Multithreading

Hi all,

I'm new to multithreaded applications and would like to write a program that has threads that write to a single log file. I wondering, if thread A does an fopen and is in the middle of writing to the file, would thread B have an error if it tries to do an fopen to also write to the file? Or would it wait for thread A to finish?

Would appreciate any pointers on multithreading that you could provide.

Many thanks in advance!

Cheers,
Ben
bcheongAsked:
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Jase-CoderConnect With a Mentor Commented:
hi if you open the file with a then with b then both threads will write to the file causing unwanted info.
You could lock a file while it is open.

http://www.flipcode.com/tutorials/tut_mthread.shtml
http://www.hpctec.mcc.ac.uk/hpctec/courses/MT/MTcourse.html

here are a couple sites that introduce threads
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Jaime OlivaresConnect With a Mentor Software ArchitectCommented:
>if you open the file with a then with b then both threads will write to the file causing unwanted info.
That will occur if you are writting information at the same position, or appending at the same time. But if you write different register, then there will be not problems with the proper file access rights.
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PaulCaswellConnect With a Mentor Commented:
The answer is to have a completely separate 'Log' thread that you send messages to. It then writes the messages to the log file. You then get ONE owner of the resource, a much safer environment.

Paul
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sunny1907Connect With a Mentor Commented:
Another solution is synchronizing the access to the log file.

       
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bigjim2000Connect With a Mentor Commented:
The key with this type of synchronization problem is the "lock" keyword.  Here is a sample to demonstrate it's use (C#):


private System.IO.StreamWriter mLogger;
private const string LOG_FILE = @"c:\log.txt";
private bool bLog = true;

private void btnStart_Click(object sender, System.EventArgs e)
{
      this.mLogger = new System.IO.StreamWriter(LOG_FILE, true);
      System.Threading.Thread t;
      t = new System.Threading.Thread(new System.Threading.ThreadStart(LogThread1));
      t.Start();
      t = new System.Threading.Thread(new System.Threading.ThreadStart(LogThread2));
      t.Start();
      System.Threading.Thread.Sleep(10000);
      this.bLog = false;
      System.Threading.Thread.Sleep(1000);
      this.mLogger.Close();
}

private void LogThread1()
{
      while( this.bLog )
      {
            lock( this.mLogger )
            {
                  this.mLogger.WriteLine("I'm in thread 1!");
            }
            System.Threading.Thread.Sleep(500);
      }
}

private void LogThread2()
{
      while( this.bLog )
      {
            lock( this.mLogger )
            {
                  this.mLogger.WriteLine("I'm in thread 2!");
            }
            System.Threading.Thread.Sleep(500);
      }
}



The lock statement "locks" a resource used by multiple threads.  Try the same code without the lock brackets and you will nodoubt get more than a few errors ;-)
If you have any more questions, let me know :-)

Hope this helped!

-Eric
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