How to use ReadFileScatter - c++

Question

I've been attempting to use ReadFileScatter today in my code (which sounds like exactly what I need), so far without a lot of luck. Google'ing the internet for what goes wrong doesn't give me much insight.
The documentation states:
The array must contain enough elements to store nNumberOfBytesToRead bytes of data, plus one element for the terminating NULL. For example, if there are 40 KB to be read and the page size is 4 KB, the array must have 11 elements that includes 10 for the data and one for the NULL.
Each buffer must be at least the size of a system memory page and must be aligned on a system memory page size boundary. The system reads one system memory page of data into each buffer.
The function stores the data in the buffers in sequential order. For example, it stores data into the first buffer, then into the second buffer, and so on until each buffer is filled and all the data is stored, or there are no more buffers.
So far I've been attempting to do just that. I allocated a bunch of bytes using VirtualAlloc (which ensures the page boundary constraint), add a terminator NULL to the list, ensure the data on disk is on the system boundary (and implicitly a disk sector size boundary) as well and issue the call.
Without further due, here's the minimum test case in C++:
// Setup: c:\tmp\test.dat is a file with at least 12K of stuff.
// I attempt to read page 2/3, e.g. offset [4096-4096+8192>
// TEST:
SYSTEM_INFO systemInfo;
GetSystemInfo(&systemInfo);
auto pageSize = systemInfo.dwPageSize;
std::cout << "Page size: "<< pageSize << std::endl;
// Allocate 2 pages that are aligned with one in the middle:
auto buffer = reinterpret_cast<char*>(VirtualAlloc(NULL, pageSize * 3, MEM_RESERVE | MEM_COMMIT, PAGE_READWRITE));
// Create read buffer:
std::vector<FILE_SEGMENT_ELEMENT> elements;
{
FILE_SEGMENT_ELEMENT element1;
element1.Buffer = buffer;
elements.push_back(element1);
}
{
FILE_SEGMENT_ELEMENT element2;
element2.Buffer = buffer + pageSize * 2;
elements.push_back(element2);
}
{
FILE_SEGMENT_ELEMENT terminator;
terminator.Buffer = NULL;
elements.push_back(terminator);
}
// [..] Physical sector size is normally checked as well. In my case it's 512 bytes,
// so I guess that's irrelevant here.
//
// Open file:
auto fileHandle = CreateFile(
"c:\\tmp\\test.dat",
GENERIC_READ | GENERIC_WRITE,
FILE_SHARE_READ,
NULL,
OPEN_ALWAYS,
FILE_FLAG_NO_BUFFERING | FILE_FLAG_WRITE_THROUGH,
NULL);
auto err = GetLastError();
if (err != ERROR_ALREADY_EXISTS && err != ERROR_SUCCESS)
{
throw std::exception(); // FIXME.
}
OVERLAPPED overlapped;
memset(&overlapped, 0, sizeof(OVERLAPPED));
LARGE_INTEGER tmp;
tmp.QuadPart = 4096; // Read from disk page 1
overlapped.Offset = tmp.LowPart;
overlapped.OffsetHigh = tmp.HighPart;
overlapped.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
auto succes = ReadFileScatter(fileHandle, elements.data(), pageSize * 2, NULL, &overlapped);
err = GetLastError();
if (!succes && err != ERROR_IO_PENDING && err != ERROR_SUCCESS)
{
throw std::exception(); // The call always ends up here with error 87: Invalid parameter
}
WaitForSingleObject(overlapped.hEvent, INFINITE);
std::cout << "Call succeeded!" << std::endl;
// FIXME: Proper exception handling.
// Clean up:
VirtualFree(buffer, pageSize * 3, MEM_DECOMMIT | MEM_RELEASE);
CloseHandle(overlapped.hEvent);
CloseHandle(fileHandle);
In the code, the error is noted with the comment // The call always ends up here with error 87: Invalid parameter. However, as far as I can see, I check all the boxes that they describe on MSDN... so...
What am I doing wrong here?