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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"> |
| — 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"> |
| — 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"> |
| — 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 timeval</code><!-- /@w --> or <code>struct 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->tv_usec < y->tv_usec) { |
| int nsec = (y->tv_usec - x->tv_usec) / 1000000 + 1; |
| y->tv_usec -= 1000000 * nsec; |
| y->tv_sec += nsec; |
| } |
| if (x->tv_usec - y->tv_usec > 1000000) { |
| int nsec = (x->tv_usec - y->tv_usec) / 1000000; |
| y->tv_usec += 1000000 * nsec; |
| y->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->tv_sec = x->tv_sec - y->tv_sec; |
| result->tv_usec = x->tv_usec - y->tv_usec; |
| |
| /* <span class="roman">Return 1 if result is negative.</span> */ |
| return x->tv_sec < y->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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