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.
Related
I have an application that creates two threads. (thread_1 for a Qt GUI and thread_2 for an app that runs a TCL interpreter).
I want thread_1 (Qt GUI) to create a command and send it to thread_2 (TCL interpreter).
I'm thinking of connecting thread_1's stdout to thread_2's stdin, and I don't know how to do it ?
if you know how to do it or can suggest different way of work, I'd appreciate your help.
The solution I propose requires to set up 2 std::queue<> or std::list to let each thread pass a message to the other one and vice versa. The simplest way is to have each thread setup its own incoming message queue, and let other threads get a pointer to it. First you need a synchronized version of the queue datatype: as I gave in the comment, there's an implementation there.
Then you only need to upgrade your thread class (or runnable class, or whatever you're using as an abstraction of a task) with one such queue available internally, and a send method publicly accessible so that other tasks may post a message to it. Your task will then have to periodically check that queue for incoming message, and eventually process it.
NB: I got that page from stack overflow itself, since the blog owner is a member of this community. See that page talking about queue synchronization issue.
I am not sure why you would like to go through standard input and output here, but I think the issue might be much simpler than you think. I would just personally use the qt signal-slot mechanism as follows:
connect(guiThreadSender, SIGNAL(sendCommand(const QByteArray&)),
tclThreadReceiver, SLOT(handleCommand(const QByteArray&)));
This is my scenario: I have a dll (with Qt in the back-end, and with no event loop started). I am able to perform the signal-slot communication with-in this dll (there is a only one thread). I would like to use the facilities of QFileSystemWatcher in this dll. But it looks like, QFileSystemWatcher starts its own thread, and it is not able to communicate to my main thread since there is no event process.
So, basically I need a way to start the event processing without being blocked !
So, basically I need a way to start the event processing without being blocked !
So, basically you want to use QEventLoop features without using the QEventLoop based on the comment discussion. The QEventLoop has to be "blocking", inherently, in order to actually have an event loop.
You could always create a "blocking" thread with the event loop inside, but then your signal-slot management might be tied to that particular thread.
This is not the usual way of using an event loop, but depending on your concrete scenario, it might be sufficient in this special case.
Hey I am not sure if this has already been asked that way. (I didn´t find anwsers to this specific questions, at least). But:
I have a program, which - at startup - creates an Login-window in a new UI-Thread.
In this window the user can enter data which has to be verified by an server.
Because the window shall still be responsive to the users actions, it (ofc it´s only a UI-thread) shall not handle the transmission and evaluation in it´s own thread.
I want the UI-thread to delegate this work back to the main thread.
In addition: The main thread (My "client" thread) shall manage all actions that go on, like logging in, handle received messages from the server etc... (not window messages)
But I am not sure of how to do this:
1.) Shall I let the UI-Thread Queue an APC to the main thread (but then the main thread does not know about the stuff going on.
2.) May I better use event objects to be waited on and queues to transmit the data from one thread to another?...
Or are there way better options?
For example: I start the client:
1. The client loads data from a file and does some intialization
The client creates a window in a new thread which handles login data input from the user.
The Window Thread shall notifiy and handle the , that has been entered by the user, over to the client.
The Client shall now pack the data and delegate the sending work to another object (e.g. CSingleConnection) which handles sending the data over a network (of course this does not require a new thread, because it can be handle with Overlapped I/O...
One special receiver thread receives the data from the server and handles it back to the client, which - in turn - evaluates the data.
If the data was correct and some special stuff was received from the server, the main thread shall signal the UI thread to close the window and terminate...
The client then creates a new window, which will handle the chatting-UI
The chatting UI thread and the Client thread shall communicate to handle messages to be sent and received...
(Hope this helps to get what I am trying)...
It all depends on what you are prepared to use. If you are developing with Qt, their signals and slots are just the thing to do such a communication. They also supply a network library, so you could easily omit the receiver thread because their network classes do asynchronous communication and will send a signal when you have data, which means your thread does not need to be blocked in the mean time.
If you don't want to use Qt, boost also supplies thread safe signals and slots, but as far as I understand it their slots will be run in the context of the calling thread...
Anyways, I have used Qt sig and slots with great satisfaction for exactly this purpose. I wholeheartedly agree GUI's shouldn't freeze, ever.
I don´t know wether this is good style or not (anwsering Your own question):
But I think I go with Event Objects and two queues (one for the connection between Client and Connection, and one to communicate Client and UI)...
I'm developing an application that gets large images from an Internet server which is the best way to download this images, without freeze the entire application? I mean background download. I have thought about download it in another thread.
Yes, you need to spawn another thread to do the network communication, and then when it is finished doing it's reading, you can use a volatile boolean flag to indicate that the work is complete and the main/application thread can take the data and incorporate it. The data may be "part" of an image if you want to show the image coming in piece by piece (as a browser does).
A background thread will work, but it's tricky to get right and not usually necessary... Qt4 makes it very easy to do non-blocking I/O in the main thread using the QTcpSocket class -- basically you connect the QTcpSocket object's readReady() signal to a slot it your program, and have your slot read out the newly available data from the QTcpSocket when it is called. For an example, have a look at the fortuneclient example in the Qt examples directory ($QTDIR/examples/network/fortuneclient).
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.