In c#, the main class created a Logger object that will be accessed by many threads. The logger object looks like (simplified)
public sealed class Logger
{
private ConcurrentQueue<string> queue = new ConcurrentQueue<string>();
public Logger()
{
// create other objects here AND a thread that extracts
// from the queue and writes to a file
// because queue is thread safe this is perfectly ok
}
public void Log(string whatToLog)
{
// Now, is this safe? This method will be called by several threads
// perhaps at the same time
string s = whatToLog + " " + DateTime.Now.ToString();
queue.Enqueue(s);
// The thread created in the constructor will extract and log
}
}
Is this OK from a design point of view? My two questions are:
Is "string s = whatToLog + " " + DateTime.Now.ToString();" ok if this method is accessed by several threads at the same time? I guess yes because any thread will have its own copy of s, right?
If the Logger object is accessed by several threads at the same time using only the Log() method, is everything safe then?
Thanks
The class is pretty thread safe.
Some suggested improvements.
The class doesn't prevent multiple instances from being instantiated, which is important if you want to have all threads log to the same object. Perhaps the singleton pattern could be applied. Quick example of a pseudo singleton using a static constructor. Please note the default constructor is private preventing any other class from creating a logger.
A performance related change is to avoid concatenating strings when logging. Creating new string is not a cheap operation. Also, once DateTime.Now is converted to a string, it is much harder to evaluate. E.g. sorting messages by creation date and time, etc. In the following, whatToLog is paired up with DateTime.Now in a Tuple.
public sealed class Logger
{
public static Logger instance {get; private set;}
static Logger()
{
instance = new Logger();
}
private ConcurrentQueue<Tuple<string, DateTime>> queue = new ConcurrentQueue<Tuple<string, DateTime>>();
private Logger() {}
public void Log(string whatToLog)
{
queue.Enqueue(new Tuple(whatToLog, DateTime.Now));
}
}
The ConcurrentQueue will make sure that the queue-part will be thread safe.
The string s you construct will not make it more or less thread-safe
In the current form, you should instantiate the logger, and pass the reference to each thread that will use this class
Although thread-safe, it does not guarantee sequentiality of the items
Queues cannot grow infinitely, make sure that your mechanism to dequeue can keep up
Improvements:
Make the class static, easier access for several threads
Separate concerns on reading and writing; this can be done by making several essential function internal and placing classes in the same namespace
use C#6 string interpolation
Code with improvements
public static class Logger
{
private static ConcurrentQueue<string> queue = new ConcurrentQueue<string>();
public static void Log(string LogMessage)
{
// thread safe logging
queue.Enqueue($"{LogMessage} {DateTime.Now}");
}
//dequeue only within namespace
internal static string Dequeue() {
string dequeuedItem;
queue.TryDequeue(out dequeuedItem);
return dequeuedItem;
}
}
public class LoggerReader
{
public LoggerReader()
{
// create other objects here AND a thread that extracts
// from the queue and writes to a file
// because queue is thread safe this is perfectly ok
string logItem = Logger.Dequeue();
}
}
I would simply use a lock in the Log method (replace ConcurrentQueue by Queue), and would not worry about each instruction anymore, especially if the original logger is more complicated than the example here!
public void Log(string whatToLog)
{
lock(queue) {
string s = whatToLog + " " + DateTime.Now.ToString();
queue.Enqueue(s);
}
}
Related
In other words, is this Singleton implementation thread safe:
public class Singleton
{
private static Singleton instance;
private Singleton() { }
static Singleton()
{
instance = new Singleton();
}
public static Singleton Instance
{
get { return instance; }
}
}
Static constructors are guaranteed to be run only once per application domain, before any instances of a class are created or any static members are accessed. https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/static-constructors
The implementation shown is thread safe for the initial construction, that is, no locking or null testing is required for constructing the Singleton object. However, this does not mean that any use of the instance will be synchronised. There are a variety of ways that this can be done; I've shown one below.
public class Singleton
{
private static Singleton instance;
// Added a static mutex for synchronising use of instance.
private static System.Threading.Mutex mutex;
private Singleton() { }
static Singleton()
{
instance = new Singleton();
mutex = new System.Threading.Mutex();
}
public static Singleton Acquire()
{
mutex.WaitOne();
return instance;
}
// Each call to Acquire() requires a call to Release()
public static void Release()
{
mutex.ReleaseMutex();
}
}
While all of these answers are giving the same general answer, there is one caveat.
Remember that all potential derivations of a generic class are compiled as individual types. So use caution when implementing static constructors for generic types.
class MyObject<T>
{
static MyObject()
{
//this code will get executed for each T.
}
}
EDIT:
Here is the demonstration:
static void Main(string[] args)
{
var obj = new Foo<object>();
var obj2 = new Foo<string>();
}
public class Foo<T>
{
static Foo()
{
System.Diagnostics.Debug.WriteLine(String.Format("Hit {0}", typeof(T).ToString()));
}
}
In the console:
Hit System.Object
Hit System.String
Using a static constructor actually is threadsafe. The static constructor is guaranteed to be executed only once.
From the C# language specification:
The static constructor for a class executes at most once in a given application domain. The execution of a static constructor is triggered by the first of the following events to occur within an application domain:
An instance of the class is created.
Any of the static members of the class are referenced.
So yes, you can trust that your singleton will be correctly instantiated.
Zooba made an excellent point (and 15 seconds before me, too!) that the static constructor will not guarantee thread-safe shared access to the singleton. That will need to be handled in another manner.
Here's the Cliffnotes version from the above MSDN page on c# singleton:
Use the following pattern, always, you can't go wrong:
public sealed class Singleton
{
private static readonly Singleton instance = new Singleton();
private Singleton(){}
public static Singleton Instance
{
get
{
return instance;
}
}
}
Beyond the obvious singleton features, it gives you these two things for free (in respect to singleton in c++):
lazy construction (or no construction if it was never called)
synchronization
Static constructors are guaranteed to fire only once per App Domain so your approach should be OK. However, it is functionally no different from the more concise, inline version:
private static readonly Singleton instance = new Singleton();
Thread safety is more of an issue when you are lazily initializing things.
The static constructor will finish running before any thread is allowed to access the class.
private class InitializerTest
{
static private int _x;
static public string Status()
{
return "_x = " + _x;
}
static InitializerTest()
{
System.Diagnostics.Debug.WriteLine("InitializerTest() starting.");
_x = 1;
Thread.Sleep(3000);
_x = 2;
System.Diagnostics.Debug.WriteLine("InitializerTest() finished.");
}
}
private void ClassInitializerInThread()
{
System.Diagnostics.Debug.WriteLine(Thread.CurrentThread.GetHashCode() + ": ClassInitializerInThread() starting.");
string status = InitializerTest.Status();
System.Diagnostics.Debug.WriteLine(Thread.CurrentThread.GetHashCode() + ": ClassInitializerInThread() status = " + status);
}
private void classInitializerButton_Click(object sender, EventArgs e)
{
new Thread(ClassInitializerInThread).Start();
new Thread(ClassInitializerInThread).Start();
new Thread(ClassInitializerInThread).Start();
}
The code above produced the results below.
10: ClassInitializerInThread() starting.
11: ClassInitializerInThread() starting.
12: ClassInitializerInThread() starting.
InitializerTest() starting.
InitializerTest() finished.
11: ClassInitializerInThread() status = _x = 2
The thread 0x2650 has exited with code 0 (0x0).
10: ClassInitializerInThread() status = _x = 2
The thread 0x1f50 has exited with code 0 (0x0).
12: ClassInitializerInThread() status = _x = 2
The thread 0x73c has exited with code 0 (0x0).
Even though the static constructor took a long time to run, the other threads stopped and waited. All threads read the value of _x set at the bottom of the static constructor.
The Common Language Infrastructure specification guarantees that "a type initializer shall run exactly once for any given type, unless explicitly called by user code." (Section 9.5.3.1.) So unless you have some whacky IL on the loose calling Singleton::.cctor directly (unlikely) your static constructor will run exactly once before the Singleton type is used, only one instance of Singleton will be created, and your Instance property is thread-safe.
Note that if Singleton's constructor accesses the Instance property (even indirectly) then the Instance property will be null. The best you can do is detect when this happens and throw an exception, by checking that instance is non-null in the property accessor. After your static constructor completes the Instance property will be non-null.
As Zoomba's answer points out you will need to make Singleton safe to access from multiple threads, or implement a locking mechanism around using the singleton instance.
Although other answers are mostly correct, there is yet another caveat with static constructors.
As per section II.10.5.3.3 Races and deadlocks of the ECMA-335 Common Language
Infrastructure
Type initialization alone shall not create a deadlock unless some code
called from a type initializer (directly or indirectly) explicitly
invokes blocking operations.
The following code results in a deadlock
using System.Threading;
class MyClass
{
static void Main() { /* Won’t run... the static constructor deadlocks */ }
static MyClass()
{
Thread thread = new Thread(arg => { });
thread.Start();
thread.Join();
}
}
Original author is Igor Ostrovsky, see his post here.
Just to be pedantic, but there is no such thing as a static constructor, but rather static type initializers, here's a small demo of cyclic static constructor dependency which illustrates this point.
Static constructor is guaranteed to be thread safe.
Also, check out the discussion on Singleton at DeveloperZen:
http://web.archive.org/web/20160404231134/http://www.developerzen.com/2007/07/15/whats-wrong-with-this-code-1-discussion/
The static constructor is locked. While the type initializer is running, any other thread which attempts to access the class in such a way that would trigger the type initializer will block.
However, the thread which is running the type initializer can access uninitialized static members. So be sure not to call Monitor.Enter() (lock(){}) or ManualResetEventSlim.Wait() from a type initializer if it is run from a UI thread—those are “interruptible” waits which result in the event loop running, executing arbitrary other parts of your program while your type initializer is still unfinished.
It is preferable for you to use managed blocking rather than unmanaged blocking. WaitHandle.WaitOne, WaitHandle.WaitAny, WaitHandle.WaitAll, Monitor.Enter, Monitor.TryEnter, Thread.Join, GC.WaitForPendingFinalizers, and so on are all responsive to Thread.Interrupt and to Thread.Abort. Also, if your thread is in a single-threaded apartment, all these managed blocking operations will correctly pump messages in your apartment while your thread is blocked:
I am trying to restrict access to an singletone object so only one thread
use it at time, Furthermore, I want to prevent from the same thread accessing twice
to the restricted code.
I tried the Lock method and i found out that its dosn't lock the thread that locked her, but only other threads..
as below:
public sealed class Singleton
{
private static readonly Singleton instance = new Singleton();
static Singleton()
{
}
private Singleton()
{
}
public static Singleton Instance
{
get
{
return instance;
}
}
}
public class SomeWorker
{
private readonly Timer _doWorkTimer = new Timer(20);
public SomeWorker()
{
InitiateTimer();
}
private void InitiateTimer()
{
_doWorkTimer .Elapsed += DoWorkElapse;
_doWorkTimer .Enabled = true;
}
private void DoWorkElapse(object source, ElapsedEventArgs e)
{
DoSomeWork();
}
private void DoSomeWork()
{
// I know that lock on string is wrong!
// Its just for the example only I
// Its just to make sure all the program is use the same lock..
lock ("ConnectionLock")
{
Console.WriteLine("Lock");
var inst = Singletone.Instance;
// Do Some Work on "inst" ...
Console.WriteLine("Unlock");
}
}
}
The result in the console for example is:
.
.
.
Lock
Unlock
Lock
Lock
Unlock
.
.
.
As we can see, 2 Lock comments shows one after another
So its mean that the "DoSomeWork()" accessed twice by the timer thread.
Anyone have any idea how to make this lock work?
Other Sync methods maby?
thanx.
You aren't doing your locking properly (and to top it off you are taking a lock on a string which is a big no-no). To save time, please read this article from Jon Skeet and implement one of the patterns to save yourself a headache.
In your code you have
public static Singletone Instance()
{
if (_instance == null)
{
lock (_instance)
{
if (_instance == null)
{
_instance = new Singletone ();
}
}
}
return _instance;;
}
Think about it. if (_instance == null) you do lock (_instance). So you lock using null. That's not good at all.
In MSDN lock Statement (C# Reference) the given example of how to use lock is:
class Account
{
decimal balance;
private Object thisLock = new Object();
public void Withdraw(decimal amount)
{
lock (thisLock)
{
if (amount > balance)
{
throw new Exception("Insufficient funds");
}
balance -= amount;
}
}
}
I guess you should follow it and have a separate object to use it as a lock.
And secondly, thread syncronization primitives are used to separate access to shared resources for different threads. If you need to separate access from one thread, you simply need to use flags. Something like this:
bool isBusy = false;
public static void Foo()
{
if (!isBusy)
{
isBusy = true;
try
{
//do the job
}
finally
{
isBusy = false;
}
}
}
Here you should understand that you simply skip the "locked-by-flag" code. On the contrary if you want to make the thread wait for itself, especially in a multithreading application, I guess it looks like it should be redesigned.
The easiest way to implement a singleton in .NET is:
public class Singleton : IDisposable
{
private readonly static Singleton _instance = new Singleton();
private readonly static object lockObject = new object();
static Singleton()
{
}
private Singleton()
{
InitiateConnection();
}
public static Singleton Instance
{
get { return _instance; }
}
/// <summary>
/// Method that accesses the DB.
/// </summary>
public void DoWork()
{
lock (lockObject)
{
//Do Db work here. Only one thread can execute these commands at a time.
}
}
~Singleton()
{
//Close the connection to DB.
//You don't want to make your singleton class implement IDisposable because
//you don't want to allow a call to Singleton.Instance.Dispose().
}
}
Read the excellent article on Singleton Pattern implementations in .NET that Bryan suggested in his answer. The above implementation is based on the fourth version described in the article. The CLR guarantees that the construction of the static field will thread-safe hence you do not need locking there. However you will need locking if your object has state (fields) that can be changed.
Note that there is a private readonly object used for ensuring mutual exclusion on the DoWork method. This way a single thread can call DoWork at a time. Also note that there is no way that the same thread can call this method twice at the same time since a thread executes instructions sequentially. The only way this method could be called twice from a single thread is if inside DoWork you call another method that eventually calls DoWork. I can't see the point of doing this and if you do then take care to avoid stack overflows. You could follow the suggestion of Konstantin and use a flag but IMHO you should redesign DoWork to do just one thing and avoid scenarios like these.
I have a Queue of items I want to process in a thread, and any instance of a class can add items to the Queue to be processed.
My idea for doing this is to have a static Thread in the class that processes the items, the only problem is that I don't know where to start this thread, since I can't start it in its initialization.
Is there a way I can start a static thread? Or should I be changing the architecture completely?
You can start it in the static constructor for the class:
private class MyClass
{
static MyClass()
{
// start thread here
}
}
You could also start it in the regular constructor of the class using a typical singleton approach.
Or you could use the new .NET 4 Lazy<T> approach to instantiating and starting it.
BUT it's generally not a good practice to do work in class constructors. A better approach would be to ensure the thread exists only when someone calls, say Execute() on an instance of the class. Within the Execute method you can use Lazy<T> or a singleton approach to creating and starting the single thread instance that will process it.
Purists will point out that actually you probably don't want to do this at all and that a Factory approach may be better for creating instances of your class and that you should separate the concerns between your class and the worker that processes it.
Other would suggest that you don't need a thread here at all, just use .NET4 Tasks and queue the items up for execution using the framework provided queue/execute methods.
Give the static queue class that you have a private inner class that handles the actual threading:
static class QueueStatic
{
public static Queue<Object> queue;
private static QueueWorker worker;
public static void DoQueueAction()
{
worker = new QueueWorker(queue);
ThreadStart t = new ThreadSTart(worker.Work);
t.Start();
}
//inner class
private class QueueWorker
{
private Queue<Object> queue;
public QueueWorker(Queue<Object> queue)
{
this.queue = queue;
}
public void Work()
{
//...
}
}
}
I am using a System.Threading.ThreadPool to manage a queue of jobs from a service. I have already implemented logging like this...
abstract class Global
{
public static LogFile LogFile = null;
}
public class LogFile : IDisposable
{
private StreamWriter sw;
public LogFile(string path){}
public void WriteEntry(string logText)
{
lock (sw)
{
sw.WriteLine(logText);
}
}
}
I want to create the log at service startup and use it from my queued worker threads.. something like this...
//On Service Start
Global.LogFile = new LogFile("log.txt");
//Kick of worker thread
ThreadPool.QueueUserWorkItem(objWrkrThread.CallbackMethod, iCount);
//Worker thread logs an entry in CallbackMethod()
Global.LogFile.WriteEntry("Hello World");
Is this safe? Will calling a method on a static instance of a class inadvertently 'synchronise' or 'block' my threads?
Michael
Nothing will 'synchronize' or 'block' unless you write code in your method. It doesn't matter whether it's an instance method or static method.
So by default, WriteEntry won't block any calls from your threads but it could very well corrupt file if you don't write the code to handle multiple simultaneous calls.
Read more on this topic here:
Are static methods thread safe
It's not safe to have multiple threads call WriteEntry at the same time unless it was designed to be safe.
What you are trying to do sounds like the perfect candidate for a Singleton class. I know it gets a bad wrap, but sometimes it's simplicity is worth it.
You can create a log class like this and you should be thread safe.
public sealed class Log
{
static Log instance=null;
static readonly object lockObject = new object();
static string path = "log.txt";
Log()
{
}
public static Log Instance
{
get
{
lock (lockObject)
{
if (instance==null)
{
instance = new Log();
}
return instance;
}
}
}
public void WriteLine(string message)
{
lock(lockObject)
{
using(StreamWriter sw = new StreamWriter(File.Open(path, FileMode.Append)))
{
sw.WriteLine(message);
}
}
}
}
Then in your code, you just call it like this:
Log executionLog = Log.Instance;
executionLog.WriteLine("this is a log message.");
You could also manage opening the file in similar thread safe methods to get rid of the over head of opening and closing the file every write.
In other words, is this Singleton implementation thread safe:
public class Singleton
{
private static Singleton instance;
private Singleton() { }
static Singleton()
{
instance = new Singleton();
}
public static Singleton Instance
{
get { return instance; }
}
}
Static constructors are guaranteed to be run only once per application domain, before any instances of a class are created or any static members are accessed. https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/static-constructors
The implementation shown is thread safe for the initial construction, that is, no locking or null testing is required for constructing the Singleton object. However, this does not mean that any use of the instance will be synchronised. There are a variety of ways that this can be done; I've shown one below.
public class Singleton
{
private static Singleton instance;
// Added a static mutex for synchronising use of instance.
private static System.Threading.Mutex mutex;
private Singleton() { }
static Singleton()
{
instance = new Singleton();
mutex = new System.Threading.Mutex();
}
public static Singleton Acquire()
{
mutex.WaitOne();
return instance;
}
// Each call to Acquire() requires a call to Release()
public static void Release()
{
mutex.ReleaseMutex();
}
}
While all of these answers are giving the same general answer, there is one caveat.
Remember that all potential derivations of a generic class are compiled as individual types. So use caution when implementing static constructors for generic types.
class MyObject<T>
{
static MyObject()
{
//this code will get executed for each T.
}
}
EDIT:
Here is the demonstration:
static void Main(string[] args)
{
var obj = new Foo<object>();
var obj2 = new Foo<string>();
}
public class Foo<T>
{
static Foo()
{
System.Diagnostics.Debug.WriteLine(String.Format("Hit {0}", typeof(T).ToString()));
}
}
In the console:
Hit System.Object
Hit System.String
Using a static constructor actually is threadsafe. The static constructor is guaranteed to be executed only once.
From the C# language specification:
The static constructor for a class executes at most once in a given application domain. The execution of a static constructor is triggered by the first of the following events to occur within an application domain:
An instance of the class is created.
Any of the static members of the class are referenced.
So yes, you can trust that your singleton will be correctly instantiated.
Zooba made an excellent point (and 15 seconds before me, too!) that the static constructor will not guarantee thread-safe shared access to the singleton. That will need to be handled in another manner.
Here's the Cliffnotes version from the above MSDN page on c# singleton:
Use the following pattern, always, you can't go wrong:
public sealed class Singleton
{
private static readonly Singleton instance = new Singleton();
private Singleton(){}
public static Singleton Instance
{
get
{
return instance;
}
}
}
Beyond the obvious singleton features, it gives you these two things for free (in respect to singleton in c++):
lazy construction (or no construction if it was never called)
synchronization
Static constructors are guaranteed to fire only once per App Domain so your approach should be OK. However, it is functionally no different from the more concise, inline version:
private static readonly Singleton instance = new Singleton();
Thread safety is more of an issue when you are lazily initializing things.
The static constructor will finish running before any thread is allowed to access the class.
private class InitializerTest
{
static private int _x;
static public string Status()
{
return "_x = " + _x;
}
static InitializerTest()
{
System.Diagnostics.Debug.WriteLine("InitializerTest() starting.");
_x = 1;
Thread.Sleep(3000);
_x = 2;
System.Diagnostics.Debug.WriteLine("InitializerTest() finished.");
}
}
private void ClassInitializerInThread()
{
System.Diagnostics.Debug.WriteLine(Thread.CurrentThread.GetHashCode() + ": ClassInitializerInThread() starting.");
string status = InitializerTest.Status();
System.Diagnostics.Debug.WriteLine(Thread.CurrentThread.GetHashCode() + ": ClassInitializerInThread() status = " + status);
}
private void classInitializerButton_Click(object sender, EventArgs e)
{
new Thread(ClassInitializerInThread).Start();
new Thread(ClassInitializerInThread).Start();
new Thread(ClassInitializerInThread).Start();
}
The code above produced the results below.
10: ClassInitializerInThread() starting.
11: ClassInitializerInThread() starting.
12: ClassInitializerInThread() starting.
InitializerTest() starting.
InitializerTest() finished.
11: ClassInitializerInThread() status = _x = 2
The thread 0x2650 has exited with code 0 (0x0).
10: ClassInitializerInThread() status = _x = 2
The thread 0x1f50 has exited with code 0 (0x0).
12: ClassInitializerInThread() status = _x = 2
The thread 0x73c has exited with code 0 (0x0).
Even though the static constructor took a long time to run, the other threads stopped and waited. All threads read the value of _x set at the bottom of the static constructor.
The Common Language Infrastructure specification guarantees that "a type initializer shall run exactly once for any given type, unless explicitly called by user code." (Section 9.5.3.1.) So unless you have some whacky IL on the loose calling Singleton::.cctor directly (unlikely) your static constructor will run exactly once before the Singleton type is used, only one instance of Singleton will be created, and your Instance property is thread-safe.
Note that if Singleton's constructor accesses the Instance property (even indirectly) then the Instance property will be null. The best you can do is detect when this happens and throw an exception, by checking that instance is non-null in the property accessor. After your static constructor completes the Instance property will be non-null.
As Zoomba's answer points out you will need to make Singleton safe to access from multiple threads, or implement a locking mechanism around using the singleton instance.
Although other answers are mostly correct, there is yet another caveat with static constructors.
As per section II.10.5.3.3 Races and deadlocks of the ECMA-335 Common Language
Infrastructure
Type initialization alone shall not create a deadlock unless some code
called from a type initializer (directly or indirectly) explicitly
invokes blocking operations.
The following code results in a deadlock
using System.Threading;
class MyClass
{
static void Main() { /* Won’t run... the static constructor deadlocks */ }
static MyClass()
{
Thread thread = new Thread(arg => { });
thread.Start();
thread.Join();
}
}
Original author is Igor Ostrovsky, see his post here.
Just to be pedantic, but there is no such thing as a static constructor, but rather static type initializers, here's a small demo of cyclic static constructor dependency which illustrates this point.
Static constructor is guaranteed to be thread safe.
Also, check out the discussion on Singleton at DeveloperZen:
http://web.archive.org/web/20160404231134/http://www.developerzen.com/2007/07/15/whats-wrong-with-this-code-1-discussion/
The static constructor is locked. While the type initializer is running, any other thread which attempts to access the class in such a way that would trigger the type initializer will block.
However, the thread which is running the type initializer can access uninitialized static members. So be sure not to call Monitor.Enter() (lock(){}) or ManualResetEventSlim.Wait() from a type initializer if it is run from a UI thread—those are “interruptible” waits which result in the event loop running, executing arbitrary other parts of your program while your type initializer is still unfinished.
It is preferable for you to use managed blocking rather than unmanaged blocking. WaitHandle.WaitOne, WaitHandle.WaitAny, WaitHandle.WaitAll, Monitor.Enter, Monitor.TryEnter, Thread.Join, GC.WaitForPendingFinalizers, and so on are all responsive to Thread.Interrupt and to Thread.Abort. Also, if your thread is in a single-threaded apartment, all these managed blocking operations will correctly pump messages in your apartment while your thread is blocked: