I worked on the synchronous libusb in my Qt project with good results and now I need the asynchronous features of this library. I understood reading here, here and here that, after I've registered my callback function using the libusb_fill_control_transfer and submitted a transfer with libusb_submit_transfer , I need to "keep live" the libusb_handle_events_completed inside a while loop to get the transfer related events since the libusb doesn't have its own thread. for example you can read a code like this
libusb_fill_control_transfer(transfer, dev, buffer, cb, &completed, 1000);
libusb_submit_transfer(transfer);
while (!completed) {
libusb_handle_events_completed(ctx, &completed);
}
Now if I want read a packet that I don't know when it occurs, I think that goes against the asynchronous nature submit a read and wait in the while with libusb_handle_events_completed until the event is triggered.
Then, do I need to create a separate thread within the libusb_handle_events_completed in an infinite while loop?
Can anyone, with experience in the asynchronous features of libusb library, give some suggestions on the right approach to handle the transfer events?
I am a newbie in using threads. I have developed a program in C++ using wxwidgets to show the count of packets received through the network interface. What I have done so far is: I have a wxbutton in a wxdialog. On clickng on the button a wxframe containing wxgrid is opened. The wxgrid contains info like interface name, packets in and packets out.
Now what I need is to constantly update the packets in and packets out fields. I have a function that calculates the packets in and packets out. So I think I need to execute the function continuously until the user closes the frame.
I hope wxthread can be utilized to perform this. But have no idea on how to start with or how a function can be executed from a thread. Can someone please guide me.. THANK YOU
A worker thread can't access any GUI object directly, so you need to post events to the main thread where you can define event handlers for them which will do whatever you need. There is a convenient base class for such events called wxThreadEvent that you may find useful.
Alternatively, and especially if you use C++11, you could use CallAfter() which allows you to execute a callback in the main thread context. This is especially nice with C++11 lambdas because it allows you to keep all your code in the same place without having to extract it into a separate event handler.
Please let me explain what is my problem:
I have a Gui application, that has to connect to remote server and keep connected to it for the time untill a user decides to quit the connection, or the server will. I wish to create the client connection mechanism in a separate thread. If the client should be able to asynchronusly receive data and in event driven style inform the main gui thread about it. The thread should also be able to receive data from gui thread to be sent to the server.
I come from a low level microcontroller place, where I would handle this task simply using interrupts and while(1) loop and flags. The problem is on a pc, this would take to much processor time. I have watched and read a lot of tutorials about sockets and threads in qt, but i still dont know what is the best aproach and how to do it properly.
For now, I have a test server on a remote target that is able to receive connections from my Qt client that I am trying to write. I have a class now for my client in Qt, that inherits from Qthread, but then I read that it is not the best aproach anymore.
I wish to create a client instance in new thread (triggered from the gui thread) that will hang forever with exec(). Now I dont know how to handle, using signals the incoming data from the server and incoming commands from the main GUI thread. In general, I would maybe know how to implement this on a low level, but i read about a lot of high level functions for this that qt delivers, i wish to use that.
I would really aprichiate help in this matter. I tried searching, but havent found any solid, working up to date code examples. Could someone please explain me how to create a client instannce in a new thread that wont disconnect after sending/ receiving some data, but instead stay connected and stay responsive to to server calls and gui thread calls in event driven style?
May be use general Qt socket mechanism instead separate thread will be better for you. Sockets is very similar to MCU interrupts and simple to use. For your application requests it must be enough.
I made a MFC application which probably has two threads, one for receiving data from a socket using UDP protocol and one is the main thread of MFC app. While any data is received some objects, created in the main thread by new operator, would be notified to fetch the data through apply the observer design pattern. The problem is that sometimes after I clicked the close system button, the GUI of the app disappeared, but its process can still be found in the Task Manager. If I stop the data source (UDP client) this problem would never happen. Other important and maybe helpful information is listed below:
The Observer design pattern was implemented with STL container list. I have used the critical section protection in the Attach, Detach and Notify functions.
I deleted the observer objects before closing the UDP socket.
The data transfer rate may be a little faster than process data, because after closing the data source the data process is still working.
I can't figure out what lead my app can not exit completely. Please give me some clues.
This is usually caused by a thread you created and not exit it programmatically when you exit the appliation. There must be a while clause in your thread. The way to find where it is still running is:
use debug mode to start you application and click the exit button the top right corner to exit it.
Check from task manager and see if it is still running
if it is, excute Debug->Break All,
Open threads windows, double click each thread, you will find where your code is still looping.
Typically a process won't terminate because there's still a foreground thread running somewhere. You must ensure that your socket library isn't running any thread when you want to close your application.
First thing, with MFC, please use the notification based methods to get notifications on message arrivals, connections etc. So you can get rid of threads if you have.
It's quite easy to attache to a debugger and Break see which threads are existing and waiting for what.
Alternatively you can use ProcessExplorer with proper symbol configuration to see the call stacks of the threads available for the particular process.
The application can two kind of issues to exit, one could be infinite loop and other might be waiting/deadlock (e.g. socket read command is a blocking call). You can easily deduce the problem by attaching to debugger.
Otherwise please provide further information about the threads, code snippet possible.
I'm programming an online game for two reasons, one to familiarize myself with server/client requests in a realtime environment (as opposed to something like a typical web browser, which is not realtime) and to actually get my hands wet in that area, so I can proceed to actually properly design one.
Anywho, I'm doing this in C++, and I've been using winsock to handle my basic, basic network tests. I obviously want to use a framelimiter and have 3D going and all of that at some point, and my main issue is that when I do a send() or receive(), the program kindly idles there and waits for a response. That would lead to maybe 8 fps on even the best internet connection.
So the obvious solution to me is to take the networking code out of the main process and start it up in its own thread. Ideally, I would call a "send" in my main process which would pass the networking thread a pointer to the message, and then periodically (every frame) check to see if the networking thread had received the reply, or timed out, or what have you. In a perfect world, I would actually have 2 or more networking threads running simultaneously, so that I could say run a chat window and do a background download of a piece of armor and still allow the player to run around all at once.
The bulk of my problem is that this is a new thing to me. I understand the concept of threading, but I can see some serious issues, like what happens if two threads try to read/write the same memory address at the same time, etc. I know that there are already methods in place to handle this sort of thing, so I'm looking for suggestions on the best way to implement something like this. Basically, I need thread A to be able to start a process in thread B by sending a chunk of data, poll thread B's status, and then receive the reply, also as a chunk of data., ideally without any major crashing going on. ^_^ I'll worry about what that data actually contains and how to handle dropped packets, etc later, I just need to get that happening first.
Thanks for any help/advice.
PS: Just thought about this, may make the question simpler. Is there a way to use the windows event handling system to my advantage? Like, would it be possible to have thread A initialize data somewhere, then trigger an event in thread B to have it pick up the data, and vice versa for thread B to tell thread A it was done? That would probably solve a lot of my problems, since I don't really need both threads to be able to work on the data at the same time, more of a baton pass really. I just don't know if this is possible between two different threads. (I know one thread can create its own messages for the event handler.)
The easiest thing
for you to do, would be to simply invoke the windows API QueueUserWorkItem. All you have to specify is the function that the thread will execute and the input passed to it. A thread pool will be automatically created for you and the jobs executed in it. New threads will be created as and when is required.
http://msdn.microsoft.com/en-us/library/ms684957(VS.85).aspx
More Control
You could have a more detailed control using another set of API's which can again manage the thread pool for you -
http://msdn.microsoft.com/en-us/library/ms686980(VS.85).aspx
Do it yourself
If you want to control all aspects of your thread creation and the pool management you would have to create the threads yourself, decide how they should end , how many to create etc (beginthreadex is the api you should be using to create threads. If you use MFC you should use AfxBeginThread function).
Send jobs to worker threads - Io completion Ports
In this case, you would also have to worry about how to communicate your jobs - i would recommend IoCOmpletionPorts to do that. It is the most scalable notification mechanism that i currently know of made for this purpose. It has the additional advantage that it is implemented in the kernel so you avoid all kinds of dead loack sitautions you would encounter if you decide to handroll something yourself.
This article will show you how with code samples -
http://blogs.msdn.com/larryosterman/archive/2004/03/29/101329.aspx
Communicate Back - Windows Messages
You could use windows messages to communicate the status back to your parent thread since it is doing the message wait anyway. use the PostMessage function to do this. (and check for errors)
ps : You could also allocate the data that needs to be sent out on a dedicated pointer and then the worker thread could take care of deleting it after sending it out. That way you avoid the return pointer traffic too.
BlodBath's suggestion of non-blocking sockets is potentially the right approach.
If you're trying to avoid using a multithreaded approach, then you could investigate the use of setting up overlapped I/O on your sockets. They will not block when you do a transmit or receive, but have the added bonus of giving you the option of waiting for multiple events within your single event loop. When your transmit has finished, you will receive an event. (see this for some details)
This is not incompatible with a multithreaded approach, so there's the option of changing your mind later. ;-)
On the design of your multithreaded app. the best thing to do is to work out all of the external activities that you want to be alerted to. For example, so far in your question you've listed network transmits, network receives, and user activity.
Depending on the number of concurrent connections you're going to be dealing with you'll probably find it conceptually simpler to have a thread per socket (assuming small numbers of sockets), where each thread is responsible for all of the processing for that socket.
Then you can implement some form of messaging system between your threads as RC suggested.
Arrange your system so that when a message is sent to a particular thread and event is also sent. Your threads can then be sent to sleep waiting for one of those events. (as well as any other stimulus - like socket events, user events etc.)
You're quite right that you need to be careful of situations where more than one thread is trying to access the same piece of memory. Mutexes and semaphores are the things to use there.
Also be aware of the limitations that your gui has when it comes to multithreading.
Some discussion on the subject can be found in this question.
But the abbreviated version is that most (and Windows is one of these) GUIs don't allow multiple threads to perform GUI operations simultaneously. To get around this problem you can make use of the message pump in your application, by sending custom messages to your gui thread to get it to perform gui operations.
I suggest looking into non-blocking sockets for the quick fix. Using non-blocking sockets send() and recv() do not block, and using the select() function you can get any waiting data every frame.
See it as a producer-consumer problem: when receiving, your network communication thread is the producer whereas the UI thread is the consumer. When sending, it's just the opposite. Implement a simple buffer class which gives you methods like push and pop (pop should be blocking for the network thread and non-blocking for the UI thread).
Rather than using the Windows event system, I would prefer something that is more portable, for example Boost condition variables.
I don't code games, but I've used a system similar to what pukku suggested. It lends nicely to doing things like having the buffer prioritize your messages to be processed if you have such a need.
I think of them as mailboxes per thread. You want to send a packet? Have the ProcessThread create a "thread message" with the payload to go on the wire and "send" it to the NetworkThread (i.e. push it on the NetworkThread's queue/mailbox and signal the condition variable of the NetworkThread so he'll wake up and pull it off). When the NetworkThread receives the response, package it up in a thread message and send it back to the ProcessThread in the same manner. Difference is the ProcessThread won't be blocked on a condition variable, just polling on mailbox.empty( ) when you want to check for the response.
You may want to push and pop directly, but a more convenient way for larger projects is to implement a toThreadName, fromThreadName scheme in a ThreadMsg base class, and a Post Office that threads register their Mailbox with. The PostOffice then has a send(ThreadMsg*); function that gets/pushes the messages to the appropriate Mailbox based on the to and from. Mailbox (the buffer/queue class) contains the ThreadMsg* = receiveMessage(), basically popping it off the underlying queue.
Depending on your needs, you could have ThreadMsg contain a virtual function process(..) that could be overridden accordingly in derived classes, or just have an ordinary ThreadMessage class with a to, from members and a getPayload( ) function to get back the raw data and deal with it directly in the ProcessThread.
Hope this helps.
Some topics you might be interested in:
mutex: A mutex allows you to lock access to specific resources for one thread only
semaphore: A way to determine how many users a certain resource still has (=how many threads are accessing it) and a way for threads to access a resource. A mutex is a special case of a semaphore.
critical section: a mutex-protected piece of code (street with only one lane) that can only be travelled by one thread at a time.
message queue: a way of distributing messages in a centralized queue
inter-process communication (IPC) - a way of threads and processes to communicate with each other through named pipes, shared memory and many other ways (it's more of a concept than a special technique)
All topics in bold print can be easily looked up on a search engine.