Uses IOKit to figure out the idle time of the system. The idle time is stored as a property of the IOHIDSystem class; the name is HIDIdleTime.

/*****************************************
 * idler.c
 *
 * Uses IOKit to figure out the idle time of the system. The idle time
 * is stored as a property of the IOHIDSystem class; the name is
 * HIDIdleTime. Stored as a 64-bit int, measured in ns.
 *
 * The program itself just prints to stdout the time that the computer has
 * been idle in seconds.
 *
 * Compile with gcc -Wall -framework IOKit -framework Carbon idler.c -o
 * idler
 *
 * Copyright (c) 2003, Stanford University
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 * Redistributions of source code must retain the above copyright notice,
 * this list of conditions and the following disclaimer.
 *
 * Redistributions in binary form must reproduce the above copyright notice,
 * this list of conditions and the following disclaimer in the documentation
 * and/or other materials provided with the distribution.
 *      
 * Neither the name of Stanford University nor the names of its contributors
 * may be used to endorse or promote products derived from this software
 * without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
 * IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#include <CoreFoundation/CoreFoundation.h>
#include <CoreServices/CoreServices.h>
#include <IOKit/IOKitLib.h>

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

/* 10^9 --  number of ns in a second */
#define NS_SECONDS 1000000000

int main(int argc, char **argv) {
  mach_port_t masterPort;
  io_iterator_t iter;
  io_registry_entry_t curObj;

  IOMasterPort(MACH_PORT_NULL, &masterPort);
  
  /* Get IOHIDSystem */
  IOServiceGetMatchingServices(masterPort,
                   IOServiceMatching("IOHIDSystem"),
                   &iter);
  if (iter == 0) {
    printf("Error accessing IOHIDSystemn");
    exit(1);
  }
  
  curObj = IOIteratorNext(iter);
  
  if (curObj == 0) {
    printf("Iterator's empty!n");
    exit(1);
  }
  
  CFMutableDictionaryRef properties = 0;
  CFTypeRef obj;

  if (IORegistryEntryCreateCFProperties(curObj, &properties,
                    kCFAllocatorDefault, 0) ==
      KERN_SUCCESS && properties != NULL) {

    obj = CFDictionaryGetValue(properties, CFSTR("HIDIdleTime"));
    CFRetain(obj);
  } else {
    printf("Couldn't grab properties of systemn");
    obj = NULL;
  }

  if (obj) {
    uint64_t tHandle;

    CFTypeID type = CFGetTypeID(obj);

    if (type == CFDataGetTypeID()) {
      CFDataGetBytes((CFDataRef) obj,
             CFRangeMake(0, sizeof(tHandle)),
             (UInt8*) &tHandle);   
    }  else if (type == CFNumberGetTypeID()) {
      CFNumberGetValue((CFNumberRef)obj,
               kCFNumberSInt64Type,
               &tHandle);
    } else {
      printf("%d: unsupported typen", (int)type);
      exit(1);
    }
   
    CFRelease(obj);    

    // essentially divides by 10^9
    tHandle >>= 30;
    printf("%qin", tHandle);
  } else {
    printf("Can't find idle timen");
  }
  
  /* Release our resources */
  IOObjectRelease(curObj);
  IOObjectRelease(iter);
  CFRelease((CFTypeRef)properties);
  
  return 0;
}