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Sharing a single streamwriter between multiple threads?

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Thursday, March 30, 2006 12:16 AM

Is it possible to open a file in 'AppendText' mode and then use a single instance of a StreamWriter between multiple threads? I am concerned if two threads call the 'WriteLine' method of StreamWriter at the same time, then what will happen and will it create a problem?

 

All replies (8)

Thursday, March 30, 2006 2:51 AM

Use a singleton design (static volatile StreamWriter writer;), and use a padlock to ensure no other threads touch it.  It does create locking contentions, but it's better than getting serious exceptions.

Thursday, March 30, 2006 8:27 AM

A couple of questions for you:

  1. Doesn't a static member automatically become thread-safe i.e. only one thread can use it at a time, and if this static member when used in a static method will also make the static method thread-safe? I thought static members are automatically thread-safe without us doing anything.
  2. Also, when you say padlock, do you mean the 'lock (obj) {    }' construct?

The reason for my initial post was that I was writing an application that writes to a log file, and since multiple threads could try to write to a single log file there could be a problem. I was opening the file only once per application session and using only one streamwriter object for the entire application, instead of each thread opening the log file and getting a new stream writer to write to the file. Any suggestions on this would be helpful?

Thanks

 

Thursday, March 30, 2006 5:46 PM

No.  If an object is declared static, that does not automatically make it thread safe.

The volatile keyword indicates that a field can be modified in the program by something such as the operating system, the hardware, or a concurrently executing thread.

volatile declaration

where:

declaration
The declaration of a field.

Remarks

The system always reads the current value of a volatile object at the point it is requested, even if the previous instruction asked for a value from the same object. Also, the value of the object is written immediately on assignment.

The volatile modifier is usually used for a field that is accessed by multiple threads without using the lock statement to serialize access. Using the volatile modifier ensures that one thread retrieves the most up-to-date value written by another thread.

The type of a field marked as volatile is restricted to the following types:

  • Any reference type.
  • Any pointer type (in an unsafe context).
  • The types sbyte, byte, short, ushort, int, uint, char, float, bool.
  • An enum type with an enum base type of byte, sbyte, short, ushort, int, or uint.

For more information on volatile, see 10.4.3 Volatile fields.

Example

The following sample shows how to declare a public field variable as volatile.

// csharp_volatile.cs
class Test
{
   public volatile int i;
   
   Test(int _i)
   {
      i = _i;
   }
   public static void Main()
   {
      
   }
}

by using volatile in declaring an object, you can ensure it's only accessed by one thread at a time.  Another means of doing this would be to create a Queue<String> object that will be Enqueued by multiple threads.  Then have another thread dedicated to dequeue, and append to that file.

Thursday, April 6, 2006 11:04 AM

  1. Doesn't a static member automatically become thread-safe i.e. only one thread can use it at a time, and if this static member when used in a static method will also make the static method thread-safe? I thought static members are automatically thread-safe without us doing anything.

I have a couple articles on that topic that might be of some help:

http://www.odetocode.com/Articles/313.aspx

http://www.odetocode.com/Articles/314.aspx

Hope you enjoy them,

Thursday, April 6, 2006 11:05 AM

 

by using volatile in declaring an object, you can ensure it's only accessed by one thread at a time. 

A volatile field can be accessed by multiple threads concurrently. Using volatile isn't a replacement for using a lock - it's for preserving the ordering of instructions in a lock-free scenario.

Thursday, April 6, 2006 6:48 PM

by using volatile in declaring an object, you can ensure it's only accessed by one thread at a time. 

A volatile field can be accessed by multiple threads concurrently. Using volatile isn't a replacement for using a lock - it's for preserving the ordering of instructions in a lock-free scenario.

You're right if the it's a reference type (class, struct, or array), but for value types with atomic operations, it won't allow multiple threads to access it.

The volatile modifier is usually used for a field that is accessed by multiple threads without using the lock Statement (C# Reference) statement to serialize access. See How to: Create and Terminate Threads (C# Programming Guide) for an example of volatile in a multi-threaded scenario.

From : How to:
The use of volatile with the _shouldStop data member allows us to safely access this member from multiple threads without the use of formal thread synchronization techniques, but only because _shouldStop is a bool. This means that only single, atomic operations are necessary to modify _shouldStop. If, however, this data member were a class, struct, or array, accessing it from multiple threads would likely result in intermittent data corruption. Consider a thread that changes the values in an array. Windows regularly interrupts threads in order to allow other threads to execute, so this thread could be halted after assigning some array elements but before assigning others. This means the array now has a state that the programmer never intended, and another thread reading this array may fail as a result.

Hence why in the first response I posted:

Use a singleton design (static volatile StreamWriter writer;), and use a padlock to ensure no other threads touch it.  It does create locking contentions, but it's better than getting serious exceptions.

Thursday, April 6, 2006 10:41 PM

Kragie:

Atomic operations are thread safe by definition, don't let volatile confuse the issue.

For example:

static int _counter = 0;

static void Main(string[] args)
{
    Thread[] threads = new Thread[100];
    for (int i = 0; i < threads.Length; i++)
    {
        threads[i] = new Thread(new ThreadStart(DoWork));    
    }

    foreach (Thread t in threads)
    {
        t.Start();
    }
    
    foreach (Thread t in threads)
    {
        t.Join();
    }

    Console.WriteLine(_counter);
}

static void DoWork()
{
    for (int i = 0; i < 100000; i++)
    {
        _counter++;
    }
}

Here is a program that spins up 100 threads. Each thread goes into a loop inside of DoWork and increments a counter 100,000 times. If 100 threads increment the counter 100,000 times, the value of counter should be 10,000,000 at the end of the program. I ran the program 3 times on a single P4 hyperthreaded machine and got these results:

9,054,022

9,371,740

9,491,573

The static counter is a value type, but the increment operator (++) isn't atomic - it requires a read and then a write. Making the field volatile won't help:

static volatile int _counter = 0;
static void DoWork()
{
     for (int i = 0; i < 100000; i++)
     {
         _counter++;
     }
}

If I run the above program 3 times, I still don't get 10,000,000 as an answer.

Instead of using ++, I can use the Interlocked class from System.Threading. This makes the operation atomic and I don't need a volatile keyword.

static int _counter = 0;
static void DoWork()
{
    for (int i = 0; i < 100000; i++)
    {
        Interlocked.Increment(ref _counter);
    }
}

With the above code I can hit 10,000,000 everytime. I could also use a lock (but it would be a tad slower):

static int _counter = 0;
static object _lock = new object();

static void DoWork()
{
    for (int i = 0; i < 100000; i++)
    {
        lock (_lock)
        {
            _counter++;
        }
    }
}

The above code also produces 10,000,000 on each run.

Volatile is about preventing the re-ordering of instructions by the compiler and hardware. Both are free to re-order non-volatile reads and writes to increase performance, but it can lead to problems if you are trying to use lock free code. The typical example for .NET 1.1 is a singleton with double-check locking (discussion here: http://discuss.develop.com/archives/wa.exe?A2=ind0203B&L=DOTNET&P=R375). My understanding is that the memory model in 2.0 changed so that particular double check locking pattern isn't a problem anymore.

Friday, April 7, 2006 2:02 AM

I see where you're going with this, and you're right about that.  If the operation isn't atomic, it won't be the correct data.  Yeah, I made an exact statement when there are chances of it not being so. 

Interlocked.Increment and Interlock.Exchange are what I use when declaring my static instances, and  you're right in that they're the explicit atomic methods.  They also don't work on any volatile types. 

However, you're right that if multiple threads are assaulting the variable, syncronization won't be exact.