Why does multi threading mess up my file watcher?

Question

I am trying to make a file watcher that watches multiple directories simultaneously and i found a function that watches one directory and it works well. So I tried to create multiple threads with the same function and just give them the different paths of the directories as parameters. And it still works, but not perfectly. It does provide me with the first change in one directory, but then it gets stuck for that one and in the other directories the changes i make are printed as if they wore made in the first directory. I think the threads are getting messed up and I don`t know why. Here is the function

void watch_directory(LPCSTR path, int i)
{

char buf[2048];
DWORD nRet;
BOOL result = TRUE;
char filename[MAX_PATH];
DirInfo[0].hDir = CreateFile(path, GENERIC_READ | FILE_LIST_DIRECTORY,
    FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
    NULL, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS | FILE_FLAG_OVERLAPPED,
    NULL);

if (DirInfo[0].hDir == INVALID_HANDLE_VALUE)
{
    return; //cannot open folder
}

lstrcpy(DirInfo[0].lpszDirName, path);
OVERLAPPED PollingOverlap;

FILE_NOTIFY_INFORMATION* pNotify;
int offset;
PollingOverlap.OffsetHigh = 0;
PollingOverlap.hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
while (result)
{
    result = ReadDirectoryChangesW(
        DirInfo[0].hDir,// handle to the directory to be watched
        &buf,// pointer to the buffer to receive the read results
        sizeof(buf),// length of lpBuffer
        TRUE,// flag for monitoring directory or directory tree
        FILE_NOTIFY_CHANGE_FILE_NAME |
        FILE_NOTIFY_CHANGE_DIR_NAME |
        FILE_NOTIFY_CHANGE_SIZE,
        &nRet,// number of bytes returned
        &PollingOverlap,// pointer to structure needed for overlapped I/O
        NULL);

    WaitForSingleObject(PollingOverlap.hEvent, INFINITE);
    offset = 0;
    int rename = 0;
    char oldName[260];
    char newName[260];
    do
    {
        pNotify = (FILE_NOTIFY_INFORMATION*)((char*)buf + offset);
        strcpy(filename, "");
        int filenamelen = WideCharToMultiByte(CP_ACP, 0, pNotify->FileName, pNotify->FileNameLength / 2, filename, sizeof(filename), NULL, NULL);
        filename[pNotify->FileNameLength / 2] = '\0';
        cout << pNotify->Action << i << filename << endl;
        offset += pNotify->NextEntryOffset;

    } while (pNotify->NextEntryOffset); //(offset != 0);
}

/*label:*/  CloseHandle(DirInfo[0].hDir);


}

And here is the way I create the threads:

    vector <string> dirs;
vector <thread> threads;
dirs.push_back("F:\\Arhitecturi\\test0");
dirs.push_back("F:\\Arhitecturi\\test1");

for (int i = 0; i < dirs.size(); i++)
{

    threads.push_back(thread(watch_directory, dirs[i].c_str(), i));
}
for (int i = 0; i < threads.size(); i++)
{
    threads[i].join();

}

for what you create several threads when not need create any thread at all ? for what you open file for asynchronous i/o when you not use it ? — RbMm, May 07 '20 at 18:13
I did not write the first function, I found it online and I just used it because it worked. I don`t know what it actually does. And I thought i needed to use the threads to watch multiple directories simultaneously — Ciprian Florin, May 07 '20 at 18:37
*I don`t know what it actually does.* i personally never understand how can use code which we not understand. for watch multiple directories need use asynchronous i/o instead create several threads — RbMm, May 07 '20 at 18:40
it looks like `DirInfo` is some kind of global/shared variable so using threads wont work. Please create a mcve and people will be able to help more. https://stackoverflow.com/help/minimal-reproducible-example — Mike Vine, May 07 '20 at 18:42
If you don't understand what the code does, then your first step should be to try to understand it. You assumed that responsibility when you copied it. It's rather presumptuous to expect others to do this work for you. — Raymond Chen, May 07 '20 at 21:43
The above sample does not reproduce the problem you said, can you provide a [complete, compileable and reproducible](https://stackoverflow.com/help/minimal-reproducible-example) sample? — Drake Wu, May 11 '20 at 07:45

Why does multi threading mess up my file watcher?

0 Answers0