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I am not able to understand the PulseEvent or race condition. But to avoid it I am trying to SetEvent instead, and ResetEvent every time before WaitForMultipleObjectsEx.
This is my flow:
Thread One - Uses CreateEvent to create an auto reseting event, I then spawn and tell Thread TWO about it.
Thread One - Tell thread TWO to run.
Thread TWO will do ResetEvent on event and then immediately start WaitForMultipleObjectsEx on the event and some other stuff for file watching. If WaitForMultipleObjectsEx returns, and it is not due to the event, then restart the loop immediately. If WaitForMultipleObjectsEx returns, due to event going to signaled, then do not restart loop.
So now imagine this case please:
Thread TWO - loop is running
Thread One - needs to add a path, so it does (1) SetEvent, and then (2) sends another message to thread 2 to add a path, and then (3) sends message to thread 2 to restart loop.
The messages of add path and restart loop will not come in to Thread TWO unless I stop the loop in TWO, which is done by the SetEvent. Thread TWO will see it was stoped due to the event, and so it wont restart the loop. So it will now get the message to add path, so it will add path, then restart loop.
Thread One - needs to stop the thread, so it does (1) SetEvent and then (2) waits for message thread 2, when it gets that message it will terminate the thread.
Will this avoid race condition?
Thank you
Suppose the loop needs to be interrupted twice in succession. You're imagining a sequence of events something like this, on thread ONE and thread TWO:
Thread ONE realizes that the first interruption is complete.
Thread ONE sends a message telling TWO to restart the wait loop.
Thread TWO reads the message "restart the wait loop".
Thread TWO resets the event.
Thread TWO starts waiting.
Thread ONE now realizes that another interruption is needed.
Thread ONE sets the event to ask for another interruption.
Thread ONE sends message related to the second interruption.
Thread TWO stops the loop, receives the message about the second interruption.
But since you don't have any control over the timing between the two threads, it might instead happen like this:
Thread ONE realizes that the first interruption is complete.
Thread ONE sends a message telling TWO to restart the wait loop.
Thread ONE now realizes that another interruption is needed.
Thread ONE sets the event to ask for another interruption.
Thread TWO reads the message "restart the wait loop".
Thread TWO resets the event.
Thread TWO starts waiting.
Thread ONE sends a message about the second interruption, but TWO isn't listening!
Even if the message passing mechanism is synchronous, so that ONE won't continue until TWO has read the message, it could happen this way:
Thread ONE realizes that the first interruption is complete.
Thread ONE sends a message telling TWO to restart the wait loop.
Thread TWO reads the message "restart the wait loop", but is then swapped out.
Thread ONE now realizes that another interruption is needed.
Thread ONE sets the event to ask for another interruption.
Thread TWO resets the event.
Thread TWO starts waiting.
Thread ONE sends a message about the second interruption, but TWO isn't listening!
(Obviously, a similar thing can happen if you use PulseEvent.)
One quick solution would be to use a second event for TWO to signal ONE at the appropriate point, i.e., after resetting the main event but before waiting on it, but that seems somewhat inelegant and also doesn't generalize very well. If you can guarantee that there will never be two interruptions in close-enough succession, you might simply choose to ignore the race condition, but note that it is difficult to reason about this because there is no theoretical limit to how long it might take for thread TWO to resume running after being swapped out.
The various alternatives depend on how the messages are being passed between the threads and any other constraints. [If you can provide more information about your current implementation I'll update my answer accordingly.]
This is an overview of some of the more obvious options.
If the message-passing mechanism is synchronous (if thread ONE waits for thread TWO to receive the message before proceeding) then using a single auto-reset event should just work. Thread ONE won't set the event until after thread TWO has received the restart-loop message. If the event is already set when thread TWO starts waiting, that just means that there were two interruptions in immediate succession; TWO will never stall waiting for a message that isn't coming. [This potential stall is the only reason I can think of why you might not want to use an auto-reset event. If you have another concern, please edit your question to provide more details.]
If is OK for sending a message to be non-blocking, and you aren't already locked in to a particular solution, any of these options would probably be sensible:
User mode APCs (the QueueUserAPC function) provide a message-passing mechanism that automatically interrupts alertable waits.
You could implement a simple queue (protected by a critical section) which uses an event to indicate whether there is a message pending or not. In this case you can safely use a manual-reset event provided that you only manipulate it when you hold the same critical section that protects the queue.
You could use an auto-reset event in combination with any sort of thread-safe queue, provided only that the queue allows you to test for emptiness without blocking. The idea here is that thread ONE would always insert the message into the queue before setting the event, and if thread TWO sees that the event is set but it turns out that the queue is empty, the event is ignored. If efficiency is a concern, you might even be able to find a suitable lock-free queue implementation. (I don't recommend attempting that yourself.)
(All of those mechanisms could also be made synchronous by using a second event object.)
I wouldn't recommend the following approaches, but if you happen to already be using one of these for messaging this is how you can make it work:
If you're using named pipes for messaging, you could use asynchronous I/O in thread TWO. Thread TWO would use an auto-reset event internally, you specify the event handle when you issue the I/O call and Windows sets it when I/O arrives. From the point of view of thread ONE, there's only a single operation. From the point of view of thread TWO, if the event is set, a message is definitely available. (I believe this is somewhat similar to your original approach, you just have to issue the I/O call in advance rather than afterwards.)
If you're using a window queue for messaging, the MsgWaitForMultipleObjectsEx() function allows you to wait for a window message and other events simultaneously.
PS:
The other problem with PulseEvent, the one mentioned in the documentation, is that this can happen:
Thread TWO starts waiting.
Thread TWO is preempted by Windows and all user code on the thread stops running.
Thread ONE pulses the event.
Thread TWO is restarted by Windows, and the wait is resumed.
Thread ONE sends a message, but TWO isn't listening.
(Personally I'm a bit disappointed that the kernel doesn't deal with this situation; I would have thought that it would be possible for it to set a flag saying that the wait shouldn't be resumed. But I can only assume that there is a good reason why this is impractical.)
The Auto-Reset Events
Would you please try to change the flow so there is just SetEvent and WaitForMultipleObjectsEx with auto-reset events? You may create more events if you need. For example, each thread will have its own pair of events: one to get notifications and another to report about its state changes - you define the scheme that best suits your needs.
Since there will be auto-reset events, there would be neither ResetEvent nor PulseEvent.
If you will be able to change the logic of the algorithm flow this way - the program will become clear, reliable, and straightforward.
I advise this because this is how our applications work since the times of Windows NT 3.51 – we manage to do everything we need with just SetEvent and WaitForMultipleObjects (without the Ex suffix).
As for the PulseEvent, as you know, it is very unreliable, even though it exists from the very first version of Windows NT - 3.1 - maybe it was reliable then, but not now.
To create the auto-reset events, use the bManualReset argument of the CreateEvent API function (if this parameter is TRUE, the function creates a manual-reset event object, which requires the use of the ResetEvent function to set the event state to non-signaled -- this is not what you need). If this parameter is FALSE, the function creates an auto-reset event object. The system will automatically reset the event state to non-signaled after a single waiting thread has been released, i.e., after WaitForMultipleObjects or WaitForSingleObject or other wait functions that explicitly wait for this event to become signaled.
These auto-reset events are very reliable and easy to use.
Let me make a few additional notes on the PulseEvent. Even Microsoft has admitted that PulseEvent is unreliable and should not be used -- see https://msdn.microsoft.com/en-us/library/windows/desktop/ms684914(v=vs.85).aspx -- because only those threads will be notified that are in the "wait" state when PulseEvent is called. If they are in any other state, they will not be notified, and you may never know for sure what the thread state is, and, even if you are responsible for the program flow, the state can be changed by the operating system contrary to your program logic. A thread waiting on a synchronization object can be momentarily removed from the wait state by a kernel-mode Asynchronous Procedure Call (APC) and returned to the wait state after the APC is complete. If the call to PulseEvent occurs during the time when the thread has been removed from the wait state, the thread will not be released because PulseEvent releases only those threads that are waiting at the moment it is called.
You can find out more about the kernel-mode APC at the following links:
https://msdn.microsoft.com/en-us/library/windows/desktop/ms681951(v=vs.85).aspx
http://www.drdobbs.com/inside-nts-asynchronous-procedure-call/184416590
http://www.osronline.com/article.cfm?id=75
The Manual-Reset Events
The Manual-Reset events are not that bad. :-) You can reliably use them when you need to notify multiple instances of a global state change that occurs only once, for example, application exit. The auto-reset events can only be used to notify one thread (because if more threads are waiting simultaneously for an auto-reset event and you set the event, one random thread will exist and will reset the event, but the behavior of the remaining threads that also wait for the event, will be undefined). From the Microsoft documentation, we may assume that one and only one thread will exit while others would definitely not exit, but this is not very explicitly articulated in the documentation. Anyway, we must take the following quote into consideration: "Do not assume a first-in, first-out (FIFO) order. External events such as kernel-mode APCs can change the wait order" Source - https://msdn.microsoft.com/en-us/library/windows/desktop/ms682655(v=vs.85).aspx
So, when you need to notify all the threads quickly – just set the manual-reset event to the signaled state, rather than signaling each auto-reset event for each thread. Once you have signaled the manual-reset event, do not call ResetEvent since then. The drawback of this solution is that the threads need to have an additional event handle passed in the array of their WaitForMultipleObjects. The array size is limited, although, to MAXIMUM_WAIT_OBJECTS, which is 64, we never reached close to this limit in practice.
You can get more ideas about auto-reset events and manual reset events from https://www.codeproject.com/Articles/39040/Auto-and-Manual-Reset-Events-Revisited
How can I terminate a thread in a proper way?
When the thread finishes, it is still in memory. I'm using Delphi 2010 (Update 5).
The way I usually describe the issues of thread termination is to stress co-operation. You should not terminate a thread. Instead you should notify the thread that you want it to terminate. You then politely wait until it has terminated.
The reasons for this are manifest. Only the thread knows how to terminate itself. Only the thread knows what locks it holds, what resources it needs to free etc.
The same arguments apply if you wish to pause or suspend a thread's execution. You should ask to it do so and then let the thread find a convenient moment when it is safe to do so.
With a Delphi TThread the standard way to request termination is to call Thread.Terminate. This does nothing more than to set a flag in the thread object. That is the request. The response is initiated by the thread code inside TThread.Execute. That should regularly check the value of its Terminated property. When that is found to be true, it should exit from the function. Naturally any tidy up (release locks, return resources etc.) should be performed before calling exit.
How exactly do you terminate a thread? If you just set Terminate, this is just a flag checked inside of the thread. If you need to terminate thread of execution (and not signal a TThread object that it needs to finish), you can use TerminateThread WinAPI function. But you should notice that this leads to resource leaks (as written in the comments in documentation for TerminateThread).
that depends on what you want to accomplish with that thread. you should provide more details about what you want to do, before we can help you.
here you have a very good tutorial on how to work with threads in Delphi:
http://www.eonclash.com/Tutorials/Multithreading/MartinHarvey1.1/Ch1.html
http://docwiki.embarcadero.com/RADStudio/en/Writing_multi-threaded_applications_Index
Can anybody give some information on timer_helper_thread() function of librt.so.1.
I am using posix timer_create() function in my application for timer functionality and i am using SIEV_THREAD for notifiction. When timeout happens, i could see in gdb that two thread are getting created. One is the thread whose start function i have specified and another is the thread whose start function is timer_help_therad() of librt.so.1. Among these two timer_helper_thread() is not exiting even after my thread is exiting. Can anbody tell me when will timer_helper_thread() exit and give some informatin on it?
Short answer: don't worry about it; it's an implementation detail and will clean up after itself when your program exits. But if you're curious...
From glibc's timer_create(2) man page:
SIGEV_THREAD:
Upon timer expiration, invoke sigev_notify_function as if it were the start function of a new thread. (Among the implementation possibilities here are that each timer notification could result in the creation of a new thread, or that a single thread is created to receive all notifications.)
And also:
The functionality for SIGEV_THREAD is implemented within glibc, rather than the kernel.
So glibc (i.e. librt.so) assumes that the kernel cannot create a thread in response to a timer event -- that all it supports is sending a signal. So someone needs to receive that signal and create the handler thread. If you wanted to muck with the details of receiving the signal yourself, you wouldn't have used SIGEV_THREAD, so glibc doesn't bother you and instead creates its own thread just for handling timer events.
This timer helper thread lasts from the fist time you call timer_create() until your program ends. Unless you're doing something unusual, you don't need to worry about it; it will clean up after itself when your program exits. The only thing it does is wait for a timer to expire, so it's not using up any extra processing power. Furthermore, it looks like there will only ever be the one helper thread, no matter how many timers you create.
#jander: Your comment is interesting here "This timer helper thread lasts from the fist time you call timer_create() until your program ends."
There are threads created on everytime a timer is timeout. Is this same as the timer_helper_thread() you mention?
I have a similar post where I observe a separate thread created only for timer_create(). Would this be the timer_helper_thread()?
Ref: New thread on invocation of timer_create()
I've got a service that I need to shut down and update. I'm having difficulties with this in two different cases:
I have some threads that sleep for large amounts of time. Obviously I can't wait for them to wake up to finish shutting down the service. I had a thought to use an AutoResetEvent that gets set by some controller thread when the sleep interval is up (by just checking every two seconds or something), and triggering it immediately at OnClose time. Is there a better way to facilitate that?
I have one thread that makes a call to a blocking method call (one which I cannot modify). How do you signal such a thread to stop?
I'm not sure if I understood your first question correctly, but have you looked at using WaitForSingleObject as an alternative to Sleep? You can specify a timeout as well as an object to wait on, so if you want it to wake up earlier, just signal the object.
What exactly do you mean by "call to a blocking thread"? Or did you just mean a blocking call? In general, there isn't a way to interrupt a thread without forcefully terminating it. However, if the call is a system call, there might be ways to return control by making the call fail, eg. cancelling I/O or closing an associated handle.
For 1. you can get your threads into an interruptable Sleep by using SleepEx rather than Sleep. Once they get this shutdown kick (initiated from your termination logic using QueueUserApc), you can detect it happened using the return code from SleepEx and terminate those threads accordingly. This is similar to the suggestion to use WaitForSingleObject, but you don't need another per-thread handle that's just used to terminate the associated thread.
The return value is zero if the
specified time interval expired.
The return value is WAIT_IO_COMPLETION
if the function returned due to one or
more I/O completion callback
functions. This can happen only if
bAlertable is TRUE, and if the thread
that called the SleepEx function is
the same thread that called the
extended I/O function.
For 2., that's a tough one unless you have access to some resource used in that thread that can cause the blocking call to abort in such a way that the calling thread can handle it cleanly. You may just have to implement code to kill that thread with extreme prejudice using TerminateThread (probably this should be the last thing you do before exiting the process) and see what happens under test.
An easy and reliable solution is to kill the service process. A process is the memory-safe abstraction of the OS, after all, so you can safely terminate one without regard for process-internal state - of course, if your process is communicating or fiddling with external state, all bets are off...
Additionally, you could implement the solution which OS's themselves commonly do: one warning signal asking the process to clean up as best possible (which sets a flag and gracefully exits what can be gracefully stopped), and then forceful termination if the process doesn't exit by itself (which ends pesky things like blocking I/O).
All services should be built such that forceful termination isn't harmful, since these processes are system managed and may be terminated by things such as a reboot - i.e., your service ideally should permit this without corrupting storage anyhow.
Oh, and one final warning; windows services may share a process (I presume for efficiency, though it strikes me as an avoidable optimization), so if you go this route, you want to make sure your service is not sharing a process with other services. You can ensure this by passing the option SERVICE_WIN32_OWN_PROCESS to ChangeServiceConfig.
Does an asynchronous call always create a new thread? What is the difference between the two?
Does an asynchronous call always create or use a new thread?
Wikipedia says:
In computer programming, asynchronous events are those occurring independently of the main program flow. Asynchronous actions are actions executed in a non-blocking scheme, allowing the main program flow to continue processing.
I know async calls can be done on single threads? How is this possible?
Whenever the operation that needs to happen asynchronously does not require the CPU to do work, that operation can be done without spawning another thread. For example, if the async operation is I/O, the CPU does not have to wait for the I/O to complete. It just needs to start the operation, and can then move on to other work while the I/O hardware (disk controller, network interface, etc.) does the I/O work. The hardware lets the CPU know when it's finished by interrupting the CPU, and the OS then delivers the event to your application.
Frequently higher-level abstractions and APIs don't expose the underlying asynchronous API's available from the OS and the underlying hardware. In those cases it's usually easier to create threads to do asynchronous operations, even if the spawned thread is just waiting on an I/O operation.
If the asynchronous operation requires the CPU to do work, then generally that operation has to happen in another thread in order for it to be truly asynchronous. Even then, it will really only be asynchronous if there is more than one execution unit.
This question is darn near too general to answer.
In the general case, an asynchronous call does not necessarily create a new thread. That's one way to implement it, with a pre-existing thread pool or external process being other ways. It depends heavily on language, object model (if any), and run time environment.
Asynchronous just means the calling thread doesn't sit and wait for the response, nor does the asynchronous activity happen in the calling thread.
Beyond that, you're going to need to get more specific.
No, asynchronous calls do not always involve threads.
They typically do start some sort of operation which continues in parallel with the caller. But that operation might be handled by another process, by the OS, by other hardware (like a disk controller), by some other computer on the network, or by a human being. Threads aren't the only way to get things done in parallel.
JavaScript is single-threaded and asynchronous. When you use XmlHttpRequest, for example, you provide it with a callback function that will be executed asynchronously when the response returns.
John Resig has a good explanation of the related issue of how timers work in JavaScript.
Multi threading refers to more than one operation happening in the same process. While async programming spreads across processes. For example if my operations calls a web service, The thread need not wait till the web service returns. Here we use async programming which allows the thread not wait for a process in another machine to complete. And when it starts getting response from the webservice it can interrupt the main thread to say that web service has completed processing the request. Now the main thread can process the result.
Windows always had asynchronous processing since the non preemptive times (versions 2.13, 3.0, 3.1, etc) using the message loop, way before supporting real threads. So to answer your question, no, it is not necessary to create a thread to perform asynchronous processing.
Asynchronous calls don't even need to occur on the same system/device as the one invoking the call. So if the question is, does an asynchronous call require a thread in the current process, the answer is no. However, there must be a thread of execution somewhere processing the asynchronous request.
Thread of execution is a vague term. In a cooperative tasking systems such as the early Macintosh and Windows OS'es, the thread of execution could simply be the same process that made the request running another stack, instruction pointer, etc... However, when people generally talk about asynchronous calls, they typically mean calls that are handled by another thread if it is intra-process (i.e. within the same process) or by another process if it is inter-process.
Note that inter-process (or interprocess) communication (IPC) is commonly generalized to include intra-process communication, since the techniques for locking, and synchronizing data are usually the same regardless of what process the separate threads of execution run in.
Some systems allow you to take advantage of the concurrency in the kernel for some facilities using callbacks. For a rather obscure instance, asynchronous IO callbacks were used to implement non-blocking internet severs back in the no-preemptive multitasking days of Mac System 6-8.
This way you have concurrent execution streams "in" you program without threads as such.
Asynchronous just means that you don't block your program waiting for something (function call, device, etc.) to finish. It can be implemented in a separate thread, but it is also common to use a dedicated thread for synchronous tasks and communicate via some kind of event system and thus achieve asynchronous-like behavior.
There are examples of single-threaded asynchronous programs. Something like:
...do something
...send some async request
while (not done)
...do something else
...do async check for results
The nature of asynchronous calls is such that, if you want the application to continue running while the call is in progress, you will either need to spawn a new thread, or at least utilise another thread you that you have created solely for the purposes of handling asynchronous callbacks.
Sometimes, depending on the situation, you may want to invoke an asynchronous method but make it appear to the user to be be synchronous (i.e. block until the asynchronous method has signalled that it is complete). This can be achieved through Win32 APIs such as WaitForSingleObject.