I have a Win32 MFC app that creates a thread which listens on the RS232 port. When new data is received that listener thread allocates memory using new and posts a message to a window using PostMessage. This carries on just fine and the window handles the incoming data and deletes the memory as necessary using delete. I'm noticing some small memory leaks right as my program closes. My suspicion is that one or two final messages are being posted and are still sitting in the message queue at the moment the user shuts the program and the thread closes before that memory gets properly deleted. Is there a way I can insure certain things happen before the program closes? Can I make sure the message queue is empty or at least has processed some of these important messages? I have tried looking at WaitForInputIdle or PeekMessage in destructors and things like that. Any ideas on a good way to solve this?
I 100% agree that all allocated memory should be explicitly free'd. (Just as you should fixed all compiler warnings). This eliminates the diagnostic noise, allowing you to quickly spot real issues.
Building on Harry Johnston's suggestion, I would push all new data into some kind of a queue and simply post a command "check the queue", removing and freeing data in the message handler. That way you can easily free everything left in the queue before exiting.
For a small utility, that leak might be acceptable - but it might cover other causes that are less benign.
PostMessage does not guarantee delivery. So other options are
using a blocking SendMessage
add the data to a deque, use Post Message to notify the receiver new data is available
(Remote code review: if PostMessage returns false, do you delete the memory right away?)
The folks arguing to not worry about it have a valid point. The process is about to end, and the OS will release all the memory, so there's not much point in spending time cleaning up first.
However, this does create noise that might obscure ongoing memory leaks that could become real problems before you application exits. It also means your program would be harder to turn into a library that could be incorporated into another app later.
I'm a fan of writing clean shutdown code, and then, in opt builds, adding an early out to skip the unnecessary work. Thus your debug builds will tell you about real leaks, and your users will get a responsive exit.
To do this cleanly:
You'll need a way for the main thread to tell the listener thread to quit (or at least to stop listening). Otherwise you'll always have a small window of opportunity where the main thread is about the quit just as the listener does another allocation. The main thread will need to know that the listener thread has received and complied with this message. Only then, can the main thread go through the queue to free up all the memory associated with the last messages and know that nothing more will arrive.
Don't use TerminateThread, or you'll end up with additional problems! If the listener thread is waiting on a handle the represents the serial port, then you can make it instead wait on two handle: the serial port handle and the handle of an event. The main thread can raise the event when it wants the listener to quit. The listener thread can raise a different event to signal that it has stopped listening.
When the main thread gets the WM_QUIT, it should raise the event to tell the listener to quit, then wait on the event that says the listener thread is done, then use PeekMessage to pull any messages that the listener posted before it stopped and free the memory associated with them.
When posting a Windows message from a worker thread back to the main thread, our process sporadically receives the "not enough quota" error. Problem is that through extensive tracing we are now quite sure that the main thread's message queue is empty.
This is the configuration:
Regular Windows process, no GUI, hidden main window.
Worker thread creates its own window and message loop.
The two are exchanging messages via PostMessage from time to time. Frequency is in the order of one message per couple of seconds.
I couldn't find any other possible reason for this error in the net than the message queue being filled with 10.000 messages, but as I said, we are quite sure that this is not the case. Are there any other known situations, where this error code can be returned from a PostMessage call?
I'm using Oracle ODP.Net for enqueue and dequeue.
Process A : Enqueue
Process B : Dequeue with MessageAvailable event
If Process A and B are running, there is no problem. On the "Process B", the event is always fired.
But, if "Process B" is off and "Process A" is on, when "Process B" restarts, the queues inserted during the off time are lost.
Is there an option for to fire the event for all queues inserted in the past ?
Many Thanks
There seem to be two approaches to address this issue:
Call the Listen() method of the OracleAQQueue class (after registering for message notification) to pick up "orphaned" messages sitting in the queue. Note that Listen() blocks until a message is received or a timeout occurs. So you'd want to specify a (short) timeout to return back to the processing thread in the event no messages are on the queue.
Call the Dequeue() method and trap Oracle error 25228 (no message available to dequeue). See the following thread from the Oracle support forums: https://forums.oracle.com/forums/thread.jspa?threadID=2186496.
I've been scratching my head on this topic. If you still have to "manually" test for new messages, what is the benefit of using the MessageAvaiable event callback in the first place? One route I've pondered is to wrap the Listen() method in an async call so that the caller isn't blocking on the thread (until a message is received or a timeout occurs). I wrapped Listen() and Dequeue() in a custom Receive() method and created my own MessageReceived event handler to pass the message details to the calling thread. Seems somewhat redundant, since ODP.NET provides the out-of-box callback, but I don't have to deal with the issue you describe (or write code to "manually" test for "orphaned" messages.
Any comments/thoughts on approach are welcomed.
I've been looking at this one too and have ended up doing something similar to Greg. I've not used the Listen() method though as I don't think it offers me anything over and above a simple Dequeue() - Listen() seems to be beneficial when you want to listen on behalf of multiple consumers, which in my instance is not relevant (see Oracle Docs).
So, in my 'Process B' I first register for notifications before initiating a polling process to check for any existing messages. It doesn't Listen(), it just calls Dequeue() within a controlled loop with a Wait period of a couple of seconds set. If the polling process encounters an Oracle timeout the wait period has expired and polling stops. I may need to consider dealing with timeouts if the wait period hasn't expired (though not 100% sure this if this is likely to happen).
I've noticed that any messages which are enqueued whilst polling will call the message notification method but by the time this connects and tries to retrieve the message the polling process always seems to have taken it. So inside the message notification method I capture and ignore any OracleExceptions with number 25263 (no message in queue <...> with message ID <...>).
I have a thread that needs to dispatch a message (a simulated mouse event that it sends using SendInput) as soon as it's generated. I want this to happen in a loop where there is no sleep; adding any sleep hurts performance as I basically want the events to go in to the event loop immediately once they have been generated. Of course I also don't want the loop in the consumer thread to hog all the cpu so I can't just keep it running, although this gives me good performance.
As far as I understand this, the task is to make the consuming thread to wait for something that signals that the producing thread has provided something to dispatch (?) but how to best do this? I think I need something like two mutexes if I want to make the two threads mutually exclusive; consumer waits for the producer and the producer continues as soon as the consumer resumes running? Didn't really get this working so far and I'm really not sure how to best do this; CriticalSections vs. mutexes, something entirely different?
The reason I don't want to call SendInput from the producer thread is that that thread (the 'main thread'?) actually runs in response to a mouse move message, intercepted by a mouse hook, and sending more mouse messages from that thread didn't allow the thread to finnish before the simulated mouse move event was processed, messing things for me. As I suspected, moving the SendInput call to another thread so that the original thread could finish out of the way fixed the problem but now I need to make the consumer more responsive; mouse messages keep coming at a good pace I suppose since just a 1 ms Sleep made the loop too slow and message processing started lagging; all works well if I have no sleep.
Thanks.
Sounds like you want to use win32 event objects instead of mutexes or critical sections. See the docs here. The event functions allow a thread to wait on a condition that can be signaled from another thread.
Windows threads support message queues - usually used for windows messages but completely usable for messaging between worker threads. PostThreadMessage can be used in the Hook Proc to post messages to another thread for processing.
The worker thread can do a normal GetMessage loop to extract messages to process - rather than passing them on to DispatchMessage as you would in a UI thread you simply check the HWND is NULL in the message structure indicating its a thread message, then process the message yourself. In the case of potential message flooding PeekMessage can be used to cull any outstanding messages from the queue.
The documentation for the Win32 API PulseEvent() function (kernel32.dll) states that this function is “… unreliable and should not be used by new applications. Instead, use condition variables”. However, condition variables cannot be used across process boundaries like (named) events can.
I have a scenario that is cross-process, cross-runtime (native and managed code) in which a single producer occasionally has something interesting to make known to zero or more consumers. Right now, a well-known named event is used (and set to signaled state) by the producer using this PulseEvent function when it needs to make something known. Zero or more consumers wait on that event (WaitForSingleObject()) and perform an action in response. There is no need for two-way communication in my scenario, and the producer does not need to know if the event has any listeners, nor does it need to know if the event was successfully acted upon. On the other hand, I do not want any consumers to ever miss any events. In other words, the system needs to be perfectly reliable – but the producer does not need to know if that is the case or not. The scenario can be thought of as a “clock ticker” – i.e., the producer provides a semi-regular signal for zero or more consumers to count. And all consumers must have the correct count over any given period of time. No polling by consumers is allowed (performance reasons). The ticker is just a few milliseconds (20 or so, but not perfectly regular).
Raymen Chen (The Old New Thing) has a blog post pointing out the “fundamentally flawed” nature of the PulseEvent() function, but I do not see an alternative for my scenario from Chen or the posted comments.
Can anyone please suggest one?
Please keep in mind that the IPC signal must cross process boundries on the machine, not simply threads. And the solution needs to have high performance in that consumers must be able to act within 10ms of each event.
I think you're going to need something a little more complex to hit your reliability target.
My understanding of your problem is that you have one producer and an unknown number of consumers all of which are different processes. Each consumer can NEVER miss any events.
I'd like more clarification as to what missing an event means.
i) if a consumer started to run and got to just before it waited on your notification method and an event occurred should it process it even though it wasn't quite ready at the point that the notification was sent? (i.e. when is a consumer considered to be active? when it starts or when it processes its first event)
ii) likewise, if the consumer is processing an event and the code that waits on the next notification hasn't yet begun its wait (I'm assuming a Wait -> Process -> Loop to Wait code structure) then should it know that another event occurred whilst it was looping around?
I'd assume that i) is a "not really" as it's a race between process start up and being "ready" and ii) is "yes"; that is notifications are, effectively, queued per consumer once the consumer is present and each consumer gets to consume all events that are produced whilst it's active and doesn't get to skip any.
So, what you're after is the ability to send a stream of notifications to a set of consumers where a consumer is guaranteed to act on all notifications in that stream from the point where it acts on the first to the point where it shuts down. i.e. if the producer produces the following stream of notifications
1 2 3 4 5 6 7 8 9 0
and consumer a) starts up and processes 3, it should also process 4-0
if consumer b) starts up and processes 5 but is shut down after 9 then it should have processed 5,6,7,8,9
if consumer c) was running when the notifications began it should have processed 1-0
etc.
Simply pulsing an event wont work. If a consumer is not actively waiting on the event when the event is pulsed then it will miss the event so we will fail if events are produced faster than we can loop around to wait on the event again.
Using a semaphore also wont work as if one consumer runs faster than another consumer to such an extent that it can loop around to the semaphore call before the other completes processing and if there's another notification within that time then one consumer could process an event more than once and one could miss one. That is you may well release 3 threads (if the producer knows there are 3 consumers) but you cant ensure that each consumer is released just the once.
A ring buffer of events (tick counts) in shared memory with each consumer knowing the value of the event it last processed and with consumers alerted via a pulsed event should work at the expense of some of the consumers being out of sync with the ticks sometimes; that is if they miss one they will catch up next time they get pulsed. As long as the ring buffer is big enough so that all consumers can process the events before the producer loops in the buffer you should be OK.
With the example above, if consumer d misses the pulse for event 4 because it wasn't waiting on its event at the time and it then settles into a wait it will be woken when event 5 is produced and since it's last processed counted is 3 it will process 4 and 5 and then loop back to the event...
If this isn't good enough then I'd suggest something like PGM via sockets to give you a reliable multicast; the advantage of this would be that you could move your consumers off onto different machines...
The reason PulseEvent is "unreliable" is not so much because of anything wrong in the function itself, just that if your consumer doesn't happen to be waiting on the event at the exact moment that PulseEvent is called, it'll miss it.
In your scenario, I think the best solution is to manually keep the counter yourself. So the producer thread keeps a count of the current "clock tick" and when a consumer thread starts up, it reads the current value of that counter. Then, instead of using PulseEvent, increment the "clock ticks" counter and use SetEvent to wake all threads waiting on the tick. When the consumer thread wakes up, it checks it's "clock tick" value against the producer's "clock ticks" and it'll know how many ticks have elapsed. Just before it waits on the event again, it can check to see if another tick has occurred.
I'm not sure if I described the above very well, but hopefully that gives you an idea :)
There are two inherent problems with PulseEvent:
if it's used with auto-reset events, it releases one waiter only.
threads might never be awaken if they happen to be removed from the waiting queue due to APC at the moment of the PulseEvent.
An alternative is to broadcast a window message and have any listener have a top-level message -only window that listens to this particular message.
The main advantage of this approach is that you don't have to block your thread explicitly. The disadvantage of this approach is that your listeners have to be STA (can't have a message queue on an MTA thread).
The biggest problem with that approach would be that the processing of the event by the listener will be delayed with the amount of time it takes the queue to get to that message.
You can also make sure you use manual-reset events (so that all waiting threads are awaken) and do SetEvent/ResetEvent with some small delay (say 150ms) to give a bigger chance for threads temporarily woken by APC to pick up your event.
Of course, whether any of these alternative approaches will work for you depends on how often you need to fire your events and whether you need the listeners to process each event or just the last one they get.
If I understand your question correctly, it seems like you can simply use SetEvent. It will release one thread. Just make sure it is an auto-reset event.
If you need to allow multiple threads, you could use a named semaphore with CreateSemaphore. Each call to ReleaseSemaphore increases the count. If the count is 3, for example, and 3 threads wait on it, they will all run.
Events are more suitable for communications between the treads inside one process (unnamed events). As you have described, you have zero ore more clients that need to read something interested. I understand that the number of clients changes dynamically. In this case, the best chose will be a named pipe.
Named Pipe is King
If you need to just send data to multiple processes, it’s better to use named pipes, not the events. Unlike auto-reset events, you don't need own pipe for each of the client processes. Each named pipe has an associated server process and one or more associated client processes (and even zero). When there are many clients, many instances of the same named pipe are automatically created by the operating system for each of the clients. All instances of a named pipe share the same pipe name, but each instance has its own buffers and handles, and provides a separate conduit for client/server communication. The use of instances enables multiple pipe clients to use the same named pipe simultaneously. Any process can act as both a server for one pipe and a client for another pipe, and vice versa, making peer-to-peer communication possible.
If you will use a named pipe, there would be no need in the events at all in your scenario, and the data will have guaranteed delivery no matter what happens with the processes – each of the processes may get long delays (e.g. by a swap) but the data will be finally delivered ASAP without your special involvement.
On The Events
If you are still interested in the events -- the auto-reset event is king! ☺
The CreateEvent function has the bManualReset argument. 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, and system automatically resets the event state to non-signaled after a single waiting thread has been released.
These auto-reset events are very reliable and easy to use.
If you wait for an auto-reset event object with WaitForMultipleObjects or WaitForSingleObject, it reliably resets the event upon exit from these wait functions.
So create events the following way:
EventHandle := CreateEvent(nil, FALSE, FALSE, nil);
Wait for the event from one thread and do SetEvent from another thread. This is very simple and very reliable.
Don’t' ever call ResetEvent (since it automatically reset) or PulseEvent (since it is not reliable and deprecated). Even Microsoft has admitted that PulseEvent should not be used. See https://msdn.microsoft.com/en-us/library/windows/desktop/ms684914(v=vs.85).aspx
This function is unreliable and should not be used, because only those threads will be notified that are in the "wait" state at the moment 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. A thread waiting on a synchronization object can be momentarily removed from the wait state by a kernel-mode Asynchronous Procedure Call, and then 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 Asynchronous Procedure Calls 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
We have never used PulseEvent in our applications. As about auto-reset events, we are using them since Windows NT 3.51 (although they appeared in the first 32-bit version of NT - 3.1) and they work very well.
Your Inter-Process Scenario
Unfortunately, your case is a little bit more complicated. You have multiple threads in multiple processes waiting for an event, and you have to make sure that all the threads did in fact receive the notification. There is no other reliable way other than to create own event for each consumer. So, you will need to have as many events as are the consumers. Besides that, you will need to keep a list of registered consumers, where each consumer has an associated event name. So, to notify all the consumers, you will have to do SetEvent in a loop for all the consumer events. This is a very fast, reliable and cheap way. Since you are using cross-process communication, the consumers will have to register and de-register its events via other means of inter-process communication, like SendMessage. For example, when a consumer process registers itself at your main notifier process, it sends SendMessage to your process to request a unique event name. You just increment the counter and return something like Event1, Event2, etc, and creating events with that name, so the consumers will open existing events. When the consumer de-registers – it closes the event handle that it opened before, and sends another SendMessage, to let you know that you should CloseHandle too on your side to finally release this event object. If the consumer process crashes, you will end up with a dummy event, since you will not know that you should do CloseHandle, but this should not be a problem - the events are very fast and very cheap, and there is virtually no limit on the kernel objects - the per-process limit on kernel handles is 2^24. If you are still concerned, you may to the opposite – the clients create the events but you open them. If they won’t open – then the client has crashed and you just remove it from the list.