MapViewOfFile read from a given position - c++

Question

I need to read a large file in parts to a limited buffer. My code works, but always reads from the beginning. I think I need to use dwFileOffsetHigh somehow and dwFileOffsetLow, but I can't figure out how. Mapper_Winapi_Uptr is a unique_ptr with a custom deleter, if necessary I can lay out its code.
System: 64bit Win10.
const std::vector<BYTE>& ReadFile(size_t pos) {
memory = Mapper_Winapi_Uptr{ static_cast<BYTE*>(MapViewOfFile(mapping, FILE_MAP_READ, 0, 0, bufferSize)) };
std::memcpy(&data[0], memory.get(), bufferSize);
return data;
}

Answer 1

You are mapping the view at file offset 0, ignoring your pos parameter which is presumably the desired file offset.
MapViewOfFile() takes a 64bit offset as input, split into 32bit low and high values. A size_t may be a 32bit or 64bit type, depending on compiler and platform. You can put your desired offset into a ULARGE_INTEGER first, that will give you the low and high values you can then give to MapViewOfFile().
Note that the file offset you give to MapViewOfFile() must be a multiple of the system allocation granularity. See Creating a View Within a File on MSDN for details on how to handle that.
Try something like this:
SYSTEM_INFO SysInfo;
GetSystemInfo(&SysInfo);
DWORD SysGran = SysInfo.dwAllocationGranularity;
...
const std::vector<BYTE>& ReadFile(size_t pos)
{
size_t MapViewStart = (pos / SysGran) * SysGran;
DWORD MapViewSize = (pos % SysGran) + bufferSize;
DWORD ViewDelta = pos - MapViewStart;
ULARGE_INTEGER ulOffset;
ulOffset.QuadPart = MapViewStart;
memory = Mapper_Winapi_Uptr{ static_cast<BYTE*>(MapViewOfFile(mapping, FILE_MAP_READ, ulOffset.HighPart, ulOffset.LowPart, bufferSize)) };
if (!memory.get()) {
// error handling ...
}
std::memcpy(&data[0], &(memory.get())[ViewDelta], bufferSize);
return data;
}