title: clock() returning a negative value in C
body: <p>I'm using a quite simple code to measure for time of execution.It works well until I am not sure may be not more than 20 minutes.But after(>20min.)it is returning negative results.I searched throughout the forums and tried everything like changing the datatype,using long unsigned (which is returning 0) but failed again.
  The following is the snippet of my code</p>
  
  <pre><code>main()
  {
      time_t start,stop;
      double time_arm;
      start = clock(); 
      /* .......  */
      stop = clock();
      time_arm=(double)(stop-start)/(double)CLOCKS_PER_SEC;
  
      printf("Time Taken by ARM only is %lf \n",time_arm);
  }
  </code></pre>
  
  <p>output is 
  Time Taken by ARM only is <strong>-2055.367296</strong></p>
  
  <p>Any help is appreciated,thanks in advance.</p>