Understanding Asynchronous / Overlapped IO - c++

Question

I am learning about Asynchronous / Overlapped IO in windows. I have written the following code, but it doesn't compile. Where is my mistake? I don't know why we need to call something as FileIoCompletionRoutine and how should I define it?
#include <windows.h>
#include <iostream>
VOID WINAPI FileIOCompletionRoutine(DWORD, DWORD, LPOVERLAPPED);
HANDLE g_HandleEvent;
wchar_t string_data[] = L"Garbage data is absloute thing";
int main(int argc, char* argv[])
{
g_HandleEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
auto file_name = L"Cayot.txt";
auto handle_file = CreateFile(file_name, GENERIC_WRITE, FILE_SHARE_WRITE, 0, CREATE_ALWAYS, FILE_FLAG_OVERLAPPED, 0);
if (handle_file == INVALID_HANDLE_VALUE)
{
std::cout << "File creation is failed." << std::endl;
return -1;
}
OVERLAPPED overlapped_instance = { 0 };
overlapped_instance.hEvent = g_HandleEvent;
WriteFileEx(handle_file, string_data, sizeof(string_data), &overlapped_instance, FileIOCompletionRoutine);
SleepEx(INFINITE, TRUE);
return 0;
}
Error compiler:
error LNK2001: unresolved external symbol "void __stdcall
FileIOCompletionRoutine(unsigned long,unsigned long,struct _OVERLAPPED
*)" (?FileIOCompletionRoutine##YGXKKPAU_OVERLAPPED###Z)

Answer 1

You have merely forward-declared the FileIOCompletionRoutine() function, which satisfies the compiler, but you have not actually implemented the function, so the linker fails to find it.
#include <windows.h>
#include <iostream>
VOID WINAPI FileIOCompletionRoutine(DWORD, DWORD, LPOVERLAPPED);
HANDLE g_HandleEvent;
wchar_t string_data[] = L"Garbage data is absloute thing";
int main(int argc, char* argv[])
{
g_HandleEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
auto file_name = L"Cayot.txt";
auto handle_file = CreateFile(file_name, GENERIC_WRITE, FILE_SHARE_WRITE, 0, CREATE_ALWAYS, FILE_FLAG_OVERLAPPED, 0);
if (handle_file == INVALID_HANDLE_VALUE)
{
std::cout << "File creation is failed." << std::endl;
return -1;
}
OVERLAPPED overlapped_instance = { 0 };
overlapped_instance.hEvent = g_HandleEvent;
WriteFileEx(handle_file, string_data, sizeof(string_data), &overlapped_instance, FileIOCompletionRoutine);
SleepEx(INFINITE, TRUE);
return 0;
}
VOID WINAPI FileIOCompletionRoutine(DWORD dwErrorCode, DWORD dwNumberOfBytesTransfered, LPOVERLAPPED lpOverlapped)
{
// Do something here...
}