Play/pause iTunes by double tapping the fn key

itunes_pp.c

// clang -framework IOKit -framework Carbon itunes_pp.c -o itunes_pp && ./itunes_pp

#include <sys/time.h>
#include <Carbon/Carbon.h>
#include <IOKit/hid/IOHIDManager.h>
#include <IOKit/hid/IOHIDValue.h>

CFMutableDictionaryRef CreateMatchingDictionary(UInt32 usage_page, UInt32 usage)
{
    CFMutableDictionaryRef dictionary = CFDictionaryCreateMutable(kCFAllocatorDefault, 0, &kCFCopyStringDictionaryKeyCallBacks, &kCFTypeDictionaryValueCallBacks);

    CFNumberRef page_number = CFNumberCreate(kCFAllocatorDefault, kCFNumberIntType, &usage_page);
    CFDictionarySetValue(dictionary, CFSTR(kIOHIDDeviceUsagePageKey), page_number);
    CFRelease(page_number);

    CFNumberRef usage_number = CFNumberCreate(kCFAllocatorDefault, kCFNumberIntType, &usage);
    CFDictionarySetValue(dictionary, CFSTR(kIOHIDDeviceUsageKey), usage_number);
    CFRelease(usage_number);

    return dictionary;
}

void HIDKeyboardCallback(void *context, IOReturn result, void *sender, IOHIDValueRef value)
{
    IOHIDElementRef elem = IOHIDValueGetElement(value);
    uint32_t usage_page = IOHIDElementGetUsagePage(elem);
    uint32_t usage = IOHIDElementGetUsage(elem);
    long pressed = IOHIDValueGetIntegerValue(value);
    static long last_pressed;

    if (usage_page == 0xFF && usage == 3 && pressed == 0) {
        struct timeval time;
        gettimeofday(&time, NULL);
        long microtime = ((unsigned long long)time.tv_sec * 1000000) + time.tv_usec;

        if (last_pressed) {
            long diff = microtime - last_pressed;
            if (diff < 500000) {
                system("osascript -l JavaScript -e 'Application(\"iTunes\").playpause()'");
            }
        }

        last_pressed = microtime;
    }
}

int main()
{
    IOHIDManagerRef hid_manager = IOHIDManagerCreate(kCFAllocatorDefault, kIOHIDOptionsTypeNone);
    CFMutableDictionaryRef matching_dictionary = CreateMatchingDictionary(kHIDPage_GenericDesktop, kHIDUsage_GD_Keyboard);
    IOHIDManagerSetDeviceMatching(hid_manager, matching_dictionary);
    IOHIDManagerRegisterInputValueCallback(hid_manager, HIDKeyboardCallback, NULL);
    IOHIDManagerScheduleWithRunLoop(hid_manager, CFRunLoopGetMain(), kCFRunLoopDefaultMode);
    IOHIDManagerOpen(hid_manager, kIOHIDOptionsTypeNone);
    CFRunLoopRun();
}