Thread deadlock when using ReadDirectoryChangesW asynchronously

Question

I want to monitor in real-time the changes of several directories in one thread, so I decided to use ReadDirectoryChangesW() method asynchronously with GetQueuedCompletionStatus. Here is my code:

static DWORD WINAPI Routine( LPVOID lParam )
{
    FileSystemWatcher* obj = (FileSystemWatcher*)lParam;

    obj->hDir[0] = CreateFile(
        obj->WatchedDir[0].c_str(),
        GENERIC_READ|GENERIC_WRITE,
        FILE_SHARE_READ|FILE_SHARE_WRITE|FILE_SHARE_DELETE,
        NULL,
        OPEN_EXISTING,
        FILE_FLAG_BACKUP_SEMANTICS|FILE_FLAG_OVERLAPPED,
        NULL
    );

    obj->hDir[1] = CreateFile(
   obj->WatchedDir[1].c_str(),
   GENERIC_READ|GENERIC_WRITE,
   FILE_SHARE_READ|FILE_SHARE_WRITE|FILE_SHARE_DELETE,
   NULL,
   OPEN_EXISTING,
   FILE_FLAG_BACKUP_SEMANTICS|FILE_FLAG_OVERLAPPED,
   NULL
    );

    if( INVALID_HANDLE_VALUE == obj->hDir[0] || INVALID_HANDLE_VALUE == obj->hDir     [1] ) 
{
 return false;
}

obj->IOCPHandle[0] = CreateIoCompletionPort(obj->hDir[0], NULL, 0, 1);
obj->IOCPHandle[1] = CreateIoCompletionPort(obj->hDir[1], NULL, 0, 1);

if (obj->IOCPHandle[0] == INVALID_HANDLE_VALUE || obj->IOCPHandle[1] ==  INVALID_HANDLE_VALUE)
{
 return false;
}

    char buf[ 2*(sizeof(FILE_NOTIFY_INFORMATION)+MAX_PATH) ];
    FILE_NOTIFY_INFORMATION* pNotify=(FILE_NOTIFY_INFORMATION *)buf;
    DWORD BytesReturned;
    LPOVERLAPPED overLap = NULL;

    m_pFileNotifyInfo = pNotify;
    dirEvents[0] = CreateEvent(NULL, TRUE, FALSE, NULL);
    dirEvents[1] = CreateEvent(NULL, TRUE, FALSE, NULL);*/

    while(true)
    {

    BOOL success = ReadDirectoryChangesW( obj->hDir[0],
 pNotify,
 sizeof(buf),
 true,
 FILE_NOTIFY_CHANGE_FILE_NAME|FILE_NOTIFY_CHANGE_DIR_NAME,
 &BytesReturned,
 overLap,
 NULL );
    BOOL success1 = ReadDirectoryChangesW( obj->hDir[1],
 pNotify,
 sizeof(buf),
 true,
 FILE_NOTIFY_CHANGE_FILE_NAME|FILE_NOTIFY_CHANGE_DIR_NAME,
 &BytesReturned,
 overLap, 
                 NULL );

    LPOVERLAPPED theOverLap;
    PULONG_PTR lpCompKey = NULL;
    if (GetQueuedCompletionStatus(obj->IOCPHandle[0], &BytesReturned, lpCompKey, &overLap, 100))
    {
 cout<<"First Dir Changed"<<endl;
    }

    if (GetQueuedCompletionStatus(obj->IOCPHandle[1], &BytesReturned, lpCompKey, &overLap, 100))
    {
 cout<<"Second Dir Changed"<<endl;
    }

    cout<<"Nothing happened yet"<<endl;

       }

    return 0;
}

Here when I run my code, my thread blocks for no reason, no matter what value I set to the last parameter of GetQueuedCompletionStatus() function. I don't know why I encountered this problem. Can anyone tell my the reason? Many thanks!

Answer 1

1. You have to specify a valid OVERLAPPED structure for asynchronous I/O to work. You are using NULL (LPOVERLAPPED overLap = NULL;).

2. Concurrent I/O operations must use separate OVERLAPPED structures, not just one.

3. Are you sure you need IOCP at all? I would just fill in hEvent in the OVERLAPPED structures and do a WaitForMultipleObjects on these events.