Ambient light sensor reader (macOS)

lightsensor.mm

// References:
// https://stackoverflow.com/questions/17625495/how-do-you-programmatically-access-the-ambient-light-sensor-on-mac-os-x-10-5
// https://stackoverflow.com/a/73955907

// Build command:
// clang -o lightsensor lightsensor.mm -F /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/System/Library/PrivateFrameworks -framework Foundation -framework IOKit -framework CoreFoundation -framework BezelServices

#include <mach/mach.h>
#import <Foundation/Foundation.h>
#import <IOKit/IOKitLib.h>
#import <IOKit/hidsystem/IOHIDServiceClient.h>

typedef struct __IOHIDEvent *IOHIDEventRef;

#define kAmbientLightSensorEvent 12

#define IOHIDEventFieldBase(type) (type << 16)

extern "C" {
  IOHIDEventRef IOHIDServiceClientCopyEvent(IOHIDServiceClientRef, int64_t, int32_t, int64_t);
  double IOHIDEventGetFloatValue(IOHIDEventRef, int32_t);

  IOHIDServiceClientRef ALCALSCopyALSServiceClient(void);
}

int main(void) {
  IOHIDServiceClientRef client = ALCALSCopyALSServiceClient();
  if (!client) {
    fprintf(stderr, "failed to find ambient light sensors\n");
    return 1;
  }

  IOHIDEventRef event = IOHIDServiceClientCopyEvent(client, kAmbientLightSensorEvent, 0, 0);
  if (!event) {
    fprintf(stderr, "failed to get ambient light sensor event\n");
    return 1;
  }

  double value = IOHIDEventGetFloatValue(event, IOHIDEventFieldBase(kAmbientLightSensorEvent));
  printf("%f\n", value);

  CFRelease(event);
  return 0;
}