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| <h4 class="subsection">21.4.1 Simple Calendar Time</h4> |
| |
| <p>This section describes the <code>time_t</code> data type for representing calendar |
| time as simple time, and the functions which operate on simple time objects. |
| These facilities are declared in the header file <samp><span class="file">time.h</span></samp>. |
| <a name="index-time_002eh-2623"></a> |
| <a name="index-epoch-2624"></a><!-- time.h --> |
| <!-- ISO --> |
| |
| <div class="defun"> |
| — Data Type: <b>time_t</b><var><a name="index-time_005ft-2625"></a></var><br> |
| <blockquote><p>This is the data type used to represent simple time. Sometimes, it also |
| represents an elapsed time. When interpreted as a calendar time value, |
| it represents the number of seconds elapsed since 00:00:00 on January 1, |
| 1970, Coordinated Universal Time. (This calendar time is sometimes |
| referred to as the <dfn>epoch</dfn>.) POSIX requires that this count not |
| include leap seconds, but on some systems this count includes leap seconds |
| if you set <code>TZ</code> to certain values (see <a href="TZ-Variable.html#TZ-Variable">TZ Variable</a>). |
| |
| <p>Note that a simple time has no concept of local time zone. Calendar |
| Time <var>T</var> is the same instant in time regardless of where on the |
| globe the computer is. |
| |
| <p>In the GNU C library, <code>time_t</code> is equivalent to <code>long int</code>. |
| In other systems, <code>time_t</code> might be either an integer or |
| floating-point type. |
| </p></blockquote></div> |
| |
| <p>The function <code>difftime</code> tells you the elapsed time between two |
| simple calendar times, which is not always as easy to compute as just |
| subtracting. See <a href="Elapsed-Time.html#Elapsed-Time">Elapsed Time</a>. |
| |
| <!-- time.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: time_t <b>time</b> (<var>time_t *result</var>)<var><a name="index-time-2626"></a></var><br> |
| <blockquote><p>The <code>time</code> function returns the current calendar time as a value of |
| type <code>time_t</code>. If the argument <var>result</var> is not a null pointer, |
| the calendar time value is also stored in <code>*</code><var>result</var>. If the |
| current calendar time is not available, the value |
| <code>(time_t)(-1)</code><!-- /@w --> is returned. |
| </p></blockquote></div> |
| |
| <!-- The GNU C library implements stime() with a call to settimeofday() on --> |
| <!-- Linux. --> |
| <!-- time.h --> |
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| <div class="defun"> |
| — Function: int <b>stime</b> (<var>time_t *newtime</var>)<var><a name="index-stime-2627"></a></var><br> |
| <blockquote><p><code>stime</code> sets the system clock, i.e., it tells the system that the |
| current calendar time is <var>newtime</var>, where <code>newtime</code> is |
| interpreted as described in the above definition of <code>time_t</code>. |
| |
| <p><code>settimeofday</code> is a newer function which sets the system clock to |
| better than one second precision. <code>settimeofday</code> is generally a |
| better choice than <code>stime</code>. See <a href="High_002dResolution-Calendar.html#High_002dResolution-Calendar">High-Resolution Calendar</a>. |
| |
| <p>Only the superuser can set the system clock. |
| |
| <p>If the function succeeds, the return value is zero. Otherwise, it is |
| <code>-1</code> and <code>errno</code> is set accordingly: |
| |
| <dl> |
| <dt><code>EPERM</code><dd>The process is not superuser. |
| </dl> |
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