Simple file lock works on unices, not on Windows

QED · #1 01-Feb-2005, 12:54

I am using a basic file-locking mechanism that works fine on Unix/Linux platform, but fails on Windows 2000 platform. The problem on Windows is that when a process is finished with the data file, it is unable to delete the lock file it created. Apparently, upon closing the lock file, the read-only permission is set for some reason. Some code below (methods inline for simplicity):

CPP / C++ / C Code:

class FileLock

{

public:

  static bool acquire(std::string const& filename)

  {

    int nAttempt(0), fd(open(filename.c_str(), O_WRONLY | O_CREAT | O_EXCL));

    while (fd < 0)

    {

      ++nAttempt;

      if (errno == EEXIST && nAttempt <= MAX_ATTEMPTS) {

#ifdef WIN32

        Sleep(10);

#else // assume POSIX-compliant platform

        usleep(10);

#endif

        fd = open(filename.c_str(), O_WRONLY | O_CREAT | O_EXCL);

      } else {

        return false; // unable to acquire lock

      }

    }

    close(fd);

    FILE_LOCK._locks.insert(filename);

    return true;

  }


  static bool release(std::string const& filename)

  {

    if (unlink(filename.c_str()) == 0)

    {

      FILE_LOCK._locks.erase(filename);

      return true;

    }

    return false;

  }


  ~FileLock()

  {

    for (std::set<std::string>::iterator i(_locks.begin()); i != _locks.end(); ++i)

    {

      release(*i);

    }

  }

private:

  static size_t const MAX_ATTEMPTS;

  static FileLock FILE_LOCK;

  std::set<std::string> _locks;

  FileLock() : _locks() {}

};

size_t const FileLock::MAX_ATTEMPTS = 1000000; // attempt at most 1M aqcuires

FileLock FileLock::FILE_LOCK = FileLock();

Any help is greatly appreciated.

P.S. Thanks to the mod/whoever edited my post. I was just coming back to fix the code tags after finding out the proper tag for C/C++ code.

QED · #2 03-Feb-2005, 07:25

Thanks to anyone who had viewed my posted and devited any time to thinking about my problem. It turns out there was a silly oversight on my part.

I had overlooked the third, optional parameter to the POSIX open function, namely the permissions mode. Apparently, the security policy on Windows NT/2000/XP makes read-only the default mode, while the Unix/Linux test boxes I used did not. The only change required to fix this is in my acquire method:

CPP / C++ / C Code:

  static bool acquire(std::string const& filename)
  {
    int nAttempt(0), fd(open(filename.c_str(), O_WRONLY | O_CREAT | O_EXCL, S_IWRITE));

    while (fd < 0) {
      ++nAttempt;

      if (errno == EEXIST && nAttempt <= MAX_ATTEMPTS) {

#ifdef WIN32
        Sleep(10);
#else // assume POSIX-compliant platform
        usleep(10);
#endif

        fd = open(filename.c_str(), O_WRONLY | O_CREAT | O_EXCL, S_IWRITE);

      } else {
        return false; // unable to acquire lock
      }
    }
    close(fd);

    FILE_LOCK._locks.insert(filename);

    return true;
  }

Note: The sleep times are not what I intended in my first code sample, so I didn't correct them here because I wanted the only change to be the relevant fix. It should read Sleep(1) for Windows and usleep(1000) otherwise; i.e., 1 millisecond.

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Simple file lock works on unices, not on Windows

Problem solved.