What is the best way to handle event with SDL/C++ - c++

I am using SDL for the view parts of my game project. And I want to handle key press events without interrupting the main thread. So I decided to run an infinite loop in another view thread to catch any events and inform the main thread. However, I am not sure that this is the best since this may cause a workload and decrease the system performace? Is there any better way to do this kind of things?
Thanks.

Don't bother with another thread. What's the point?
What does your main thread do? I imagine something like this:
Update Logic
Render
Goto 1
If you receive input after (or during) the update cycle then you have to wait till the next update cycle before you'll see the effects. The same is true during rendering. You might as well just check for input before the update cycle and do it all singlethreaded.
Input
Update Logic
Render
Goto 1
Multithreading gains nothing here and just increases complexity.
For some added reading, check out Christer Ericson's blog post about input latency (he's the director of technology for the team that makes God of War).

And I want to handle key press events without interrupting the main thread.
SDL is not inherently an interrupt or event driven framework. IO occurs by reading events off of the event queue by calling SDL_WaitEvent or SDL_PollEvent. This must occur in the "main" thread, the one that called SDL_SetVideoMode.
That's not to say you cannot use multiple threads, and there's good justification for doing so, for instance, it can simplify network communication if it doesn't have to rely on the SDL event loop. If you want the simulation to occur in a separate thread, then it can pass information back and forth through synchronized shared objects. In particular, you can always put events into the SDL event queue safely from any thread.

Related

Converting a Console Program into an MFC app (Thread issues) (Pleora SDK)

Back to stackoverflow with another question after hours of trying on my own haha.
Thank you all for reading this and helping in advance.
Please note the console program has following functionalities:
connect to a frame grabber
apply some configs
store the incoming data (640 * 480 16-bit grayscale imgs) in a stream of buffers inside a while loop
Exits the while loop upon a key press.
disconnect from device
And I'm only adding the displaying the images functionality on the MFC GUI app. In short,
i) Converting a console app to an MFC app (dialog based)
ii) decided to use thread for displaying images, but DK how to properly exit from thread when there are certain tasks to be done (such as call disconnectFromDevice(); freeBuffers();, etc) before exiting the thread.
iii) have tried making the while loop condition false but didn't work
( I actually want this to be a callback function that's called repeatedly but IDK how to implement it inside a thread)
iv) forcing AfxEndThread didn't work and it's not even the way it should be done (I think).
So my question is,
1. Are you supposed to use a while loop to excuete a certain job that should repeatedly be done? If not, do you have to implement a callback inside a thread? Or use Windows message loop? Why and how? Please provide a hello-world-like sample code example
(for example, you are printing "hello world" repeatedly inside a thread with a condtion in an MFC GUI app. How do you update or check the condition to end the thread if you can't just AfxEndThread() inside the threadproc)
2. If it's ok with a while, how do you exit from the while loop, in other words how do you properly update the exit condition outside the thread the while loop's in?
Please refer to the source code in the provided link
ctrl+F OnBnClickedConnectButton, AcquireImages and OnBnClickedDisconnectButton
https://github.com/MetaCortex728/img_processing/blob/main/IR140Dlg.cpp
Worker threads do not have message-queues, the (typically one and only) UI one does. The message-queue for a thread is created by the first call of the GetMessage() function. Why use messages to control processing in a worker thread? You would have to establish a special protocol for this, defining custom messages and posting them to the queue.
Worker threads can be implemented as a loop. The loop can be terminated based on various conditions, like failures to retrieve any data or request from the user. You can simply exit the thread proc to terminate the thread's execution. If the thread doesn't respond it may have stuck (unless it performs a really lengthy operation) and the UI thread must provide some mechanism to kill it. That is first request termination and if it doesn't respond within some set time then kill it.
The condition mechanism to terminate should best be some synchronization object (I would recommend a manual-reset event), interlocked variable or a simple boolean which you should access and set using a critical section.
Some considerations:
You pass a parameter block to the thread. Make sure that it remains alive throughout the thread's lifetime. For example, it should NOT be a local variable in a function that exits before the thread's termination.
The loop must be "efficient", ie do not loop infinitely if data are not available. Consider using blocking functions with timeouts, if available.
Resource management (eg connecting/disconnecting, allocating/releasing etc) should best be performed by the same thread.
An alternative implementation can be APCs. Then the thread's proc function is a while(!bTerminate) { SleepEx(INFINITE, TRUE); } loop, and other threads issue requests using a the QueueUserAPC() function.
The AfxEndThread(0) call in OnBnClickedDisconnectButton() is wrong, it terminates the current thread, which in this case is the main (UI) thread. Check the documentation.
A sidenote, my suggestion about the project type is not a dialog-based application but instead a normal MFC application without a document class (uncheck the Document/View architecture support option), as it offers features like menus, toolbars and the like, and most importantly the ON_UPDATE_COMMAND_UI handlers.

C++ - Execute function every X milliseconds

I can't seem to find a good answer to this:
I'm making a game, and I want the logic loop to be separate from the graphics loop. In other words I want the game to go through a loop every X milliseconds regardless of how many frames/second it is displaying.
Obviously they will both be sharing a lot of variables, so I can't have a thread/timer passing one variable back and forth... I'm basically just looking for a way to have a timer in the background that every X milliseconds sends out a flag to execute the logic loop, regardless of where the graphics loop is.
I'm open to any suggestions. It seems like the best option is to have 2 threads, but I'm not sure what the best way to communicate between them is, without constantly synchronizing large amounts of data.
You can very well do multithreading by having your "world view" exchanged every tick. So here is how it works:
Your current world view is pointed to by a single smart pointer and is read only, so no locking is necessary.
Your logic creates your (first) world view, publishes it and schedules the renderer.
Your renderer grabs a copy of the pointer to your world view and renders it (remember, read-only)
In the meantime, your logic creates a new, slightly different world view.
When it's done it exchanges the pointer to the current world view, publishing it as the current one.
Even if the renderer is still busy with the old world view there is no locking necessary.
Eventually the renderer finishes rendering the (old) world. It grabs the new world view and starts another run.
In the meantime, ... (goto step 4)
The only locking you need is for the time when you publish or grab the pointer to the world. As an alternative you can do atomic exchange but then you have to make sure you use smart pointers that can do that.
Most toolkits have an event loop (built above some multiplexing syscall like poll(2) -or the obsolete select-...), e.g. GTK has g_application_run (which is above:) gtk_main which is built above Glib main event loop (which in fact does a poll or something similar). Likewise, Qt has QApplication and its exec methods.
Very often, you can register timers within the event loop. For GTK, use GTimers, g_timeout_add etc. For Qt learn about its timers.
Very often, you can also register some idle or background processing, which is one of your function which is started by the event loop after other events and timeouts have been processed. Your idle function is expected to run quickly (usually it does a small step of some computation in a few milliseconds, to keep the GUI responsive). For GTK, use g_idle_add etc. IIRC, in Qt you can use a timer with a 0 delay.
So you could code even a (conceptually) single threaded application, using timeouts and idle processing.
Of course, you could use multi-threading: generally the main thread is running the event loop, and other threads can do other things. You have synchronization issues. On POSIX systems, a nice synchronization trick could be to use a pipe(7) to self: you set up a pipe before running the event loop, and your computation threads may write a few bytes on it, while the main event loop is "listening" on it (with GTK, using g_source_add_poll or async IO or GUnixInputStream etc.., with Qt, using QSocketNotifier etc....). Then, in the input handler running in the main loop for that pipe, you could access traditional global data with mutexes etc...
Conceptually, read about continuations. It is a relevant notion.
You could have a Draw and Update Method attached to all your game components. That way you can set it that while your game is running the update is called and the draw is ignored or any combination of the two. It also has the benefit of keeping logic and graphics completely separate.
Couldn't you just have a draw method for each object that needs to be drawn and make them globals. Then just run your rendering thread with a sleep delay in it. As long as your rendering thread doesn't write any information to the globals you should be fine. Look up sfml to see an example of it in action.
If you are running on a unix system you could use usleep() however that is not available on windows so you might want to look here for alternatives.

SDL_PollEvent vs SDL_WaitEvent

So I was reading this article which contains 'Tips and Advice for Multithreaded Programming in SDL' - https://vilimpoc.org/research/portmonitorg/sdl-tips-and-tricks.html
It talks about SDL_PollEvent being inefficient as it can cause excessive CPU usage and so recommends using SDL_WaitEvent instead.
It shows an example of both loops but I can't see how this would work with a game loop. Is it the case that SDL_WaitEvent should only be used by things which don't require constant updates ie if you had a game running you would perform game logic each frame.
The only things I can think it could be used for are programs like a paint program where there is only action required on user input.
Am I correct in thinking I should continue to use SDL_PollEvent for generic game programming?
If your game only updates/repaints on user input, then you could use SDL_WaitEvent. However, most games have animation/physics going on even when there is no user input. So I think SDL_PollEvent would be best for most games.
One case in which SDL_WaitEvent might be useful is if you have it in one thread and your animation/logic on another thread. That way even if SDL_WaitEvent waits for a long time, your game will continue painting/updating. (EDIT: This may not actually work. See Henrik's comment below)
As for SDL_PollEvent using 100% CPU as the article indicated, you could mitigate that by adding a sleep in your loop when you detect that your game is running more than the required frames-per-second.
If you don't need sub-frame precision in your input, and your game is constantly animating, then SDL_PollEvent is appropriate.
Sub-frame precision can be important for, eg. games where the player might want very small increments in movement - quickly tapping and releasing a key has unpredictable behavior if you use the classic lazy method of keydown to mean "velocity = 1" and keyup to mean "velocity = 0" and then you only update position once per frame. If your tap happens to overlap with the frame render then you get one frame-duration of movement, if it does not you get no movement, where what you really want is an amount of movement smaller than the length of a frame based on the timestamps at which the events occurred.
Unfortunately SDL's events don't include the actual event timestamps from the operating system, only the timestamp of the PumpEvents call, and WaitEvent effectively polls at 10ms intervals, so even with WaitEvent running in a separate thread, the most precision you'll get is 10ms (you could maybe approximate smaller by saying if you get a keydown and keyup in the same poll cycle then it's ~5ms).
So if you really want precision timing on your input, you might actually need to write your own version of SDL_WaitEventTimeout with a smaller SDL_Delay, and run that in a separate thread from your main game loop.
Further unfortunately, SDL_PumpEvents must be run on the thread that initialized the video subsystem (per https://wiki.libsdl.org/SDL_PumpEvents ), so the whole idea of running your input loop on another thread to get sub-frame timing is nixed by the SDL framework.
In conclusion, for SDL applications with animation there is no reason to use anything other than SDL_PollEvents. The best you can do for sub-framerate input precision is, if you have time to burn between frames, you have the option of being precise during that time, but then you'll get weird render-duration windows each frame where your input loses precision, so you end up with a different kind of inconsistency.
In general, you should use SDL_WaitEvent rather than SDL_PollEvent to release the CPU to the operating system to handle other tasks, like processing user input. This will manifest to you users as sluggish reaction to user input, since this can cause a delay between when they enter a command and when your application processes the event. By using SDL_WaitEvent instead, the OS can post events to your application more quickly, which improves the perceived performance.
As a side benefit, users on battery powered systems, like laptops and portable devices should see slightly less battery usage since the OS has the opportunity to reduce overall CPU usage since your game isn't using it 100% of the time - it would only be using it when an event actually occurs.
This is a very late response, I know. But this is the thread that tops a Google search on this, so it seems the place to add an alternative suggestion to dealing with this that some might find useful.
You could write your code using SDL_WaitEvent, so that, when your application is not actively animating anything, it'll block and hand the CPU back to the OS.
But then you can send a user-defined message to the queue, from another thread (e.g. the game logic thread), to wake up the main rendering thread with that message. And then it goes through the loop to render a frame, swap and returns back to SDL_WaitEvent again. Where another of these user-defined messages can be waiting to be picked up, to tell it to loop once more.
This sort of structure might be good for an application (or game) where there's a "burst" of animation, but otherwise it's best for it to block and go idle (and save battery on laptops).
For example, a GUI where it animates when you open or close or move windows or hover over buttons, but it's otherwise static content most of the time.
(Or, for a game, though it's animating all the time in-game, it might not need to do that for the pause screen or the game menus. So, you could send the "SDL_ANIMATEEVENT" user-defined message during gameplay, but then, in the game menus and pause screen, just wait for mouse / keyboard events and actually allow the CPU to idle and cool down.)
Indeed, you could have self-triggering animation events. In that the rendering thread is woken up by a "SDL_ANIMATEEVENT" and then one more frame of animation is done. But because the animation is not complete, the rendering thread itself posts a "SDL_ANIMATEEVENT" to its own queue, that'll trigger it to wake up again, when it reaches SDL_WaitEvent.
And another idea there is that SDL events can carry data too. So you could supply, say, an animation ID in "data1" and a "current frame" counter in "data2" with the event. So that when the thread picks up the "SDL_ANIMATEEVENT", the event itself tells it which animation to do and what frame we're currently on.
This is a "best of both worlds" solution, I feel. It can behave like SDL_WaitEvent or SDL_PollEvent at the application's discretion by just sending messages to itself.
For a game, this might not be worth it, as you're updating frames constantly, so there's no big advantage to this and maybe it's not worth bothering with (though even games could benefit from going to 0% CPU usage in the pause screen or in-game menus, to let the CPU cool down and use less laptop battery).
But for something like a GUI - which has more "burst-y" animation - then a mouse event can trigger an animation (e.g. opening a new window, which zooms or slides into view) that sends "SDL_ANIMATEEVENT" back to the queue. And it keeps doing that until the animation is complete, then falls back to normal SDL_WaitEvent behaviour again.
It's an idea that might fit what some people need, so I thought I'd float it here for general consumption.
You could actually initialise the SDL and the window in the main thread and then create 2 more threads for updates(Just updates game states and variables as time passes) and rendering(renders the surfaces accordingly).
Then after all that is done, use SDL_WaitEvent in your main thread to manage SDL_Events. This way you could ensure that event is managed in the same thread that called the sdl_init.
I have been using this method for long to make my games work in windows and linux and have been able to successfully run 3 threads at the same time as mentioned above.
I had to use mutex to make sure that textures/surfaces can be transformed/changed in the update thread as well by pausing the render thread, and the lock is called every once 60 frames, so its not going to cause major perf issues.
This model works best to create event driven games, run time games, or both.

Question on using multithreading to periodically and forcefully check for updates on software

I'm working on an application that has a main thread performing some work (message loop of the UI etc.), but I would also like a second thread, which would periodically test if there are any updates available to download. I would also like the possibility for the main thread to ask the secondary thread to force checking for updates, and for the secondary thread to ask the main thread for confirmation on downloading updates.
I don't have that much experience with IPC and multithreading in real life situations, so I'm not sure how I should go about designing this. I would like to eventually have this work on both Windows and POSIX, but let us focus on POSIX for now. Here's my idea so far:
Secondary thread pseudocode:
repeat forever:
check_for_updates()
if (are_any_updates()) {
put the list of available updates on some message queue
send signal SIGUSER1 to main thread
wait for response from that message queue
if (response is positive) download_updates()
}
unblock signal SIGUSER1 on secondary thread
Sleep(one hour)
block signal SIGUSER1
if (any_signal_was_received_while_sleeping)
any_signal_was_received_while_sleeping := false
Sleep(one more hour)
SIGUSER1 handler on secondary thread (main thread has requested us to check for updates):
block signal SIGUSER1 (making sure we don't get signal in signal)
any_signal_was_received_while_sleeping := true
check_for_updates()
...
unblock signal SIGUSER1
Basically, main thread uses SIGUSER1 to ask the secondary thread to force checking for updates, while secondary thread uses SIGUSER1 to ask the main thread to look into the message queue for the available updates and to confirm whether they should be downloaded or not.
I'm not sure if this is a good design or if it would even work properly. One of my problems is related to handling SIGUSER1 received in the main thread, because it's a pretty big application and I'm not really sure when is the right time to block and unblock it (I assume it should be somewhere in the message loop).
Any opinion is appreciated, including advice on what IPC features should I use on Windows (maybe RPC instead of signals?). I could completely remove the use of message queue if I settled on threads, but I might consider using processes instead. I'll clearly use threads on Windows, but I'm not sure about POSIX yet.
You should strongly consider using boost::thread to solve your problem. It is far more comprehensible than directly using posix and is cross platform. Take the time to use a better tool and you will end up saving yourself a great deal of effort.
In particular I think you will find that a condition variable would neatly facilitate your simple interaction.
EDIT:
You can do almost anything with the correct use of mutexes and condition variables. Another piece of advice would be to encapsulate your threads inside class objects. This allows you to write functions that act on the thread and it's data. In your case the main thread could have a method like requestUpdateConfirmation(), inside this you can block the calling thread and wait for the main thread to deal with the request before releasing the caller.

Processing messages is too slow, resulting in a jerky, unresponsive UI - how can I use multiple threads to alleviate this?

I'm having trouble keeping my app responsive to user actions. Therefore, I'd like to split message processing between multiple threads.
Can I simply create several threads, reading from the same message queue in all of them, and letting which ever one is able process each message?
If so, how can this be accomplished?
If not, can you suggest another way of resolving this problem?
You cannot have more than one thread which interacts with the message pump or any UI elements. That way lies madness.
If there are long processing tasks which can be farmed out to worker threads, you can do it that way, but you'll have to use another thread-safe queue to manage them.
If this were later in the future, I would say use the Asynchronous Agents APIs (plug for what I'm working on) in the yet to be released Visual Studio 2010 however what I would say given todays tools is to separate the work, specifically in your message passing pump you want to do as little work as possible to identify the message and pass it along to another thread which will process the work (hopefully there isn't Thread Local information that is needed). Passing it along to another thread means inserting it into a thread safe queue of some sort either locked or lock-free and then setting an event that other threads can watch to pull items from the queue (or just pull them directly). You can look at using a 'work stealing queue' with a thread pool for efficiency.
This will accomplish getting the work off the UI thread, to have the UI thread do additional work (like painting the results of that work) you need to generate a windows message to wake up the UI thread and check for the results, an easy way to do this is to have another 'work ready' queue of work objects to execute on the UI thread. imagine an queue that looks like this: threadsafe_queue<function<void(void)> basically you can check if it to see if it is non-empty on the UI thread, and if there are work items then you can execute them inline. You'll want the work objects to be as short lived as possible and preferably not do any blocking at all.
Another technique that can help if you are still seeing jerky movement responsiveness is to either ensure that you're thread callback isn't executing longer that 16ms and that you aren't taking any locks or doing any sort of I/O on the UI thread. There's a series of tools that can help identify these operations, the most freely available is the 'windows performance toolkit'.
Create the separate thread when processing the long operation i.e. keep it simple, the issue is with some code you are running that is taking too long, that's the code that should have a separate thread.