Write reliably to same file from different processes

Question

I did create a small C++ Tracing solution which works very well. Within one process all is well but when I open the output file from diferent processes the data gets not correctly written. I did open the file with FILE_SHARE_WRITE to be able to write to the file when it is already open. Then I did create a named mutex to ensure proper synchronisation between processes. But it appears that this is not enough. According to the MSDN this does work within one process but not between different processes. Next I tried to call FlushFileBuffers after every write while the mutex was still held but data was still distorted like this

The format is time process id/Thread id method enter/leave/severity namespace+method and then the message text.

10:29:42.994 7448/2236       }} Dll2.Test.fndll2 L1 -> Duration: 0.094s
10:29:43.040 7448/2236 {{       Dll2.DllMain L1
10:29:43.134 7448/2236 Info     Dll2.DllMain L1 Process detach
10:29:43.181 7448/2236       }} Dll2.DllMain L1 -> Duration: 0.141s
     }} Dll2.DllMain L1 -10:29:42.681 7448/2236 Info     Dll1.DllMain L1 Process attach
10:29:42.728 7448/2236       }} Dll1.DllMain L1 -10:29:42.744 2216/5510:29:42.775 7448/2236 {{       Dll1.Test.fndll1 10:210:29:42.822 7448/2236 Info     Dll1.Test.fndll1 10:29:42.837 2216/557610:29:42.853 7448/2236       }} Dll1.Test.fndll1 L110:29:42.884 2216/557610:29:43.306 7448/2236 {{       Dll1.DllMain L1
10:29:43.353 7448/2236 Info     Dll1.DllMain L1 Process detach
10:29:43.400 7448/2236       }} Dll1.DllMain L1 -> Duration: 0.094s

I have looked at FILE_FLAG_NO_BUFFERING but it has severe limitations and it seems not easy to use.

Does anybody know the right way to write synchronized to the same file without distoriting the output?

Yours,

Alois Kraus

Answer 1

I finally got it working. The trick was to Seek at the end of the file before very write. Otherwise I would overwrite about half of my output although I do lock with a cross process mutex before every write.

The code looks like this

__int64 Seek (HANDLE hf, __int64 distance, DWORD MoveMethod)  // from MSDN 
{
   LARGE_INTEGER li;
   li.QuadPart = distance;
   li.LowPart = SetFilePointer (hf, 
                                li.LowPart, 
                                &li.HighPart, 
                                MoveMethod);

   if (li.LowPart == INVALID_SET_FILE_POINTER && GetLastError() != NO_ERROR)
   {
      li.QuadPart = -1;
   }

   return li.QuadPart;
}



void WriteToFile(TCHAR *pData)
{
    DWORD dwWritten = 0;

    if( FALSE == ::WriteFile(_hFile, pData, _tcslen(pData)*sizeof(TCHAR), &dwWritten, NULL) )
    {
        _LastError = ::GetLastError();
        ASSERT__(FALSE);
    }
}

virtual void Write(TCHAR *pStr)
{
    if( _hWriteMutex != NULL )
    {
        DWORD res = ::WaitForSingleObject(_hWriteMutex, 120*1000);
        if( res == WAIT_OBJECT_0 || res == WAIT_ABANDONED ) // another process might have crashed while holding the mutex
        {
            // Ensure that we are really writing at the end of the file 
            __int64 fPos = Seek(_hFile, 0, FILE_END);
            WriteToFile(pStr);
            ::ReleaseMutex(_hWriteMutex);
        }
        else
        {
            ASSERT__(FALSE);
        }
    }
    else
    {
        WriteToFile(pStr);
    }
}