUE)
{
STARTING_VCN_INPUT_BUFFER vcnStartBuffer = {};
RETRIEVAL_POINTERS_BUFFER pointsBuffer = {};
DWORD deviceIoControlDataritten = 0;
do
{
if (DeviceIoControl(file, FSCTL_GET_RETRIEVAL_POINTERS, &vcnStartBuffer, sizeof(vcnStartBuffer), &pointsBuffer, sizeof(pointsBuffer), &deviceIoControlDataritten, NULL))
{
Extent extent(pointsBuffer.Extents->NextVcn.QuadPart - pointsBuffer.StartingVcn.QuadPart, pointsBuffer.Extents[0].Lcn);
output.push_back(extent);
CloseHandle(file);
return true;
}
if (pointsBuffer.ExtentCount == 0) //small files could be stroed in master file table, so this part shouldn't always return false
{
CloseHandle(file);
return false;
}
Extent extent(pointsBuffer.Extents->NextVcn.QuadPart - pointsBuffer.StartingVcn.QuadPart, pointsBuffer.Extents[0].Lcn);
output.push_back(extent);
vcnStartBuffer.StartingVcn = pointsBuffer.Extents->NextVcn;
}
while (ERROR_MORE_DATA == GetLastError());
CloseHandle(file);
}
return false;
}
bool PermanentDeleteFile/*for now just read...*/(const std::wstring& filePath)
{
ExtentsVector extents;
if (!GetFileExtentPoints(filePath, extents))
return false;
DWORD sectorsPerCluster = 0;
DWORD bytesPerSector = 0;
LARGE_INTEGER fileSize = {};
HANDLE file = CreateFile(filePath.c_str(), FILE_READ_ATTRIBUTES, (FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE), NULL, OPEN_EXISTING, 0, 0);
if (file != INVALID_HANDLE_VALUE)
{
TCHAR drive[] = L"?:\\";
drive[0] = filePath[0];
if (!GetDiskFreeSpace(drive, &sectorsPerCluster, &bytesPerSector, NULL, NULL))
{
CloseHandle(file);
return false;
}
if (!GetFileSizeEx(file, &fileSize))
{
CloseHandle(file);
return false;
}
CloseHandle(file);
}
LONGLONG clusterSize = sectorsPerCluster * bytesPerSector;
LONGLONG clustersCount = fileSize.QuadPart / clusterSize + ((fileSize.QuadPart % clusterSize == 0) ? 0 : 1);
TCHAR rawDrive[] = L"\\\\.\\?:";
rawDrive[4] = filePath[0];
HANDLE driveHandle = CreateFile(rawDrive, /*GENERIC_WRITE*/GENERIC_READ/**/, FILE_SHARE_READ | FILE_SHARE_WRITE, NULL, OPEN_EXISTING, 0, 0);
if (driveHandle != INVALID_HANDLE_VALUE)
{
for (ExtentsVector::iterator it = extents.begin(); it != extents.end(); ++it)
{
LARGE_INTEGER distance = {};
distance.QuadPart = it->Lcn.QuadPart * clusterSize;
BOOL b = SetFilePointerEx(driveHandle, distance, NULL, FILE_BEGIN);
if (b)
{
std::string buffer;
buffer.resize(clusterSize * it->ClustersCount);
DWORD read = 0;
BOOL B = ReadFile(driveHandle, &buffer[0], clusterSize * it->ClustersCount, &read, NULL);
B = FALSE;//here I have breakpoint and for FAT type drives buffer contains invalid data
}
else
{
CloseHandle(driveHandle);
return false;
}
}
}
return false;
}
GetFileExtentPoints should collect clusters entries and cluster chain size for latter actions according to given file.
The place where the problem is seen is in PermanentDeleteFile I have marked that place with comment.
I have tried to read all my FAT32 drive and found that data I'm looking for is in drive but in other place so, as I understand FAT uses some kind of data offset. After that read this. As I think the offset is described by first 3 bytes in drive, but don't understand how to use it. In my case it is 0xEB 0x58 0x90. Want to ask, how to decode file system header into data offset.
BTW. I'm coding with C++ and using WinAPI.
EDIT: I have change my line into distance.QuadPart = it->Lcn.QuadPart * clusterSize + 0x01000000;, this works for FAT, however I don't want to have hardcoded constants, so can someone tell, if it is always this value, or it is calculated some way.