How to use OVERLAPPED to write data at specific offset (win32 programming) - c++

Question

(I'm not native English speaker so sorry for potential mistake :p)
I want to write a Windows tool that can accelerate big files copying just like FastCopy. But get stuck at the very first step: Write data to specific offset of a file via OVERLAPPED (with WriteFile).
Here is my code:
#define _CRT_SECURE_NO_WARNINGS
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <Windows.h>
#include <fileapi.h>
LPOVERLAPPED SetupOvl(DWORD OffsetHigh, DWORD Offset) {
printf("Ovl??: Got Offset %d\n", Offset);
LPOVERLAPPED Ovl = (LPOVERLAPPED)malloc(sizeof(OVERLAPPED));
Ovl->Internal = 0;
Ovl->InternalHigh = 0;
Ovl->OffsetHigh = OffsetHigh;
Ovl->Offset = Offset;
Ovl->Pointer = NULL;
Ovl->hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
return Ovl;
}
int main()
{
HANDLE hf,hf2;
void* buffer2 = (void*)malloc(2 << 16);
void* buffer = (void*)malloc(2 << 16);
int readed_number = 0, writed_number = 0;
DWORD written_number = 0;
DWORD wn1 = 0, wn2 = 0;
// OpenThread
DeleteFileW(L"C:\\Users\\Lev\\Downloads\\dst.txt");
hf = CreateFileW(
L"C:\\Users\\Lev\\Downloads\\src.txt",
GENERIC_READ,
FILE_SHARE_READ,
NULL,
OPEN_EXISTING,
FILE_ATTRIBUTE_NORMAL,
0);
hf2 = CreateFileW(
L"C:\\Users\\Lev\\Downloads\\dst.txt",
GENERIC_READ | GENERIC_WRITE,
0,
NULL,
CREATE_NEW,
FILE_ATTRIBUTE_NORMAL | FILE_FLAG_OVERLAPPED,
NULL);
// ReadThread
BOOL ret = ReadFile(
hf,
buffer,
2 << 15,
(LPDWORD)&readed_number,
NULL
);
SetFilePointer(hf2, 16, 0, FILE_BEGIN);
SetEndOfFile(hf2);
LPOVERLAPPED Ovl1 = SetupOvl(0, 2);
WriteFile(hf2, buffer, 6, NULL, Ovl1);
LPOVERLAPPED Ovl2 = SetupOvl(0, 12);
buffer = (void*)((char*)buffer + (unsigned int)(10));
WriteFile(hf2, buffer, 4, NULL, Ovl2);
ret = GetOverlappedResult(hf2, Ovl1, &wn1, TRUE);
ret = GetOverlappedResult(hf2, Ovl2, &wn2, TRUE);
printf("Write_A: Write %d Bytes!\n", wn1);
printf("Write_B: Write %d Bytes!\n", wn2);
free(Ovl1);
free(Ovl2);
CloseHandle(hf);
CloseHandle(hf2);
return 0;
}
Output from console:
Ovl??: Got Offset 2
Ovl??: Got Offset 8
Write_A: Write 4 Bytes!
Write_B: Write 5 Bytes!
Content of src.txt: 1234567890abcdefghijklmn
Content of dst.txt: abcd56 (6 charactors and lots of 0x00)
In the code I try to write "123456" at 3rd byte and "abcd" at 13th byte. But it didn't work. I have look up MSDN and some books but found not code examples I need.
By the way, which is the best way to make a multi-thread acceleration? Multi-thread reading or multi-thread writing? Is it possible to use OVERLAPPED I/O to implement multiple threads writing to the same file in parallel? Or does it just write sequentially?
Any helpful suggestion is appreciated!