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<h3 class="section">21.2 Elapsed Time</h3>

<p><a name="index-elapsed-time-2597"></a>
One way to represent an elapsed time is with a simple arithmetic data
type, as with the following function to compute the elapsed time between
two calendar times.  This function is declared in <samp><span class="file">time.h</span></samp>.

<!-- time.h -->
<!-- ISO -->
<div class="defun">
&mdash; Function: double <b>difftime</b> (<var>time_t time1, time_t time0</var>)<var><a name="index-difftime-2598"></a></var><br>
<blockquote><p>The <code>difftime</code> function returns the number of seconds of elapsed
time between calendar time <var>time1</var> and calendar time <var>time0</var>, as
a value of type <code>double</code>.  The difference ignores leap seconds
unless leap second support is enabled.

        <p>In the GNU system, you can simply subtract <code>time_t</code> values.  But on
other systems, the <code>time_t</code> data type might use some other encoding
where subtraction doesn't work directly. 
</p></blockquote></div>

   <p>The GNU C library provides two data types specifically for representing
an elapsed time.  They are used by various GNU C library functions, and
you can use them for your own purposes too.  They're exactly the same
except that one has a resolution in microseconds, and the other, newer
one, is in nanoseconds.

<!-- sys/time.h -->
<!-- BSD -->
<div class="defun">
&mdash; Data Type: <b>struct timeval</b><var><a name="index-struct-timeval-2599"></a></var><br>
<blockquote><p><a name="index-timeval-2600"></a>The <code>struct timeval</code> structure represents an elapsed time.  It is
declared in <samp><span class="file">sys/time.h</span></samp> and has the following members:

          <dl>
<dt><code>long int tv_sec</code><dd>This represents the number of whole seconds of elapsed time.

          <br><dt><code>long int tv_usec</code><dd>This is the rest of the elapsed time (a fraction of a second),
represented as the number of microseconds.  It is always less than one
million.

        </dl>
        </p></blockquote></div>

<!-- sys/time.h -->
<!-- POSIX.1 -->
<div class="defun">
&mdash; Data Type: <b>struct timespec</b><var><a name="index-struct-timespec-2601"></a></var><br>
<blockquote><p><a name="index-timespec-2602"></a>The <code>struct timespec</code> structure represents an elapsed time.  It is
declared in <samp><span class="file">time.h</span></samp> and has the following members:

          <dl>
<dt><code>long int tv_sec</code><dd>This represents the number of whole seconds of elapsed time.

          <br><dt><code>long int tv_nsec</code><dd>This is the rest of the elapsed time (a fraction of a second),
represented as the number of nanoseconds.  It is always less than one
billion.

        </dl>
        </p></blockquote></div>

   <p>It is often necessary to subtract two values of type <code>struct&nbsp;timeval</code><!-- /@w --> or <code>struct&nbsp;timespec</code><!-- /@w -->.  Here is the best way to do
this.  It works even on some peculiar operating systems where the
<code>tv_sec</code> member has an unsigned type.

<pre class="smallexample">     /* <span class="roman">Subtract the `struct timeval' values X and Y,</span>
        <span class="roman">storing the result in RESULT.</span>
        <span class="roman">Return 1 if the difference is negative, otherwise 0.</span>  */
     
     int
     timeval_subtract (result, x, y)
          struct timeval *result, *x, *y;
     {
       /* <span class="roman">Perform the carry for the later subtraction by updating </span><var>y</var><span class="roman">.</span> */
       if (x-&gt;tv_usec &lt; y-&gt;tv_usec) {
         int nsec = (y-&gt;tv_usec - x-&gt;tv_usec) / 1000000 + 1;
         y-&gt;tv_usec -= 1000000 * nsec;
         y-&gt;tv_sec += nsec;
       }
       if (x-&gt;tv_usec - y-&gt;tv_usec &gt; 1000000) {
         int nsec = (x-&gt;tv_usec - y-&gt;tv_usec) / 1000000;
         y-&gt;tv_usec += 1000000 * nsec;
         y-&gt;tv_sec -= nsec;
       }
     
       /* <span class="roman">Compute the time remaining to wait.</span>
          <code>tv_usec</code><span class="roman"> is certainly positive.</span> */
       result-&gt;tv_sec = x-&gt;tv_sec - y-&gt;tv_sec;
       result-&gt;tv_usec = x-&gt;tv_usec - y-&gt;tv_usec;
     
       /* <span class="roman">Return 1 if result is negative.</span> */
       return x-&gt;tv_sec &lt; y-&gt;tv_sec;
     }
</pre>
   <p>Common functions that use <code>struct timeval</code> are <code>gettimeofday</code>
and <code>settimeofday</code>.

   <p>There are no GNU C library functions specifically oriented toward
dealing with elapsed times, but the calendar time, processor time, and
alarm and sleeping functions have a lot to do with them.

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