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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 <time.h> |
| |
| 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"> |
| — 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"> |
| — 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"> |
| — 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"> |
| — 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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