# Basic design - using SDL with joysticks that come and go during runtime (fwd)

**URL:** <https://discourse.libsdl.org/t/basic-design-using-sdl-with-joysticks-that-come-and-go-during-runtime-fwd/22803>\
**Category:** SDL Development\
**Created:** [July 14, 2017, 3:39pm UTC](https://discourse.libsdl.org/t/basic-design-using-sdl-with-joysticks-that-come-and-go-during-runtime-fwd/22803 "2017-07-14T15:39:56Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Ed\_Phillips](https://discourse.libsdl.org/letter_avatar_proxy/v4/letter/e/57b2e6/32.png) [@Ed\_Phillips](https://discourse.libsdl.org/u/Ed_Phillips)\
**Post date:** [July 14, 2017, 3:39pm UTC](https://discourse.libsdl.org/t/basic-design-using-sdl-with-joysticks-that-come-and-go-during-runtime-fwd/22803/1 "2017-07-14T15:39:56Z")

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I’m in the process of making my (Win/Linux) game detect add/remove events for  
joysticks. The original code assumed that joysticks would be plugged in before  
the game was launched and would stay connected. If you needed to swap out  
controllers, you had to relaunch the game. This is antiquated.

Now that I bring device added/removed events into the mix, a few new "gotchas"  
come into play:

1. If an SDL\_Joystick pointer is opened, it keeps pointing to the same  
structure even when the physical device is disconnected from the system. You  
can keep making calls using the pointer and things like axis readings appear to  
continue working but the SDL joystick basically “goes dead”. That’s a good  
thing, IMHO… you don’t want to have to deal with “fatal error” handling for  
every single call to do anything with an SDL\_Joystick.

2. If you had an SDL\_Joystick pointer opened, and the device got disconnected  
and reconnected, the pointer would remain pointing to an old structure that is  
"dead". If you call SDL\_JoystickGetAttached, it will return SDL\_FALSE but it  
will never return SDL\_TRUE even after the device gets reconnected. This was  
suprising. SDL doesn’t “reacquire” a device that was temporarily lost (like  
XINPUT would do for play #1 whose battery died or who got excited and pulled  
their USB out).

3. If a device is disconnected and reconnected, the same joystick gets a new,  
unique instance ID… so any events are for a “different” device now. I had  
thought the “instance ID” was the solution to the problem of figuring out which  
device is which, but it makes sense now… it’s just a auto-incremented number  
for each device encountered (whether it was seen before or not).

4. The index numbers get shuffled around when devices are added/removed - which  
is as expected according to the docs. However, this leads to the situation  
where if you can’t use the same index number for the same device, and you can’t  
use the same instance ID for the same device, then you can never tell  
"definitively" that even a single device was disconnected and reconnected,  
except by maybe using the GUID for a DINPUT device (not very good for 3 of the  
same device type), or looking for “#1”, “#2”, etc., for an XINPUT device.

So given all that, I have to take a step back and ask, how are we supposed to  
design our code to deal with devices coming and going?

So far, I’ve basically rewritten my code to close out all controllers and  
reopen all controllers every time there is a device added/removed event (and  
deal with matching the game code structures with the proper SDL\_Joystick and  
getting my game’s event mapping system re-setup properly, etc.). This certainly  
works, but it somehow seems like the… “wrong way”.

```
Ed

```

Ed Phillips [ed@udel.edu](mailto:ed@udel.edu) University of Delaware (302) 831-6082  
Systems Programmer IV, Network and Systems Services

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**Author:** ![icculus](https://discourse.libsdl.org/user_avatar/discourse.libsdl.org/icculus/32/81_2.png) [@icculus](https://discourse.libsdl.org/u/icculus)\
**Post date:** [July 14, 2017, 8:11pm UTC](https://discourse.libsdl.org/t/basic-design-using-sdl-with-joysticks-that-come-and-go-during-runtime-fwd/22803/2 "2017-07-14T20:11:31Z")

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The problem is, even on Windows, we don’t _know_ it’s the same device. It might be in a different part of the USB tree, it might be someone’s friend coming in with the same PS4 controller model, etc. It’s possible Xinput, behind the scenes, uses a device-unique serial number to reassociate the connection, but literally no other API offers that.

In any case, a user might unplug a controller because the controller sucks and she wants to try a totally different one, and that’s worth handling too.

It might be useful to have a “Press Start to Join!” message somewhere on the screen when you see a new device added (like a old coin op arcade machine might say “press Player 2 button!” when there are extra quarters inserted). If they press it, either drop them in immediately or pause the game and let them get sorted. Let whatever available controller presses Start first become the user’s new controller.

If an active player disconnects, maybe you should pause the game until they come back in any case.

Ymmv by game design, of course, but the tl;dr on this, in my opinion, is: “it’s a UI problem.”

(But better solutions, including API changes, are welcome!)

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**Author:** ![Ed\_Phillips](https://discourse.libsdl.org/letter_avatar_proxy/v4/letter/e/57b2e6/32.png) [@Ed\_Phillips](https://discourse.libsdl.org/u/Ed_Phillips)\
**Post date:** [July 14, 2017, 8:32pm UTC](https://discourse.libsdl.org/t/basic-design-using-sdl-with-joysticks-that-come-and-go-during-runtime-fwd/22803/3 "2017-07-14T20:32:36Z")

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Okay Ryan… that makes sense. It’s good to know I haven’t completely  
lost my mind and am missing the “good way” to handle when devices are  
added/removed.

Thanks,

```
Ed
```

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**Author:** ![bogomancer](https://discourse.libsdl.org/user_avatar/discourse.libsdl.org/bogomancer/32/2921_2.png) [@bogomancer](https://discourse.libsdl.org/u/bogomancer)\
**Post date:** [July 15, 2017, 1:36pm UTC](https://discourse.libsdl.org/t/basic-design-using-sdl-with-joysticks-that-come-and-go-during-runtime-fwd/22803/4 "2017-07-15T13:36:31Z")

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Yup, you’re not the only one who’s been left scratching his head over this issue. I eventually settled on something similar to what Ryan suggested.