Asynchronous readdirectorychangesw call blocks the thread from exiting

See more:

C++

multi-threading

WinAPI

INTRODUCTION:

I am writing small app that monitors certain directory for newly added files.

I would like to put monitoring code in a separate thread, so I can leave main thread free for other stuff, and cancel monitoring thread when I need to.

RELEVANT INFORMATION:

• I am using ReadDirectoryChangesW[^] to do the monitoring
• I am using raw WIN32 API for thread creation/synchronization
• I am trying to support Windows XP onward;

PROBLEM:

I was able to code everything properly, except one thing:

I can not exit monitoring thread properly, hence this post.

I am signaling an event object in main thread, wait for thread to exit and then do cleanup.

The problem lies in my usage of ReadDirectoryChangesW since everything works fine after I comment out that piece of code.

Once the event handle is signaled, ReadDirectoryChangesW blocks the thread which prevents it to "catch" the event and exit. If I add new file in the directory it "unblocks" ReadDirectoryChangesW, thread "catches" the event and exits.

In order to help further, I have made small MVCE below, that illustrates what I have stated so far.

MVCE:

#include <iostream>
#include <windows.h>
#include 

struct SThreadParams
{
    HANDLE hEvent;
    HANDLE hDir;
    int processDirectoryChanges(const char *buffer)
    {
        if (NULL == buffer) return -1;

        DWORD offset = 0;
        char fileName[MAX_PATH] = "";
        FILE_NOTIFY_INFORMATION *fni = NULL;

        do
        {
            fni = (FILE_NOTIFY_INFORMATION*)(&buffer[offset]);
            // since we do not use UNICODE, 
            // we must convert fni->FileName from UNICODE to multibyte
            int ret = ::WideCharToMultiByte(CP_ACP, 0, fni->FileName,
                fni->FileNameLength / sizeof(WCHAR),
                fileName, sizeof(fileName), NULL, NULL);

            switch (fni->Action)
            {
            case FILE_ACTION_ADDED:     
            {
                std::cout << "FILE_ACTION_ADDED " << fileName << std::endl;
            }
            break;
            case FILE_ACTION_REMOVED:
            {
                std::cout << "FILE_ACTION_REMOVED " << fileName << std::endl;
            }
            break;
            case FILE_ACTION_MODIFIED:
            {
                std::cout << "FILE_ACTION_MODIFIED " << fileName << std::endl;
            }
            break;
            case FILE_ACTION_RENAMED_OLD_NAME:
            {
                std::cout << "FILE_ACTION_RENAMED_OLD_NAME " << fileName << std::endl;
            }
            break;
            case FILE_ACTION_RENAMED_NEW_NAME:
            {
                std::cout << "FILE_ACTION_RENAMED_NEW_NAME " << fileName << std::endl;
            }
            break;
            default:
                break;
            }
            // clear string so we can reuse it
            ::memset(fileName, '\0', sizeof(fileName));
            // advance to next entry
            offset += fni->NextEntryOffset;

        } while (fni->NextEntryOffset != 0);

        return 0;
    }
};

DWORD WINAPI thread(LPVOID arg)
{
    SThreadParams p = *((SThreadParams *)arg);
    OVERLAPPED ovl = { 0 };
    DWORD bytesTransferred = 0, error = 0;
    char buffer[1024];

    if (NULL == (ovl.hEvent = ::CreateEvent(NULL, TRUE, FALSE, NULL)))
    {
        std::cout << "CreateEvent error = " << ::GetLastError() << std::endl;
        return ::GetLastError();
    };

    do {

        if (::ReadDirectoryChangesW(p.hDir, buffer, sizeof(buffer), FALSE,
            FILE_NOTIFY_CHANGE_FILE_NAME,
            NULL, &ovl, NULL))
        {
            if (::GetOverlappedResult(p.hDir, &ovl, &bytesTransferred, TRUE))
            {
                for (int i = 0; i < 5; ++i) std::cout << '=';
                std::cout << std::endl;

                if (-1 == p.processDirectoryChanges(buffer))
                    std::cout << "processDirectoryChanges error = " << std::endl;
            }
            else
            { 
                bytesTransferred = 0;
                std::cout << "GetOverlappedResult error = " << ::GetLastError() << std::endl;
            }

            if (0 == ::ResetEvent(ovl.hEvent))
            {
                std::cout << "ResetEvent error = " << ::GetLastError() << std::endl;
                ::CloseHandle(ovl.hEvent);
                return ::GetLastError();
            }
        }
        else
        {
            // we shall just output the error, and try again...
            std::cout << "ReadDirectoryChangesW error =  " << ::GetLastError() << std::endl;
        }

        error = ::WaitForSingleObject(p.hEvent, 2000);

    } while (WAIT_TIMEOUT == error);

    ::CloseHandle(ovl.hEvent);

    return 0;
}

int main()
{
    SThreadParams s;

    s.hDir = ::CreateFile(SOME_DIRECTORY,
            FILE_LIST_DIRECTORY, FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
            NULL, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, NULL);

    if (INVALID_HANDLE_VALUE == s.hDir)
    {
        std::cout << "CreateFile error = " << ::GetLastError() << std::endl;
        return 1;
    }

    s.hEvent = ::CreateEvent(NULL, TRUE, FALSE, NULL);

    if (NULL == s.hEvent)
    {
        std::cout << "CreateEvent error = " << ::GetLastError() << std::endl;
        ::CloseHandle(s.hDir);
        return 1;
    }

    HANDLE hThread = ::CreateThread(NULL, 0, thread, (LPVOID)&s, 0, NULL);

    if (NULL == hThread)
    {
        std::cout << "CreateThread error = " << ::GetLastError() << std::endl;
        ::CloseHandle(s.hDir);
        ::CloseHandle(s.hEvent);
        return 1;
    }

    std::cout << "press any key to close program..." << std::endl;
    std::cin.get();

    if (0 == ::CancelIoEx(s.hDir, NULL))
    {
        std::cout << "CancelIoEx error = " << ::GetLastError() << std::endl;
        ::CloseHandle(s.hDir);
        ::CloseHandle(s.hEvent);
        return 1;
    }

    if (0 == ::SetEvent(s.hEvent))
    {
        std::cout << "SetEvent error = " << ::GetLastError() << std::endl;
        ::CloseHandle(s.hDir);
        ::CloseHandle(s.hEvent);
        return 1;
    }

    // wait for thread to exit
    DWORD error = ::WaitForSingleObject(hThread, INFINITE);
    std::cout << "Thread exited with error code = " << error << std::endl;

    ::CloseHandle(s.hEvent);
    ::CloseHandle(s.hDir);
    ::CloseHandle(hThread);

    return 0;
}

What I have tried:

• I have moved out OVERLAPPED structure out of thread into structure that was passed to thread. Then I set OVERLAPPED.hEvent to forcibly "unblock" ReadDirectoryChangesW. This seems to work, but scares me because I think it is not safe/error prone since it is undocumented.
• I have tried to use completion routines but got no success since I am new with all this. I was able to receive notifications, but content of the buffer (the one filled with ReadDirectoryChangesW) was not read properly after the first pass. I am still trying to make this work on my own, but could use help.
• I could use I/o completion port, but since I will monitor only one directory I think this is a bit of an overkill. If I am mistaken, please instruct me how to use I/o completion port for my case, I would love to try them out.