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<h4 class="subsection">21.3.1 CPU Time Inquiry</h4>

<p>To get a process' CPU time, you can use the <code>clock</code> function.  This
facility is declared in the header file <samp><span class="file">time.h</span></samp>. 
<a name="index-time_002eh-2606"></a>
In typical usage, you call the <code>clock</code> function at the beginning
and end of the interval you want to time, subtract the values, and then
divide by <code>CLOCKS_PER_SEC</code> (the number of clock ticks per second)
to get processor time, like this:

<pre class="smallexample">     #include &lt;time.h&gt;
     
     clock_t start, end;
     double cpu_time_used;
     
     start = clock();
     ... /* <span class="roman">Do the work.</span> */
     end = clock();
     cpu_time_used = ((double) (end - start)) / CLOCKS_PER_SEC;
</pre>
   <p>Do not use a single CPU time as an amount of time; it doesn't work that
way.  Either do a subtraction as shown above or query processor time
directly.  See <a href="Processor-Time.html#Processor-Time">Processor Time</a>.

   <p>Different computers and operating systems vary wildly in how they keep
track of CPU time.  It's common for the internal processor clock
to have a resolution somewhere between a hundredth and millionth of a
second.

<!-- time.h -->
<!-- ISO -->
<div class="defun">
&mdash; Macro: int <b>CLOCKS_PER_SEC</b><var><a name="index-CLOCKS_005fPER_005fSEC-2607"></a></var><br>
<blockquote><p>The value of this macro is the number of clock ticks per second measured
by the <code>clock</code> function.  POSIX requires that this value be one
million independent of the actual resolution. 
</p></blockquote></div>

<!-- time.h -->
<!-- POSIX.1 -->
<div class="defun">
&mdash; Macro: int <b>CLK_TCK</b><var><a name="index-CLK_005fTCK-2608"></a></var><br>
<blockquote><p>This is an obsolete name for <code>CLOCKS_PER_SEC</code>. 
</p></blockquote></div>

<!-- time.h -->
<!-- ISO -->
<div class="defun">
&mdash; Data Type: <b>clock_t</b><var><a name="index-clock_005ft-2609"></a></var><br>
<blockquote><p>This is the type of the value returned by the <code>clock</code> function. 
Values of type <code>clock_t</code> are numbers of clock ticks. 
</p></blockquote></div>

<!-- time.h -->
<!-- ISO -->
<div class="defun">
&mdash; Function: clock_t <b>clock</b> (<var>void</var>)<var><a name="index-clock-2610"></a></var><br>
<blockquote><p>This function returns the calling process' current CPU time.  If the CPU
time is not available or cannot be represented, <code>clock</code> returns the
value <code>(clock_t)(-1)</code>. 
</p></blockquote></div>

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