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POReal-time awareness of timezone change in localtime() vs localtime_r()
 

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  1. POReal-time awareness of timezone change in localtime() vs localtime_r()
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    <p>Working on an Ubuntu 12.04.3 LTS box, I just noticed that localtime() and localtime_r() behave differently when the system's timezone changes during the lifetime of a process: localtime() picks up the timezone change immediately, whereas localtime_r() does not, it seems to stick to what was the timezone at the launch of the process. Is this expected behavior? I haven't seen this covered anywhere.</p> <p>More precisely, when I use the following code ...</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;sys/time.h&gt; #include &lt;time.h&gt; #include &lt;unistd.h&gt; int main() { while (1) { time_t t = time(NULL); struct tm *tm = localtime(&amp;t); printf("localtime:%02d/%02d/%02d-%02d:%02d:%02d\n", tm-&gt;tm_mon + 1, tm-&gt;tm_mday, tm-&gt;tm_year + 1900, tm-&gt;tm_hour, tm-&gt;tm_min, tm-&gt;tm_sec); sleep(1); } return 0; } </code></pre> <p>... and change the timezone from UTC via ...</p> <pre><code># echo 'Europe/Berlin' &gt; /etc/timezone # sudo dpkg-reconfigure --frontend noninteractive tzdata </code></pre> <p>... then the code produces the following, ...</p> <pre><code>localtime:10/04/2013-01:11:33 localtime:10/04/2013-01:11:34 localtime:10/04/2013-01:11:35 localtime:10/03/2013-23:11:36 localtime:10/03/2013-23:11:37 localtime:10/03/2013-23:11:38 </code></pre> <p>... but if I use:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;sys/time.h&gt; #include &lt;time.h&gt; #include &lt;unistd.h&gt; int main() { while (1) { time_t t = time(NULL); struct tm local_tm; struct tm *tm = localtime_r(&amp;t, &amp;local_tm); printf("localtime_r:%02d/%02d/%02d-%02d:%02d:%02d\n", tm-&gt;tm_mon + 1, tm-&gt;tm_mday, tm-&gt;tm_year + 1900, tm-&gt;tm_hour, tm-&gt;tm_min, tm-&gt;tm_sec); sleep(1); } return 0; } </code></pre> <p>... then there's no change when doing a similar timezone change:</p> <pre><code>localtime_r:10/04/2013-01:15:37 localtime_r:10/04/2013-01:15:38 localtime_r:10/04/2013-01:15:39 localtime_r:10/04/2013-01:15:40 localtime_r:10/04/2013-01:15:41 localtime_r:10/04/2013-01:15:42 </code></pre> <p>UPDATE: adding a call to tzset() before invoking localtime_r() produces the expected behavior. Whether that's clear from the spec/manpage or not (see discussion below) is a question for mentalhealth.stackexchange.com...</p>
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