C++ Copy Raw std::bytes to std::vector<std::byte>

Question

I have an FILE_NOTIFY_INFORMATION struct which i have filled like this:

FILE_NOTIFY_INFORMATION* fni = new FILE_NOTIFY_INFORMATION;
fni->Action = 1;
wcscpy_s(fni->FileName,12, L"example.txt");
fni->FileNameLength = 12;
fni->NextEntryOffset = 0;

I have then castet this Struct to an std::byte*.

auto fni_as_byte = reinterpret_cast<std::byte*>(fni);

Now i want to put this fni_as_byte into an vector of std::vector<std::byte>. Because i need this for testing purpose.

Normally you receive the FILE_NOTIFY_INFORMATION for example from the ReadDirectoryChangesW function. And it's called like this:

std::vector<std::byte> notifyBuffer(1024);
res = ReadDirectoryChangesW(handle, notifyBuffer.data(), static_cast<DWORD>(notifyBuffer.size()), false, FILE_NOTIFY_CHANGE_FILE_NAME, nullptr, &overlapped, nullptr);

So how can i manually copy the castet FILE_NOTIFY_INFORMATION into the std::vector<std::byte>?

Answer 1

Your code is corrupting memory, as the actual size of FILE_NOTIFY_INFORMATION is variable length. See Why do some structures end with an array of size 1?. You are not allocating enough memory to hold the string you are copying into the FileName. To allocate your FILE_NOTIFY_INFORMATION correctly, you would need to do something more like this instead:

FILE_NOTIFY_INFORMATION* fni = (FILE_NOTIFY_INFORMATION*) new std::byte[offsetof(FILE_NOTIFY_INFORMATION, FileName[12])];
fni->Action = 1;
wcscpy_s(fni->FileName, 12, L"example.txt");
fni->FileNameLength = 11 * sizeof(WCHAR); // <— expressed in BYTEs, not including the null terminator!
fni->NextEntryOffset = 0;
...
delete (std::byte*) fni;

That being said, to copy the FILE_NOTIFY_INFORMATION into a std::vector, you can do it like this:

FILE_NOTIFY_INFORMATION* fni = ...;
// fill fni as needed...
std::vector<std::byte> buffer(offsetof(FILE_NOTIFY_INFORMATION, FileName) + fni->FileNameLength);
std::memcpy(buffer.data(), fni, buffer.size());
// use buffer as needed...

Alternatively, just use the std::vector up front as the memory for the original FILE_NOTIFY_INFORMATION:

std::vector<std::byte> buffer(offsetof(FILE_NOTIFY_INFORMATION, FileName[12]));
auto fni = reinterpret_cast<FILE_NOTIFY_INFORMATION*>(buffer.data());
// fill fni as needed....
// use buffer as needed...