stopping dll loop

stopping dll loop - c#

I have a multi-thread C# application that uses some recursive functions in a dll. The problem that I have is how to cleanly stop the recursive functions.
The recursive functions are used to traverse our SCADA system's hierarchical 'SCADA Object' data. Traversing the data takes a long time (10s of minutes) depending on the size of our system and what we need to do with the data.
When I start the work I create a background thread so the GUI stays responsive. Then the background worker handles the calling of the recursive function in the dll.
I can send a cancel request to the background worker using CancelAsync but the background worker can't check the CancellationPending flag because it is blocked waiting of the dll's recursive function to finish.
Typically there is only 1 recursive function active at a time but there are dozens of recursive functions that are used at various times by different background workers.
As a quick (and really shameful) hack I added a global 'CodeEnabled' flag to the dll. So when the GUI does the CancelAsync it also sets the 'CodeEnabled' flag to false. (I know I need some of those bad code offsets). Then the dll's recursive loop checks the 'CodeEnabled' flag and returns to the background worker which is finally able to stop.
I don't want to move the recursive logic to the background worker thread because I need it in other places (e.g. other background workers).
What other approach should be used for this type of problem?

It depends on the design, really. Much recursion can be replaced with (for example) a local stack (Stack<>) or queue (Queue<>), in which case a cancel flag can be held locally without too much pain. Another option is to use some kind of progress event that allows subscribers to set a cancel flag. A third option is to pass some kind of context class into the function(s), with a (volatile or synchronized) flag that can be set.
In any of these cases you should have relatively easy access to a cancel flag to exit the recursion.
FooContext ctx = new FooFontext();
BeginSomeRecursiveFunction(ctx);
...
ctx.Cancel = true; // or ctx.Cancel(), whatever
with (in your function that accepts the context):
if(ctx.Cancel) return; // or maybe throw something
// like an OperationCancelledException();
blah...
CallMyself(ctx); // and further down the rabbit hole we go...
Another interesting option is to use iterator blocks for your long function rather than regular code; then your calling code can simply stop iterating when it has had enough.

Well, it seems to me that you need to propagate the "stop now" state down the recursive calls. You could have some sort of cancellation token which you pass down the recursive calls, and also keep hold of in the UI thread. Something as simple as this:
public class CancellationToken
{
private volatile bool cancelled;
public bool IsCancelled { get { return cancelled; } }
public void Cancel() { cancelled = true; }
}
(I'm getting increasingly wary of volatility and lock-free coding; I would be tempted to use a lock here instead of a volatile variable, but I've kept it here for the sake of simplicity.)
So you'd create the cancellation token, pass it in, and then at the start of each recursive method call you'd have:
if (token.IsCancelled)
{
return null; // Or some other dummy value, or throw an exception
}
Then you'd just call Cancel() in the UI thread. Basically it's a just a way of sharing the state of "should this task continue".
The choice of whether to propagate a dummy return value back or throw an exception is an interesting one. In some ways this isn't exceptional - you must be partially expecting it, or you wouldn't pass the cancellation token in the first place - but at the same time exceptions have the behaviour you want in terms of unwinding the stack to somewhere that can recognise the cancellation easily.

I like the previous answers, but here's another.
I think you're asking how to have different cancel flag for different threads.
Assuming that the threads which you might want to cancel each have some kind of ThreadId then, instead of having a single global 'CodeEnabled' flag, you could have a global thread-safe dictionary of flags, where the TheadId values are used as the dictionary's keys.
A thread would then query the dictionary to see whether its flag has been set.

Related

How can I make a interrupt in C#? [duplicate]

I understand Thread.Abort() is evil from the multitude of articles I've read on the topic, so I'm currently in the process of ripping out all of my abort's in order to replace it for a cleaner way; and after comparing user strategies from people here on stackoverflow and then after reading "How to: Create and Terminate Threads (C# Programming Guide)" from MSDN both which state an approach very much the same -- which is to use a volatile bool approach checking strategy, which is nice, but I still have a few questions....
Immediately what stands out to me here, is what if you do not have a simple worker process which is just running a loop of crunching code? For instance for me, my process is a background file uploader process, I do in fact loop through each file, so that's something, and sure I could add my while (!_shouldStop) at the top which covers me every loop iteration, but I have many more business processes which occur before it hits it's next loop iteration, I want this cancel procedure to be snappy; don't tell me I need to sprinkle these while loops every 4-5 lines down throughout my entire worker function?!
I really hope there is a better way, could somebody please advise me on if this is in fact, the correct [and only?] approach to do this, or strategies they have used in the past to achieve what I am after.
Thanks gang.
Further reading: All these SO responses assume the worker thread will loop. That doesn't sit comfortably with me. What if it is a linear, but timely background operation?

Unfortunately there may not be a better option. It really depends on your specific scenario. The idea is to stop the thread gracefully at safe points. That is the crux of the reason why Thread.Abort is not good; because it is not guaranteed to occur at safe points. By sprinkling the code with a stopping mechanism you are effectively manually defining the safe points. This is called cooperative cancellation. There are basically 4 broad mechanisms for doing this. You can choose the one that best fits your situation.
Poll a stopping flag
You have already mentioned this method. This a pretty common one. Make periodic checks of the flag at safe points in your algorithm and bail out when it gets signalled. The standard approach is to mark the variable volatile. If that is not possible or inconvenient then you can use a lock. Remember, you cannot mark a local variable as volatile so if a lambda expression captures it through a closure, for example, then you would have to resort to a different method for creating the memory barrier that is required. There is not a whole lot else that needs to be said for this method.
Use the new cancellation mechanisms in the TPL
This is similar to polling a stopping flag except that it uses the new cancellation data structures in the TPL. It is still based on cooperative cancellation patterns. You need to get a CancellationToken and the periodically check IsCancellationRequested. To request cancellation you would call Cancel on the CancellationTokenSource that originally provided the token. There is a lot you can do with the new cancellation mechanisms. You can read more about here.
Use wait handles
This method can be useful if your worker thread requires waiting on an specific interval or for a signal during its normal operation. You can Set a ManualResetEvent, for example, to let the thread know it is time to stop. You can test the event using the WaitOne function which returns a bool indicating whether the event was signalled. The WaitOne takes a parameter that specifies how much time to wait for the call to return if the event was not signaled in that amount of time. You can use this technique in place of Thread.Sleep and get the stopping indication at the same time. It is also useful if there are other WaitHandle instances that the thread may have to wait on. You can call WaitHandle.WaitAny to wait on any event (including the stop event) all in one call. Using an event can be better than calling Thread.Interrupt since you have more control over of the flow of the program (Thread.Interrupt throws an exception so you would have to strategically place the try-catch blocks to perform any necessary cleanup).
Specialized scenarios
There are several one-off scenarios that have very specialized stopping mechanisms. It is definitely outside the scope of this answer to enumerate them all (never mind that it would be nearly impossible). A good example of what I mean here is the Socket class. If the thread is blocked on a call to Send or Receive then calling Close will interrupt the socket on whatever blocking call it was in effectively unblocking it. I am sure there are several other areas in the BCL where similiar techniques can be used to unblock a thread.
Interrupt the thread via Thread.Interrupt
The advantage here is that it is simple and you do not have to focus on sprinkling your code with anything really. The disadvantage is that you have little control over where the safe points are in your algorithm. The reason is because Thread.Interrupt works by injecting an exception inside one of the canned BCL blocking calls. These include Thread.Sleep, WaitHandle.WaitOne, Thread.Join, etc. So you have to be wise about where you place them. However, most the time the algorithm dictates where they go and that is usually fine anyway especially if your algorithm spends most of its time in one of these blocking calls. If you algorithm does not use one of the blocking calls in the BCL then this method will not work for you. The theory here is that the ThreadInterruptException is only generated from .NET waiting call so it is likely at a safe point. At the very least you know that the thread cannot be in unmanaged code or bail out of a critical section leaving a dangling lock in an acquired state. Despite this being less invasive than Thread.Abort I still discourage its use because it is not obvious which calls respond to it and many developers will be unfamiliar with its nuances.

Well, unfortunately in multithreading you often have to compromise "snappiness" for cleanliness... you can exit a thread immediately if you Interrupt it, but it won't be very clean. So no, you don't have to sprinkle the _shouldStop checks every 4-5 lines, but if you do interrupt your thread then you should handle the exception and exit out of the loop in a clean manner.
Update
Even if it's not a looping thread (i.e. perhaps it's a thread that performs some long-running asynchronous operation or some type of block for input operation), you can Interrupt it, but you should still catch the ThreadInterruptedException and exit the thread cleanly. I think that the examples you've been reading are very appropriate.
Update 2.0
Yes I have an example... I'll just show you an example based on the link you referenced:
public class InterruptExample
{
private Thread t;
private volatile boolean alive;
public InterruptExample()
{
alive = false;
t = new Thread(()=>
{
try
{
while (alive)
{
/* Do work. */
}
}
catch (ThreadInterruptedException exception)
{
/* Clean up. */
}
});
t.IsBackground = true;
}
public void Start()
{
alive = true;
t.Start();
}
public void Kill(int timeout = 0)
{
// somebody tells you to stop the thread
t.Interrupt();
// Optionally you can block the caller
// by making them wait until the thread exits.
// If they leave the default timeout,
// then they will not wait at all
t.Join(timeout);
}
}

If cancellation is a requirement of the thing you're building, then it should be treated with as much respect as the rest of your code--it may be something you have to design for.
Lets assume that your thread is doing one of two things at all times.
Something CPU bound
Waiting for the kernel
If you're CPU bound in the thread in question, you probably have a good spot to insert the bail-out check. If you're calling into someone else's code to do some long-running CPU-bound task, then you might need to fix the external code, move it out of process (aborting threads is evil, but aborting processes is well-defined and safe), etc.
If you're waiting for the kernel, then there's probably a handle (or fd, or mach port, ...) involved in the wait. Usually if you destroy the relevant handle, the kernel will return with some failure code immediately. If you're in .net/java/etc. you'll likely end up with an exception. In C, whatever code you already have in place to handle system call failures will propagate the error up to a meaningful part of your app. Either way, you break out of the low-level place fairly cleanly and in a very timely manner without needing new code sprinkled everywhere.
A tactic I often use with this kind of code is to keep track of a list of handles that need to be closed and then have my abort function set a "cancelled" flag and then close them. When the function fails it can check the flag and report failure due to cancellation rather than due to whatever the specific exception/errno was.
You seem to be implying that an acceptable granularity for cancellation is at the level of a service call. This is probably not good thinking--you are much better off cancelling the background work synchronously and joining the old background thread from the foreground thread. It's way cleaner becasue:
It avoids a class of race conditions when old bgwork threads come back to life after unexpected delays.
It avoids potential hidden thread/memory leaks caused by hanging background processes by making it possible for the effects of a hanging background thread to hide.
There are two reasons to be scared of this approach:
You don't think you can abort your own code in a timely fashion. If cancellation is a requirement of your app, the decision you really need to make is a resource/business decision: do a hack, or fix your problem cleanly.
You don't trust some code you're calling because it's out of your control. If you really don't trust it, consider moving it out-of-process. You get much better isolation from many kinds of risks, including this one, that way.

The best answer largely depends on what you're doing in the thread.
Like you said, most answers revolve around polling a shared boolean every couple lines. Even though you may not like it, this is often the simplest scheme. If you want to make your life easier, you can write a method like ThrowIfCancelled(), which throws some kind of exception if you're done. The purists will say this is (gasp) using exceptions for control flow, but then again cacelling is exceptional imo.
If you're doing IO operations (like network stuff), you may want to consider doing everything using async operations.
If you're doing a sequence of steps, you could use the IEnumerable trick to make a state machine. Example:
<
abstract class StateMachine : IDisposable
{
public abstract IEnumerable<object> Main();
public virtual void Dispose()
{
/// ... override with free-ing code ...
}
bool wasCancelled;
public bool Cancel()
{
// ... set wasCancelled using locking scheme of choice ...
}
public Thread Run()
{
var thread = new Thread(() =>
{
try
{
if(wasCancelled) return;
foreach(var x in Main())
{
if(wasCancelled) return;
}
}
finally { Dispose(); }
});
thread.Start()
}
}
class MyStateMachine : StateMachine
{
public override IEnumerabl<object> Main()
{
DoSomething();
yield return null;
DoSomethingElse();
yield return null;
}
}
// then call new MyStateMachine().Run() to run.
>
Overengineering? It depends how many state machines you use. If you just have 1, yes. If you have 100, then maybe not. Too tricky? Well, it depends. Another bonus of this approach is that it lets you (with minor modifications) move your operation into a Timer.tick callback and void threading altogether if it makes sense.
and do everything that blucz says too.

Perhaps the a piece of the problem is that you have such a long method / while loop. Whether or not you are having threading issues, you should break it down into smaller processing steps. Let's suppose those steps are Alpha(), Bravo(), Charlie() and Delta().
You could then do something like this:
public void MyBigBackgroundTask()
{
Action[] tasks = new Action[] { Alpha, Bravo, Charlie, Delta };
int workStepSize = 0;
while (!_shouldStop)
{
tasks[workStepSize++]();
workStepSize %= tasks.Length;
};
}
So yes it loops endlessly, but checks if it is time to stop between each business step.

You don't have to sprinkle while loops everywhere. The outer while loop just checks if it's been told to stop and if so doesn't make another iteration...
If you have a straight "go do something and close out" thread (no loops in it) then you just check the _shouldStop boolean either before or after each major spot inside the thread. That way you know whether it should continue on or bail out.
for example:
public void DoWork() {
RunSomeBigMethod();
if (_shouldStop){ return; }
RunSomeOtherBigMethod();
if (_shouldStop){ return; }
//....
}

Instead of adding a while loop where a loop doesn't otherwise belong, add something like if (_shouldStop) CleanupAndExit(); wherever it makes sense to do so. There's no need to check after every single operation or sprinkle the code all over with them. Instead, think of each check as a chance to exit the thread at that point and add them strategically with this in mind.

All these SO responses assume the worker thread will loop. That doesn't sit comfortably with me
There are not a lot of ways to make code take a long time. Looping is a pretty essential programming construct. Making code take a long time without looping takes a huge amount of statements. Hundreds of thousands.
Or calling some other code that is doing the looping for you. Yes, hard to make that code stop on demand. That just doesn't work.

Question about terminating a thread cleanly in .NET

I understand Thread.Abort() is evil from the multitude of articles I've read on the topic, so I'm currently in the process of ripping out all of my abort's in order to replace it for a cleaner way; and after comparing user strategies from people here on stackoverflow and then after reading "How to: Create and Terminate Threads (C# Programming Guide)" from MSDN both which state an approach very much the same -- which is to use a volatile bool approach checking strategy, which is nice, but I still have a few questions....
Immediately what stands out to me here, is what if you do not have a simple worker process which is just running a loop of crunching code? For instance for me, my process is a background file uploader process, I do in fact loop through each file, so that's something, and sure I could add my while (!_shouldStop) at the top which covers me every loop iteration, but I have many more business processes which occur before it hits it's next loop iteration, I want this cancel procedure to be snappy; don't tell me I need to sprinkle these while loops every 4-5 lines down throughout my entire worker function?!
I really hope there is a better way, could somebody please advise me on if this is in fact, the correct [and only?] approach to do this, or strategies they have used in the past to achieve what I am after.
Thanks gang.
Further reading: All these SO responses assume the worker thread will loop. That doesn't sit comfortably with me. What if it is a linear, but timely background operation?

Well, unfortunately in multithreading you often have to compromise "snappiness" for cleanliness... you can exit a thread immediately if you Interrupt it, but it won't be very clean. So no, you don't have to sprinkle the _shouldStop checks every 4-5 lines, but if you do interrupt your thread then you should handle the exception and exit out of the loop in a clean manner.
Update
Even if it's not a looping thread (i.e. perhaps it's a thread that performs some long-running asynchronous operation or some type of block for input operation), you can Interrupt it, but you should still catch the ThreadInterruptedException and exit the thread cleanly. I think that the examples you've been reading are very appropriate.
Update 2.0
Yes I have an example... I'll just show you an example based on the link you referenced:
public class InterruptExample
{
private Thread t;
private volatile boolean alive;
public InterruptExample()
{
alive = false;
t = new Thread(()=>
{
try
{
while (alive)
{
/* Do work. */
}
}
catch (ThreadInterruptedException exception)
{
/* Clean up. */
}
});
t.IsBackground = true;
}
public void Start()
{
alive = true;
t.Start();
}
public void Kill(int timeout = 0)
{
// somebody tells you to stop the thread
t.Interrupt();
// Optionally you can block the caller
// by making them wait until the thread exits.
// If they leave the default timeout,
// then they will not wait at all
t.Join(timeout);
}
}

If cancellation is a requirement of the thing you're building, then it should be treated with as much respect as the rest of your code--it may be something you have to design for.
Lets assume that your thread is doing one of two things at all times.
Something CPU bound
Waiting for the kernel
If you're CPU bound in the thread in question, you probably have a good spot to insert the bail-out check. If you're calling into someone else's code to do some long-running CPU-bound task, then you might need to fix the external code, move it out of process (aborting threads is evil, but aborting processes is well-defined and safe), etc.
If you're waiting for the kernel, then there's probably a handle (or fd, or mach port, ...) involved in the wait. Usually if you destroy the relevant handle, the kernel will return with some failure code immediately. If you're in .net/java/etc. you'll likely end up with an exception. In C, whatever code you already have in place to handle system call failures will propagate the error up to a meaningful part of your app. Either way, you break out of the low-level place fairly cleanly and in a very timely manner without needing new code sprinkled everywhere.
A tactic I often use with this kind of code is to keep track of a list of handles that need to be closed and then have my abort function set a "cancelled" flag and then close them. When the function fails it can check the flag and report failure due to cancellation rather than due to whatever the specific exception/errno was.
You seem to be implying that an acceptable granularity for cancellation is at the level of a service call. This is probably not good thinking--you are much better off cancelling the background work synchronously and joining the old background thread from the foreground thread. It's way cleaner becasue:
It avoids a class of race conditions when old bgwork threads come back to life after unexpected delays.
It avoids potential hidden thread/memory leaks caused by hanging background processes by making it possible for the effects of a hanging background thread to hide.
There are two reasons to be scared of this approach:
You don't think you can abort your own code in a timely fashion. If cancellation is a requirement of your app, the decision you really need to make is a resource/business decision: do a hack, or fix your problem cleanly.
You don't trust some code you're calling because it's out of your control. If you really don't trust it, consider moving it out-of-process. You get much better isolation from many kinds of risks, including this one, that way.

The best answer largely depends on what you're doing in the thread.
Like you said, most answers revolve around polling a shared boolean every couple lines. Even though you may not like it, this is often the simplest scheme. If you want to make your life easier, you can write a method like ThrowIfCancelled(), which throws some kind of exception if you're done. The purists will say this is (gasp) using exceptions for control flow, but then again cacelling is exceptional imo.
If you're doing IO operations (like network stuff), you may want to consider doing everything using async operations.
If you're doing a sequence of steps, you could use the IEnumerable trick to make a state machine. Example:
<
abstract class StateMachine : IDisposable
{
public abstract IEnumerable<object> Main();
public virtual void Dispose()
{
/// ... override with free-ing code ...
}
bool wasCancelled;
public bool Cancel()
{
// ... set wasCancelled using locking scheme of choice ...
}
public Thread Run()
{
var thread = new Thread(() =>
{
try
{
if(wasCancelled) return;
foreach(var x in Main())
{
if(wasCancelled) return;
}
}
finally { Dispose(); }
});
thread.Start()
}
}
class MyStateMachine : StateMachine
{
public override IEnumerabl<object> Main()
{
DoSomething();
yield return null;
DoSomethingElse();
yield return null;
}
}
// then call new MyStateMachine().Run() to run.
>
Overengineering? It depends how many state machines you use. If you just have 1, yes. If you have 100, then maybe not. Too tricky? Well, it depends. Another bonus of this approach is that it lets you (with minor modifications) move your operation into a Timer.tick callback and void threading altogether if it makes sense.
and do everything that blucz says too.

Perhaps the a piece of the problem is that you have such a long method / while loop. Whether or not you are having threading issues, you should break it down into smaller processing steps. Let's suppose those steps are Alpha(), Bravo(), Charlie() and Delta().
You could then do something like this:
public void MyBigBackgroundTask()
{
Action[] tasks = new Action[] { Alpha, Bravo, Charlie, Delta };
int workStepSize = 0;
while (!_shouldStop)
{
tasks[workStepSize++]();
workStepSize %= tasks.Length;
};
}
So yes it loops endlessly, but checks if it is time to stop between each business step.

You don't have to sprinkle while loops everywhere. The outer while loop just checks if it's been told to stop and if so doesn't make another iteration...
If you have a straight "go do something and close out" thread (no loops in it) then you just check the _shouldStop boolean either before or after each major spot inside the thread. That way you know whether it should continue on or bail out.
for example:
public void DoWork() {
RunSomeBigMethod();
if (_shouldStop){ return; }
RunSomeOtherBigMethod();
if (_shouldStop){ return; }
//....
}

Instead of adding a while loop where a loop doesn't otherwise belong, add something like if (_shouldStop) CleanupAndExit(); wherever it makes sense to do so. There's no need to check after every single operation or sprinkle the code all over with them. Instead, think of each check as a chance to exit the thread at that point and add them strategically with this in mind.

All these SO responses assume the worker thread will loop. That doesn't sit comfortably with me
There are not a lot of ways to make code take a long time. Looping is a pretty essential programming construct. Making code take a long time without looping takes a huge amount of statements. Hundreds of thousands.
Or calling some other code that is doing the looping for you. Yes, hard to make that code stop on demand. That just doesn't work.

Best practices for moving objects to a separate thread

We have an implementation for an Ultrasound machine application current where the Ultrasound object is created on the UI's thread. A Singleton implementation would have been good here, but regardless, isn't.
Recently, the set methods changed such that they automatically stop and restart the ultrasound machine, which can take between 10-100ms depending on the state of the machine. For most cases, this isn't too bad of a problem, however it's still causing the UI thread to block for 100ms. Additionally, these methods are not thread-safe and must be called on the same thread where the object was initialized.
This largest issue this is now causing is unresponsive buttons in the UI, especially sliders which may try to update variables many times as you slide the bar. As a result, sliders especially will stutter and update very slowly as it makes many set calls through databound propeties.
What is a good way to create a thread specifically for the creation and work for this Ultrasound object, which will persist through the lifetime of the application?
A current temporary workaround involves spawning a Timer, and invoking a parameter update once we have detected the slider hasn't moved for 200ms, however a Timer would then have to be implemented for every slider and seems like a very messy solution which solves unresponsive sliders, but still blocks the UI thread occasionally.

One thing that's really great about programming the GUI is that you don't have to worry about multiple threads mucking things up for you (assuming you've got CheckForIllegalCrossThreadCalls = true, as you should). It's all single-threaded, operating by means of a message pump (queue) that processes incoming messages one-by-one.
Since you've indicated that you need to synchronize method calls that are not written to be thread-safe (totally understandable), there's no reason you can't implement your own message pump to deal with your Ultrasound object.
A naive, very simplistic version might look something like this (the BlockingCollection<T> class is great if you're on .NET 4.0 or have installed Rx extensions; otherwise, you can just use a plain vanilla Queue<T> and do your own locking). Warning: this is just a quick skeleton I've thrown together just now; I make no promises as to its robustness or even correctness.
class MessagePump<T>
{
// In your case you would set this to your Ultrasound object.
// You could just as easily design this class to be "object-agnostic";
// but I think that coupling an instance to a specific object makes it clearer
// what the purpose of the MessagePump<T> is.
private T _obj;
private BlockingCollection<Action<T>> _workItems;
private Thread _thread;
public MessagePump(T obj)
{
_obj = obj;
// Note: the default underlying data store for a BlockingCollection<T>
// is a FIFO ConcurrentQueue<T>, which is what we want.
_workItems = new BlockingCollection<Action<T>>();
_thread = new Thread(ProcessQueue);
_thread.IsBackground = true;
_thread.Start();
}
public void Submit(Action<T> workItem)
{
_workItems.Add(workItem);
}
private void ProcessQueue()
{
for (;;)
{
Action<T> workItem = _workItems.Take();
try
{
workItem(_obj);
}
catch
{
// Put in some exception handling mechanism so that
// this thread is always running. One idea would be to
// raise an event containing the Exception object on a
// threadpool thread. You definitely don't want to raise
// the event from THIS thread, though, since then you
// could hit ANOTHER exception, which would defeat the
// purpose of this catch block.
}
}
}
}
Then what would happen is: every time you want to interact with your Ultrasound object in some way, you do so through this message pump, by calling Submit and passing in some action that works with your Ultrasound object. The Ultrasound object then receives all messages sent to it synchronously (by which I mean, one at a time), while operating on its own non-GUI thread.

You should maintain a dedicated UltraSound thread, which creates the UltraSound object and then listens for callbacks from other threads.
You should maintain a thread-safe queue of delegates and have the UltraSound thread repeatedly execute and remove the first delegate in the queue.
This way, the UI thread can post actions to the queue, which will then be executed asynchronously by the UltraSound thread.

I'm not sure I fully understand the setup, but here is my attempt at a solution:
How about having the event handler for the slider check the last event time, and wait for 50ms before processing a user adjustment (only process the most recent value).
Then have a thread using a while loop and waiting on an AutoResetEvent trigger from the GUI. It would then create the object and set it?

How can a child thread notify a parent thread of its status/progress?

I have a service responsible for many tasks, one of which is to launch jobs (one at a time) on a separate thread (threadJob child), these jobs can take a fair amount of time and
have various phases to them which I need to report back.
Ever so often a calling application requests the status from the service (GetStatus), this means that somehow the service needs to know at what point the job (child thread) is
at, my hope was that at some milestones the child thread could somehow inform (SetStatus) the parent thread (service) of its status and the service could return that information
to the calling application.
For example - I was looking to do something like this:
class Service
{
private Thread threadJob;
private int JOB_STATUS;
public Service()
{
JOB_STATUS = "IDLE";
}
public void RunTask()
{
threadJob = new Thread(new ThreadStart(PerformWork));
threadJob.IsBackground = true;
threadJob.Start();
}
public void PerformWork()
{
SetStatus("STARTING");
// do some work //
SetStatus("PHASE I");
// do some work //
SetStatus("PHASE II");
// do some work //
SetStatus("PHASE III");
// do some work //
SetStatus("FINISHED");
}
private void SetStatus(int status)
{
JOB_STATUS = status;
}
public string GetStatus()
{
return JOB_STATUS;
}
};
So, when a job needs to be performed RunTask() is called and this launches the thread (threadJob). This will run and perform some steps (using SetStatus to set the new status at
various points) and finally finish. Now, there is also function GetStatus() which should return the STATUS whenever requested (from a calling application using IPC) - this status
should reflect the current status of the job running by threadJob.
So, my problem is simple enough...
How can threadJob (or more specifically PerformWork()) return to Service the change in status in a thread-safe manner (I assume my example above of SetStatus/GetStatus is
unsafe)? Do I need to use events? I assume I cannot simply change JOB_STATUS directly ... Should I use a LOCK (if so on what?)...

You may have already looked into this, but the BackgroundWorker class gives you a nice interface for running tasks on background threads, and provides events to hook into for notifications that progress has changed.

You could create an event in the Service class and then invoke it in a thread-safe manner. Pay very close attention to how I have implemented the SetStatus method.
class Service
{
public delegate void JobStatusChangeHandler(string status);
// Event add/remove auto implemented code is already thread-safe.
public event JobStatusChangeHandler JobStatusChange;
public void PerformWork()
{
SetStatus("STARTING");
// stuff
SetStatus("FINISHED");
}
private void SetStatus(string status)
{
JobStatusChangeHandler snapshot;
lock (this)
{
// Get a snapshot of the invocation list for the event handler.
snapshot = JobStatusChange;
}
// This is threadsafe because multicast delegates are immutable.
// If you did not extract the invocation list into a local variable then
// the event may have all delegates removed after the check for null which
// which would result in a NullReferenceException when you attempt to invoke
// it.
if (snapshot != null)
{
snapshot(status);
}
}
}

I'd have the child thread raise a 'statusupdate' event, passing a struct with the information necessary for the parent and have the parent subscribe to it when launching it.

You can use the Event-Based Async Pattern.

I would go with delegate/event from the thread to the caller. If caller was UI or somewhere on that lines, I would be nice to the message pump and use appropriate Invoke()s to serialize notifications with the UIs thread when required.

I once wrote an app that needed a marker showing the progress a thread was making. I just used a shared global variable between them. The parent would just read the value, and the thread would just update it. No need to synchronize as only the parent read it, and only the child wrote it atomically. As it happened the parent was redrawing things frequently enough anyhow that it didn't even need to be poked by the child when the child updated the variable. Sometimes the simplest possible way works well.

Your current code mixes strings and ints for JOB_STATUS, which can't work. I'm assuming strings here, but it doesn't really matter, as I'll explain.
You current implementation is thread safe in the sense that no memory corruption will occur, since all assignments to reference type fields are guaranteed to be atomic. The CLR demands this, otherwise you could potentially access unmanaged memory if you could somehow access partially updated references. Your processor gives you that atomicity for free, however.
So as long as you're using reference types like strings, you won't get any memory corruption. The same is true for primitives like ints (and smaller) and enums based on them. (Just avoid longs and bigger, and non-primitive value types such as nullable integers.)
But, that is not the end of the story: this implementation is not guaranteed to always represent the current state. The reason for this is that the thread that calls GetStatus might be looking at a stale copy of the JOB_STATUS field, because the assignment in SetState contains no so-called memory barrier. That is: the new value for JOB_STATUS need not be sent to your main RAM right away. There are several reasons why this can be delayed:
Writing to main RAM is inherently slow (relatively speaking), which is the reason your processor has all kinds of buffers and L-something caches in the first place, so the processor usually delays memory synchronization. Not for very long, but it will probably delay. This can be quite noticeable on multicore processors, as these usually have separate caches per core.
The JIT might have stored the value of JOB_STATUS in a register earlier on, as part of some optimization strategy. Again, registers are far more efficient to use than your main RAM. However, this does mean that it might not see changes early enough, as it's still looking at the old copy in the register. (We're not talking minutes here, but still.)
So, if you want to be 100% certain that each thread & processor core is immediately aware of the changed status, declare your field as volatile:
private volatile int JOB_STATUS;
Now, GetStatus/SetStatus, without any locking constructs, is truly thread safe, as volatile demands that the value is read from and written to main RAM immediately (or something 100% equivalent, if the processor can do that more efficiently).
Note that if you don't declare your field as volatile you must use synchronization primitives, such as lock, but generally speaking you need to use the synchronization primitives both Get and Set, otherwise you won't solve the problem that volatile fixes.
Mind you, as you're doing IPC calls to get the status, I'd wager that you won't ever actually be able to observe any difference between non-volatile and volatile, given the overhead of the IPC calls and the thread synchronizations undoubtedly performed behind the scenes.
For more information on volatile, see volatile (C#) on MSDN.

EventWaitHandle behavior for pthread_cond_t

I've recently seen the light of EventWaitHandle's powerful behavior in C# and decided to move some functionality in a sister application to do the same. The only problem is that the sister app is written in C.
No big deal, I'm using pthreads, which have a pthread_cond_t datatype that allows for signalling. My only question is, is it possible for a cond to be 'signalled' before something is waiting on it?
Right now my testing says no. That is, if ThreadA fires a signal before ThreadB is waiting, ThreadB will wait indefinately. Is there another pthread type that I can use that behaves closer to the functionality of the EventWaitHandle in C#? An object is signalled, meaning that the first thread to wait on it, will pass immediately, and set it to unsignalled.
Wrapping the pthread_cond into another data structure wouldn't be too hard to achieve this. But again, is this functionality already available in the pthread library?

If you're using condition variables correctly, this won't matter.
The basic flow of your code should be (in pseudocode):
lock(lockobj);
while (!signalled) {
wait(condvar);
}
signalled = false;
unlock(lockobj);
on the waiting side, and:
lock(lockobj);
signalled = true;
notify(condvar);
unlock(lockobj);
on the signalling side. (Of course, the lock object and condition variable used have to be the same on both sides.) Hope this helps!

Alternative answer (also in pseudocode) if you want multiple signallings (i.e., if signalled twice, then two threads can wait before the state is unsignalled again).
Waiting side:
lock(lockobj);
while (signalled != 0) {
wait(condvar);
}
--signalled;
unlock(lockobj);
Signalling side:
lock(lockobj);
++signalled;
notify(condvar);
unlock(lockobj);

I ended up just wrapping a condition type in a new structure and created some simple functions to behave much like the EventWaitHandle from C#. I needed two mutexes to acheive proper serialized access.
The the cond_mutex is used for waiting on the conditional variable, while the data_mutex is used when setting the state from signaled to not signaled.
The reset mode is the same from C#. AUTO or MANUAL. This allows for the event_wait_t to reset itself automatically after waiting. Or letting the programmer manually do it with a call to event_wait_reset(event_wait_t *ewh);

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C# (C-Sharp) is a programming language developed by Microsoft that runs on the .NET Framework.