WriteFile returns invalid handle - c++

Question

I am attempting to use Windows WriteFile API as follows:
class FileWriter
{
public:
FileWriter(const std::string & path);
~FileWriter();
bool write(unsigned char * buffer, int num_bytes, unsigned __int64 offset);
private:
HANDLE file_h;
};
The implementation is simple as:
FileWriter::FileWriter(const std::string & path)
{
file_h = CreateFileA(path.c_str(), GENERIC_WRITE,
FILE_SHARE_READ, 0, CREATE_ALWAYS,
FILE_ATTRIBUTE_NORMAL | FILE_FLAG_OVERLAPPED, 0);
if (file_h == INVALID_HANDLE_VALUE) {
throw std::runtime_error("Could not open the output file");
}
}
FileWriter::~FileWriter()
{
if (file_h != INVALID_HANDLE_VALUE) {
CloseHandle(file_h);
}
}
bool FileWriter::write(unsigned char * buffer, int num_bytes,
unsigned __int64 offset)
{
if (file_h != INVALID_HANDLE_VALUE && buffer) {
OVERLAPPED overlapped;
overlapped.Offset = offset & 0xFFFFFFFF;
overlapped.OffsetHigh = offset >> 32;
BOOL e = WriteFile(file_h, buffer, num_bytes,
0, &overlapped);
DWORD c = GetLastError();
LPSTR messageBuffer = nullptr;
size_t size = FormatMessageA(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS,
NULL, c, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT), (LPSTR)&messageBuffer, 0, NULL);
std::string message(messageBuffer, size);
//Free the buffer.
LocalFree(messageBuffer);
if (e == FALSE && c == ERROR_IO_PENDING) {
return true;
}
else {
if ((e == TRUE) && (c == ERROR_SUCCESS))
return true;
}
}
return false;
}
This always fails with The handle is invalid error. However, the file handle is always valid. I even check for it. So, I am not sure what I am doing wrong.