C语言如何使用OVERLAPPED进行重叠IO？

OVERLAPPED结构体就没有预留用户自定义参数，通过GetQueuedCompletionStatus得到OVERLAPPED*之后，怎么得到与其对应的用户参数？举个例子

 HANDLE hCompletionPort = NULL;
char buf1[4 * 1024];
…………
char bufn[4 * 1024];
void f1()
{
    hCompletionPort = CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0, 0);
     
    hFile1 = CreateFile…………WithOverlappedFlag
    hFile2 …………
    …………
    hFilen …………
     
    CreateIoCompletionPort(hCompletionPort, hFile1, 0, 0);
    CreateIoCompletionPort(hCompletionPort, hFile2, 0, 0);
    …………
    CreateIoCompletionPort(hCompletionPort, hFilen, 0, 0);
     
    OVERLAPPED* op1 = new OVERLAPPED;
    ReadFileEx(hFile1, buf1, sizeof(buf1), op1, NULL);
    OVERLAPPED* op2 = new OVERLAPPED;
    ReadFileEx(hFile2, buf2, sizeof(buf2), op2, NULL);
    …………
    OVERLAPPED* op3 = new OVERLAPPED;
    ReadFileEx(hFilen, bufn, sizeof(bufn), op3, NULL);
}
void f2()
{
    DWORD dwNumberOfBytes = 0;
    ULONG_PTR key = 0;
    OVERLAPPED* op = NULL;
    GetQueuedCompletionStatus(hCompletionPort, &dwNumberOfBytes, &key, &op, INFINITE);//1
}

注释1（//1）处得到op，但是此op是读的哪个文件呢？

http://www.cnblogs.com/skyofbitbit/p/3650140.html