IOHIDEventSystemClientScheduleWithRunLoop with EXC_BAD_ACCESS

Question

I'm trying to get touch events in my application. So I used the IOHIDFamily callback to get the events. My code is like this:

void handle_event(void* target, void* refcon, IOHIDServiceRef service, IOHIDEventRef event)
{
    printf("Received event of type %2d from service %p.\n", 
           IOHIDEventGetType(event), service);
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    void *ioHIDEventSystem = IOHIDEventSystemClientCreate(kCFAllocatorDefault);
    IOHIDEventSystemClientScheduleWithRunLoop(system, CFRunLoopGetCurrent(), kCFRunLoopDefaultMode);
    IOHIDEventSystemClientRegisterEventCallback(system, handle_event, NULL, NULL);
    CFRunLoopRun();
}

I got an error when executing:

IOHIDEventSystemClientScheduleWithRunLoop(system, CFRunLoopGetCurrent(), kCFRunLoopDefaultMode);

IOKit`IOHIDEventSystemClientScheduleWithRunLoop:
...
0x32f8fd14: cmp.w r10, #0
0x32f8fd18: strd r10, r11, [r4, #116] <---EXC_BAD_ACCESS(code=EXC_ARM_DA_ALIGN)
0x32f8fd1c: beq 0x32f8fdac ; IOHIDEventSystemClientScheduleWithRunLoop + 168
0x32f8fd1e: ldr r1, [r4, #96]
0x32f8fd20: cbz r1, 0x32f8fd2a ; IOHIDEventSystemClientScheduleWithRunLoop + 38
0x32f8fd22: mov r0, r10
...

Did I use IOHIDFamily in the wrong way?

Answer 1

There's at least a couple problems I see in the code posted:

First, you are calling

CFRunLoopRun();

in the viewDidLoad method, which is going to be called on the main/UI thread. I see no reason for that, so just remove that line. I'd normally expect to see that call if you had a method that you were running on a background thread, and you needed to start a background run loop. Or, if you were registering for callbacks directly in main(), as in this answer.

Then, you have this:

void *ioHIDEventSystem = IOHIDEventSystemClientCreate(kCFAllocatorDefault);
IOHIDEventSystemClientScheduleWithRunLoop(system, CFRunLoopGetCurrent(), kCFRunLoopDefaultMode);

I'm guessing the second line should be

IOHIDEventSystemClientScheduleWithRunLoop(ioHIDEventSystem, CFRunLoopGetCurrent(), kCFRunLoopDefaultMode);

I don't know what the system variable actually refers to, but it doesn't look right.

Take a look at this recent answer, as it seems to use IOKit correctly.