FileSystemWatcher cannot access file because its being used by another process

Question

Here my situation. I'm still pretty new at this. I'm generating some .txt files in my linux virtual box and using a c# winform application in windows to load the data into a chart. There are around 20 files or so. What I need to happen is to load the first file, display it on the chart, reload the file and display it on the chart, reload the file, display it on the chart, etc. My c code is writing over the same file every time with new values. So in the winforms when you are looking at the chart you will kind of see the data transforming as each new dataset is loaded. This should happen fairly quickly. Some of the new data loads but then I get the error about the file being used by another process. I've been reading about the error but I guess I still don't understand how to fix it. I need my c code that is generating the files to line up with the c# chart loading code so that it happens smoothly. In C I am also getting an error message that the file couldn't be opened for writing.

Here is what I'e tried so far. I tried waiting for a moment in the c code before opening the file for writing which didn't work. I've also tried just trying to open the file repeatedly until it opens without error, but it didn't work. My only other solution I could think of was to just create files like 1.txt 2.txt 3.txt... and then load them in succession. That might be the way I have to go. But I'm still new and don't actually know if I'm doing this the best way to start.

C code

for(i = 0; i < FILES; i++){
    fd = fopen(path, "w+");
    if( fd == NULL ){
        printf( "Could not open file for writing. Exiting...\n");
        exit(-1);
    }
    for(j = 0; j < ROW; j++){
        fprintf(fd, "%.4f", vector[i][j]);
        fprintf(fd, "%s", "\n");
    }
    fclose(fd);
}

C# Code

// Define the event handlers. 
private void OnChanged(object source, FileSystemEventArgs e)
{
    FinalList.Clear();
    chart4.Series[0].Points.Clear();
    loadData(FinalPath, FinalList, chart4);
 }

private void loadData(String path, List<double> list, System.Windows.Forms.DataVisualization.Charting.Chart chart)
{
    string line = null;
    double value = 0;        
    using (TextReader reader = File.OpenText(path))
    {
        line = reader.ReadLine();
        while (line != null)
        {
                value = System.Convert.ToDouble(line);
                list.Add(value);
                line = reader.ReadLine();
            }
        }

        for (int i = 0; i < ROW; i++)
        {
            chart.Series["Graph"].Points.AddXY(i + 1, list[i]);

        }
    }

Can you access Windows API functions from the C program? If so, you could use CreateEvent() to create a sync event, and access that same event from the C# program using a ManualResetEvent. Then you can do proper synchronisation between the programs. — Matthew Watson
– Matthew Watson, Commented Mar 23, 2013 at 19:35

corny · Accepted Answer · 2013-03-23 21:08:45Z

2

I guess using a (network?) filesystem is a bad way to do inter-process communication between different platforms. You will have synchronization problems if you continue this way. Just imagine the the moment file is only half-written by the C-code. Because of a full buffer, the Linux system might sync this file to disk. Now your windows C# application reads the file, but will only get half of it. This is not what you want, because your c# application will never know when the C program is done with writing the complete file.

I suggest you use a socket to transport the data between Linux and Windows.

On the linux side, you could you could use a very very cheap socket implementation. You could write your data to e.g., file.txt and use the system command in conjunction with netcat (nc on ubuntu) to publish the file.

cat file.txt | nc -l 1234

I seems like you can also download netcat for windows. You can fetch your file like this (replace 1.2.3.4 with the ip address of your Linux VM):

nc 1.2.3.4 1234 > file.txt

Make sure your windows can reach the Linux VM. With the VirtualBox default network mode (NAT), this is not possible, but you can easily change it.

edited Mar 23, 2013 at 21:08

answered Mar 23, 2013 at 19:48

corny

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2 Comments

Matthew The Terrible Over a year ago

Thanks for the suggestions. I'm going to look into this. I'm doing an internship with right now but nobody I work with is a computer science major so I can't ask any questions really. This is definitely not stuff we really talked about in any classes I've taken, but thank god for the interwebs.

corny Over a year ago

If you plan to continue in this area, make sure you learn about sockets. This cheap netcat-solution is not great but some starting point. nc stackoverflow.com 80 ;)

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