How to find that Mutex in C# is acquired?

Question

How can I find from mutex handle in C# that a mutex is acquired?

When mutex.WaitOne(timeout) timeouts, it returns false. However, how can I find that from the mutex handle? (Maybe using p/invoke.)

UPDATE:

public class InterProcessLock : IDisposable
{
    readonly Mutex mutex;

    public bool IsAcquired { get; private set; }

    public InterProcessLock(string name, TimeSpan timeout)
    {
        bool created;
        var security = new MutexSecurity();
        security.AddAccessRule(new MutexAccessRule(new SecurityIdentifier(WellKnownSidType.WorldSid, null), MutexRights.Synchronize | MutexRights.Modify, AccessControlType.Allow));
        mutex = new Mutex(false, name, out created, security);
        IsAcquired = mutex.WaitOne(timeout);
    }

    #region IDisposable Members

    public void Dispose()
    {
        if (IsAcquired)
        {
            mutex.ReleaseMutex();
            IsAcquired = false;
        }
    }

    #endregion
}

Currently, I am using my own property IsAcquired to determine whether I should release a mutex. Not essential but clearer, would be not to use a secondary copy of the information represented by IsAcquired property, but rather to ask directly the mutex whether it is acquired by me. Since calling mutex.ReleaseMutex() throws an exception if it is not acquired by me.

(By acquired state I mean that the mutex is in not-signaled state when I am owning the mutex.)

(EDIT: I have added IsAcquired = false; thanks to mattdekrey's post.)

Solution 1

As you may found, there are no public members on Mutex class: http://msdn.microsoft.com/en-us/library/system.threading.mutex_members.aspx

There is also no public native functions for that: http://msdn.microsoft.com/en-us/library/ms686360%28v=VS.85%29.aspx

However, there are some undocumented/unsupported functions especially in ntdll.dll. These allow accessing system objects. However, these functions may change or not be available in future versions of operating system.

So, the answer is: It is not possible using conventional means.

Solution 2

The reason there is no clean way to do this is because it is not a good idea and the reason is because race conditions are -very- easy to introduce when you rely on this type of logic. So your design needs to change.

First, you should not acquire a lock in a constructor. Turn this class into a factory that returns a properly initialized mutex object. That way you can know if you acquired the lock or not.

DO NOT rely on Dispose to release locks, this is asking for deadlock ridden code that is hard to maintain. Use a try/finally block to ensure it is released.

Timeouts are a bit sketchy. Only use timeouts when not acquiring the lock would be considered normal operation. Not being able to acquire the lock is usually a bug and merely avoiding it with timeouts hides the bug. If you need timeouts, consider using an event (maybe AutoResetEvent), this may be more appropriate.

Solution 3

Well, it's not exactly what you're asking for, but I think it would solve your problem: why not just add some error handling specifically for the exception that occurs if the Mutex is aquired by someone else?

public void Dispose()
{
    if (IsAcquired)
        try
        { mutex.ReleaseMutex(); }
        catch (System.Threading.SynchronizationLockException)
        {
            // Handle the exception, assuming you need to do anything.
            // All other exceptions would still be passed up the stack.
        }
}

Solution 4

Why can't you use Mutex.OpenExisting

try
{
    Mutex foundMutex = Mutex.OpenExisting("MyTestingMutex");

    // Found Mutex
    foundMutex.ReleaseMutex();
}
catch (System.Threading.WaitHandleCannotBeOpenedException)
{
    //   System.Threading.WaitHandleCannotBeOpenedException:
    //     The named mutex does not exist.
}

EDIT

I am guessing some of this.

It seems like you are trying to develop an API. One of the items you are offering in your API is an InterProcessLock.

I am going to assume you are sharing a collection across threads and you are using the Mutex to make sure only one operation is on it a time.

using (InterProcessLock myLock = new InterProcessLock("LockMutex", TimeSpan.FromMilliseconds(100.0)))
{
    if(myLock.IsAcquired)
    {
        // I have control then I can delete, add to the collection.
    }
}

I would reconsider this design. What if I never wraped InterProcessLock myLock = new InterProcessLock("LockMutex", TimeSpan.FromMilliseconds(100.0)) in a using? Dispose would not be called. What if the user never calls the Dispose at all?

There would be an abandoned Mutex

From MSDN

Caution An abandoned mutex often indicates a serious error in the code. When a thread exits without releasing the mutex, the data structures protected by the mutex might not be in a consistent state. The next thread to request ownership of the mutex can handle this exception and proceed, if the integrity of the data structures can be verified.

If you are trying to protect your users you might want to help them by controlling the Mutex for them so they never have to worry about it.

A possible example is

public static bool PerformLockedProcess(Action process, string commonLockName, TimeSpan timeout)
{
    Mutex mutex = null;

    // Get the Mutex for the User
    try
    {
        bool created;
        var security = new MutexSecurity();
        security.AddAccessRule(new MutexAccessRule(new SecurityIdentifier(WellKnownSidType.WorldSid, null), MutexRights.Synchronize | MutexRights.Modify, AccessControlType.Allow));

        mutex = new Mutex(false, commonLockName, out created, security);

        bool acquired = mutex.WaitOne(timeout);

        if (acquired)
        {
            process();

            return true;
        }

        return false;
    }
    finally
    {
        // Make sure we do not abandon the Mutex
        if (mutex != null)
        {
            try
            {
                mutex.ReleaseMutex();
            }
            catch (ApplicationException)
            {
                // In case that failes
            }
        }
    }
}

This is one possible way. It all depends on what the goal is. I would NOT relay on the final user to call a Dispose since a Mutex is an operating system construct. And if the name is not unquie it could effect other processes using the same mutex name.

Solution 5

This won’t benefit the original poster of the question but here it goes.

While I do not disagree with other posters on proper use of Mutexes, I had an application where I needed to test whether someone owns a mutex without taking ownership myself. As mentioned by others the only way is to use an undocumented NtQueryMutant system call from ntdll.dll. I created an extension method for the Mutex class that can be used like this:

        bool createdNew = true;
        var m = new Mutex(false, MutexName, out createdNew);
        if ( m != null)
        {
            int currentCount;
            bool ownedByCaller, abandonedState;
            if (m.TryQuery(out currentCount, out ownedByCaller, out abandonedState))
            {
                Console.WriteLine(string.Format("Created New: {3}, Count: {0}, OwvedByMe: {1}, Abandoned: {2}",
                    currentCount, ownedByCaller, abandonedState, createdNew));
            }
            m.Close();
        }

And here is the implementation

public static class MutexExtensionMethods
{
    public static bool TryQuery(this Mutex m, out int currentCount, out bool ownedByCaller, out bool abandonedState)
    {
        currentCount = -1;
        ownedByCaller = abandonedState = false;
        try
        {
            var handle = m.SafeWaitHandle;
            if (handle != null)
            {
                var h = handle.DangerousGetHandle();
                MutantBasicInformation mbi;
                int retLength;
                var ntStatus = NtQueryMutant(
                    h,
                    MutantInformationClass.MutantBasicInformation,
                    out mbi, 
                    Marshal.SizeOf(typeof(MutantBasicInformation)),
                    out retLength);
                GC.KeepAlive(handle); // Prevent "handle" from being collected before NtQueryMutant returns
                if (ntStatus == 0)
                {
                    currentCount   = mbi.CurrentCount;
                    ownedByCaller  = mbi.OwnedByCaller;
                    abandonedState = mbi.AbandonedState;
                    return true;
                }
            }
        }
        catch
        {
        }
        return false;
    }

    #region NTDLL.DLL

    [DllImport("ntdll.dll")]
    public static extern uint NtQueryMutant(
        [In] IntPtr MutantHandle,
        [In] MutantInformationClass MutantInformationClass,
        [Out] out MutantBasicInformation MutantInformation,
        [In] int MutantInformationLength,
        [Out] [Optional] out int ReturnLength
        );

    public enum MutantInformationClass : int
    {
        MutantBasicInformation
    }

    [StructLayout(LayoutKind.Sequential)]
    public struct MutantBasicInformation
    {
        public int CurrentCount;
        [MarshalAs(UnmanagedType.U1)]
        public bool OwnedByCaller;
        [MarshalAs(UnmanagedType.U1)]
        public bool AbandonedState;
    }

    #endregion

}

View more solutions

Comments

[Abimael López]

Thanks, but I don't want to acquire the mutex if it is not already acquired. Is there any function for that?

[Alex F]

No, there is no such way both in .NET mutex and native API. Mutex may be acquired only if it is not acquired by any other thread. Possibly you need some other synchronization type. Define your requirements, maybe some other synchronization type, like event or semaphore, meets them.

[Abimael López]

I put the answer into the post (since my answer is longer).

[Abimael López]

Yes, thanks, also a solution, but I am just trying to solve the problem "better" way, if possible:)

[Abimael López]

If programmers call Dispose() twice, it will throw an exception. So they can find that there is a bug in their code. Earlier, I was also throwing an exception in the constructor. But later I have changed that, since programmers may decide in certain situations to run the code even if the lock is not acquired. (For instance, there are independent processes and maybe one got stuck, so the second is waiting a while and then it will run despite this.)

[Matt DeKrey]

The IDisposable design documentation specifies that "If an object's Dispose method is called more than once, the object must ignore all calls after the first one. The object must not throw an exception if its Dispose method is called multiple times." msdn.microsoft.com/en-us/library/…

[Abimael López]

+1 Ah, ok. Thank you for the information. I have not read that:(

[Matt DeKrey]

Karl - I agree with all of your statements except the constructor/dispose parts; Dispose is not the same as a destructor - the using statement is implemented by a try/finally block when it comes down to the IL. (As stated below, I think the constructor would be fine if it ALWAYS threw an exception when it failed to acquire the lock to guarantee that the critical section did not run.)

[Karl Strings]

Matt - msdn.microsoft.com/en-us/library/b1yfkh5e.aspx: "Do not assume that Dispose will be called." You are right, as long as the devs scope it in a using block or manually call dispose this would be fine. But it's awkward. Many people will assume a finalizer (or more likely just have no clue...) and let it fall off scope. This won't work with locks because the GC may or may not collect the object right away, causing weirdness in the locking. It creates hard to find bugs. It will work, but it adds "do it this way" process overhead for a simple lock. Best to make locks act like locks.

[Matt DeKrey]

Hmm, I haven't read that. I wonder why that isn't mentioned on the IDisposable.Dispose documentation page. +1 - I think yours should be accepted as the answer.

[Abimael López]

Karl, I may not understand properly - if I add a Release() method that will call Dispose(), will it be ok? Next, how can I use AutoResetEvent to synchronize processes?

[Karl Strings]

I am saying to not use the dispose interface at all. Have a static method that creates the mutex objects you need (factory pattern). Then use those mutexes directly rather than wrapping it. I miss quoted, it is not an AutoResetEvent, it is an EventWaitHandle. You can create or wait on a system event until another process signals it. You may want to clarify what problem you are trying to solve, I think you may get better answers.

[Abimael López]

I am sorry, maybe I have a brain problem:) I still don't understand:( You said: 'First, you should not acquire a lock in a constructor.' Could you provide a rationale for that? Since, it is similar to streams/files. Files are also used for inter-process locking. My current problem: I have system where multiple processes runs 'randomly' (based on external events). I would like to avoid them run at same time. So they are acquiring a lock. (But if a process have a problem - runs longer, the next waiting process should report that and continue - without killing the locked process.)

[Karl Strings]

1) Acquiring locks in a constructor. Your cleanup logic will get complicated - there is no good way to release the lock. Streams/Files do not acquire locks in a constructor. You may be confused as to what a lock is and what it does for you. 2) These types of problems are difficult to solve and it will become a maintenance issue. it is better to write a service application that controls the processes or write a service that combines all the functionality of the other processes. Central management is much simpler to debug and maintain.

[Abimael López]

1, Your cleanup logic will get complicated - there is no good way to release the lock. - Calling Release or Dispose is simple. Or what complication you see there? 2, Streams/Files do not acquire locks in a constructor. No, but they are allocating a system handle as well. What is the difference between locking a file and locking a mutex? 3, Central management is much simpler to debug and maintain. I can't agree more:) Centralized vs. distributed systems...

[Spence]

From a security point of view, this is the only way to prevent a race condition. Any checking you could do in a multithreaded environment would be defeated as you could do all the checking in the world, then the thread acquires just as you release causing an exception. This is probably the safest pattern.

[Abimael López]

Not sure. Since, I think that using interprocess lock among threads in a same process is not a good idea. So, in case that one thread acquires a global process lock when another thread is just releasing it, I want to throw an exception.

[Abimael López]

What for? This is just a check whether the mutex exists. Not whether it is acquired.

[Abimael López]

Thank you for the hint that passing closures is safer against user mistakes than IDisposable.Dispose or a custom Release method.

[Paul Groke]

I'm strongly against releasing locks in a finally block, because most developers do it wrong. And doing it correctly is a big pain. Also IMO it isn't even possible with most mutex classes - they lack the necessary locking method (a method similar to the object obj, ref bool lockTaken overload of Monitor.Enter). I'd strongly suggest to use a lock() block instead, and if that isn't possible, a using() block with a helper object that unlocks in its Dispose method. That way you only have zero (lock) or one place (Dispose) where you have to write the correct locking/unlocking code.

[Paul Groke]

I added a missing (required) GC.KeepAlive call. Better yet would be to just declare NtQueryMutant to use a SafeWaitHandle parameter.