CreateFile2, WriteFile, and ReadFile: how can I enforce 16 byte alignment?

Question

I'm creating and writing a file with CreateFile2 and WriteFile, then later using readfile with the to read 16 bytes at a time into an __m128i and then performing simd operations on it. Works fine in debug mode, but throws the access denied (0xc0000005) error code in release mode. In my experience, that happens when I'm trying to shove non 16-byte-aligned stuff into 16-byte-aligned stuff. However, I'm unsure where the lack of 16-byte-alignment is first rearing its ugly head.

#define simd __m128i

Is it in the CreateFile2() call?

_CREATEFILE2_EXTENDED_PARAMETERS extend = { 0 };
extend.dwSize = sizeof(CREATEFILE2_EXTENDED_PARAMETERS);
extend.dwFileAttributes = FILE_ATTRIBUTE_NORMAL;
extend.dwFileFlags = /*FILE_FLAG_NO_BUFFERING |*/ FILE_FLAG_OVERLAPPED;
extend.dwSecurityQosFlags = SECURITY_ANONYMOUS;
extend.lpSecurityAttributes = nullptr;
extend.hTemplateFile = nullptr;

hMappedFile = CreateFile2(
    testFileName.c_str(),
    GENERIC_READ | GENERIC_WRITE,
    0,
    OPEN_ALWAYS,
    &extend);

...in the WriteFile() call?

_OVERLAPPED positionalData;
positionalData.Offset = 0;
positionalData.OffsetHigh = 0;
positionalData.hEvent = 0;

bool writeCheck = WriteFile(
    hMappedFile,
    &buffer[0],
    vSize,
    NULL,
    &positionalData);

...in the later ReadFile() call?

const simd* FileNodePointer(
    _In_ const uint32_t index) const throw()
{
    std::vector<simd> Node(8);

    _OVERLAPPED positionalData;
    positionalData.Offset = index;
    positionalData.OffsetHigh = 0;
    positionalData.hEvent = 0;

    ReadFile(
        hMappedFile,
        (LPVOID)&Node[0],
        128,
        NULL,
        &positionalData);

    return reinterpret_cast<const simd*>(&Node[0]);
}

How can I enforce 16-byte-alignment here?

Thanks!

Answer 1

TL;DR You have a classic "use after free" error.

---

None of these functions require 16 byte alignment. If buffering is enabled, they don't care about alignment at all, and if direct I/O is enabled, they require page alignment which is much more restrictive than 16 bytes.

If your data buffer is unaligned, it's because you created it that way. The file I/O is not moving your buffer in memory.

But your access violation is not caused by alignment problems at all, it is the dangling pointer you return from FileNodePointer:

return reinterpret_cast<const simd*>(&Node[0]);

That's a pointer into content of a vector with automatic lifetime, the vector destructor runs during the function return process and frees the memory containing the data you just read from the file.