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POc++ is clock_gettime() overflowing?
 

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  1. POc++ is clock_gettime() overflowing?
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    <p>When using the following example code, the tv_nsec value, I presume is looping round because it's <em>only</em> an <code>long</code>;</p> <pre><code>#include &lt;iostream&gt; using namespace std; #include &lt;time.h&gt; int main(int argc, char *argv[]) { timespec time1, time2; timespec timens = {0}; timens.tv_sec = 1; clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &amp;time1); nanosleep(&amp;timens, (struct timespec *)NULL); clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &amp;time2); cout &lt;&lt; time1.tv_nsec &lt;&lt; " : " &lt;&lt; time2.tv_nsec &lt;&lt; endl; cout &lt;&lt; (time2.tv_nsec-time1.tv_nsec) &lt;&lt; endl; return 0; } $ ./microtime 2754095 : 2766801 12706 </code></pre> <p>The difference between the start and end time is 12706ns (12.7us), it didn't take 12us to sleep for 1 second, I'm <em>reasonably</em> sure of that! So what has happened here, has tv_nsec looped round? Assuming it's an <code>singed long</code> to give a smaller range of positive values (0 to 2147483647) instead of an <code>unsigned long</code> (0 to 4294967295), 2147483647 nanoseconds is still just over 2 full seconds!</p> <p>If I change the code to the following I get a seemingly invalid output still;</p> <pre><code>timens.tv_sec = 0; timens.tv_nsec = 100000; $ ./microtime 2743552 : 2754327 10775 </code></pre> <p>The sleep duration was set for 100,000ns but according to that output the program has slept for 10,000ns. I know that is a small amount of time but I'm trying to show here that whether it is a long time like 1 second or a short time like 100us, there seems to be no accuracy here, or anywhere in between these values. Am I doing something wrong, have I implemented this incorrectly somehow? Why can't I get the actual duration of the event in between the calls to clock_gettime()?</p> <p>Many thanks for your time.</p> <p><strong>Update</strong></p> <p>@Oli; I can test your suggestion with the following code but it doesn't seem to work:</p> <pre><code>int main(int argc, char *argv[]) { timespec time1, time2; timespec timens = {0}; timens.tv_sec = 1; timens.tv_nsec = 0; clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &amp;time1); nanosleep(&amp;timens, (struct timespec *)NULL); clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &amp;time2); cout &lt;&lt;time1.tv_sec&lt;&lt;"s:"&lt;&lt;time1.tv_nsec&lt;&lt;"ns : "&lt;&lt; time2.tv_sec&lt;&lt;"s:"&lt;&lt;time2.tv_nsec&lt;&lt;"ns"&lt;&lt;endl; cout &lt;&lt;(time2.tv_nsec-time1.tv_nsec)&lt;&lt;endl; return 0; } $ ./microtime 0s:2801478ns : 0s:2813732ns 12254 </code></pre>
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