ReadFileEx方法读文件,为何回调函数没被触发?解决办法

#include<iostream>
#include<string>
#include<Windows.h>
using namespace std;

VOID CALLBACK FileIOCompletionRoutine(DWORD dwErrorCode,DWORD dwNumberOfBytesTransfered,LPOVERLAPPED lpOverlapped)
{

 cout<<"enter call back"<<endl;
}

int main(void)
{
    HANDLE hEvent=NULL;
    HANDLE hFile=NULL;
    const int buffSize=512;
         char *dataBuffer=new  char[buffSize];

        hFile = CreateFile(
                TEXT("a.txt"),     
                GENERIC_READ,        
                0,                 
                NULL,               
                OPEN_EXISTING,          
                FILE_FLAG_OVERLAPPED,  
                NULL
              );
                
  if(hFile==INVALID_HANDLE_VALUE)   return 0;

  hEvent=CreateEvent(NULL,true,false,NULL);

  if(hEvent==NULL) return 0;
   
    OVERLAPPED ovlap;
    ZeroMemory(&ovlap,sizeof(ovlap));
    ovlap.hEvent=hEvent;

       bool  fNotEnd=ReadFileEx
            (
                hFile,           
                dataBuffer,     
                buffSize,  
                &ovlap,
               (LPOVERLAPPED_COMPLETION_ROUTINE)  FileIOCompletionRoutine
            );


       while(true)
       {
       cout<<"loop"<<endl;
        Sleep(1000);
       }
   return 0;
}