Programmatically check if caps lock is on in macOS using C++

capslock.cpp

#include <IOKit/IOKitLib.h>
#include <IOKit/hidsystem/IOHIDLib.h>
#include <IOKit/hidsystem/IOHIDParameter.h>
#include <CoreFoundation/CoreFoundation.h>

// Prints 0 if capslock is off. Prints 1 if capslock is on
// Modified from: https://discussions.apple.com/thread/7094207

// Compile with:
// clang -framework IOKit -framework CoreFoundation -o capslock capslock.cpp

int main(int argc, char **argv) {
  kern_return_t kr;
  io_service_t ios;
  io_connect_t ioc;
  CFMutableDictionaryRef mdict;
    
  mdict = IOServiceMatching(kIOHIDSystemClass);
  ios = IOServiceGetMatchingService(kIOMasterPortDefault, (CFDictionaryRef) mdict);
  if (!ios) {
    if (mdict) {
      CFRelease(mdict);
    }
    fprintf(stderr, "IOServiceGetMatchingService() failed\n");
    return 1;
  }

  kr = IOServiceOpen(ios, mach_task_self(), kIOHIDParamConnectType, &ioc);
  IOObjectRelease(ios);
  if (kr != KERN_SUCCESS) {
    fprintf(stderr, "IOServiceOpen() failed: %x\n", kr);
    return (int) kr;
  }

  // CAPSLOCK_QUERY
  bool state;
  kr = IOHIDGetModifierLockState(ioc, kIOHIDCapsLockState, &state);
  if (kr != KERN_SUCCESS) {
    IOServiceClose(ioc);
    fprintf(stderr, "IOHIDGetModifierLockState() failed: %x\n", kr);
    return (int) kr;
  }
  IOServiceClose(ioc);
  fprintf(stdout, "%d\n", (int) state);
  return 0;
}