How do you programmatically access the ambient light sensor on Mac OS X 10.5+?

Question

I'm trying to programmatically access the ambient light sensor in a Mac application running on OS X 10.5 and above, but can't find a way to do this.

Two other questions had been posed about this here, "Accessing mac's sensor data" and "Disable ambient-light sensor screen dimming programmatically on OS X", but they either didn't address this or present solutions that break on 10.5 and up.

What private API does Apple use to access the ambient light-sensor data on OS X and/or how would I find it?

Answer 1

I've found the closest thing I can -- example code from a Firefox bug report last modified in April 2013. The following works, producing a simple CLI program to query the sensor (taken freely from https://bugzilla.mozilla.org/show_bug.cgi?id=793728#attach_664102). The service polled is "AppleLMUController", which you can then extract relevant information from -- the snippet below creates a serviceObject=IOServiceGetMatchingService(kIOMasterPortDefault, IOServiceMatching("AppleLMUController")), which is then used.

// lmutracker.mm
//
// clang -o lmutracker lmutracker.mm -framework IOKit -framework CoreFoundation

#include <mach/mach.h>
#import <IOKit/IOKitLib.h>
#import <CoreFoundation/CoreFoundation.h>

static double updateInterval = 0.1;
static io_connect_t dataPort = 0;

void updateTimerCallBack(CFRunLoopTimerRef timer, void *info) {
  kern_return_t kr;
  uint32_t outputs = 2;
  uint64_t values[outputs];

  kr = IOConnectCallMethod(dataPort, 0, nil, 0, nil, 0, values, &outputs, nil, 0);
  if (kr == KERN_SUCCESS) {
    printf("\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b\b%8lld %8lld", values[0], values[1]);
    return;
  }

  if (kr == kIOReturnBusy) {
    return;
  }

  mach_error("I/O Kit error:", kr);
  exit(kr);
}

int main(void) {
  kern_return_t kr;
  io_service_t serviceObject;
  CFRunLoopTimerRef updateTimer;

  serviceObject = IOServiceGetMatchingService(kIOMasterPortDefault, IOServiceMatching("AppleLMUController"));
  if (!serviceObject) {
    fprintf(stderr, "failed to find ambient light sensors\n");
    exit(1);
  }

  kr = IOServiceOpen(serviceObject, mach_task_self(), 0, &dataPort);
  IOObjectRelease(serviceObject);
  if (kr != KERN_SUCCESS) {
    mach_error("IOServiceOpen:", kr);
    exit(kr);
  }

  setbuf(stdout, NULL);
  printf("%8ld %8ld", 0L, 0L);

  updateTimer = CFRunLoopTimerCreate(kCFAllocatorDefault,
                  CFAbsoluteTimeGetCurrent() + updateInterval, updateInterval,
                  0, 0, updateTimerCallBack, NULL);
  CFRunLoopAddTimer(CFRunLoopGetCurrent(), updateTimer, kCFRunLoopDefaultMode);
  CFRunLoopRun();

  exit(0);
}