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| <h4 class="subsection">21.4.3 Broken-down Time</h4> |
| |
| <p><a name="index-broken_002ddown-time-2634"></a><a name="index-calendar-time-and-broken_002ddown-time-2635"></a> |
| Calendar time is represented by the usual GNU C library functions as an |
| elapsed time since a fixed base calendar time. This is convenient for |
| computation, but has no relation to the way people normally think of |
| calendar time. By contrast, <dfn>broken-down time</dfn> is a binary |
| representation of calendar time separated into year, month, day, and so |
| on. Broken-down time values are not useful for calculations, but they |
| are useful for printing human readable time information. |
| |
| <p>A broken-down time value is always relative to a choice of time |
| zone, and it also indicates which time zone that is. |
| |
| <p>The symbols in this section are declared in the header file <samp><span class="file">time.h</span></samp>. |
| |
| <!-- time.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Data Type: <b>struct tm</b><var><a name="index-struct-tm-2636"></a></var><br> |
| <blockquote><p>This is the data type used to represent a broken-down time. The structure |
| contains at least the following members, which can appear in any order. |
| |
| <dl> |
| <dt><code>int tm_sec</code><dd>This is the number of full seconds since the top of the minute (normally |
| in the range <code>0</code> through <code>59</code>, but the actual upper limit is |
| <code>60</code>, to allow for leap seconds if leap second support is |
| available). |
| <a name="index-leap-second-2637"></a> |
| <br><dt><code>int tm_min</code><dd>This is the number of full minutes since the top of the hour (in the |
| range <code>0</code> through <code>59</code>). |
| |
| <br><dt><code>int tm_hour</code><dd>This is the number of full hours past midnight (in the range <code>0</code> through |
| <code>23</code>). |
| |
| <br><dt><code>int tm_mday</code><dd>This is the ordinal day of the month (in the range <code>1</code> through <code>31</code>). |
| Watch out for this one! As the only ordinal number in the structure, it is |
| inconsistent with the rest of the structure. |
| |
| <br><dt><code>int tm_mon</code><dd>This is the number of full calendar months since the beginning of the |
| year (in the range <code>0</code> through <code>11</code>). Watch out for this one! |
| People usually use ordinal numbers for month-of-year (where January = 1). |
| |
| <br><dt><code>int tm_year</code><dd>This is the number of full calendar years since 1900. |
| |
| <br><dt><code>int tm_wday</code><dd>This is the number of full days since Sunday (in the range <code>0</code> through |
| <code>6</code>). |
| |
| <br><dt><code>int tm_yday</code><dd>This is the number of full days since the beginning of the year (in the |
| range <code>0</code> through <code>365</code>). |
| |
| <br><dt><code>int tm_isdst</code><dd><a name="index-Daylight-Saving-Time-2638"></a><a name="index-summer-time-2639"></a>This is a flag that indicates whether Daylight Saving Time is (or was, or |
| will be) in effect at the time described. The value is positive if |
| Daylight Saving Time is in effect, zero if it is not, and negative if the |
| information is not available. |
| |
| <br><dt><code>long int tm_gmtoff</code><dd>This field describes the time zone that was used to compute this |
| broken-down time value, including any adjustment for daylight saving; it |
| is the number of seconds that you must add to UTC to get local time. |
| You can also think of this as the number of seconds east of UTC. For |
| example, for U.S. Eastern Standard Time, the value is <code>-5*60*60</code>. |
| The <code>tm_gmtoff</code> field is derived from BSD and is a GNU library |
| extension; it is not visible in a strict ISO C<!-- /@w --> environment. |
| |
| <br><dt><code>const char *tm_zone</code><dd>This field is the name for the time zone that was used to compute this |
| broken-down time value. Like <code>tm_gmtoff</code>, this field is a BSD and |
| GNU extension, and is not visible in a strict ISO C<!-- /@w --> environment. |
| </dl> |
| </p></blockquote></div> |
| |
| <!-- time.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: struct tm * <b>localtime</b> (<var>const time_t *time</var>)<var><a name="index-localtime-2640"></a></var><br> |
| <blockquote><p>The <code>localtime</code> function converts the simple time pointed to by |
| <var>time</var> to broken-down time representation, expressed relative to the |
| user's specified time zone. |
| |
| <p>The return value is a pointer to a static broken-down time structure, which |
| might be overwritten by subsequent calls to <code>ctime</code>, <code>gmtime</code>, |
| or <code>localtime</code>. (But no other library function overwrites the contents |
| of this object.) |
| |
| <p>The return value is the null pointer if <var>time</var> cannot be represented |
| as a broken-down time; typically this is because the year cannot fit into |
| an <code>int</code>. |
| |
| <p>Calling <code>localtime</code> has one other effect: it sets the variable |
| <code>tzname</code> with information about the current time zone. See <a href="Time-Zone-Functions.html#Time-Zone-Functions">Time Zone Functions</a>. |
| </p></blockquote></div> |
| |
| <p>Using the <code>localtime</code> function is a big problem in multi-threaded |
| programs. The result is returned in a static buffer and this is used in |
| all threads. POSIX.1c introduced a variant of this function. |
| |
| <!-- time.h --> |
| <!-- POSIX.1c --> |
| <div class="defun"> |
| — Function: struct tm * <b>localtime_r</b> (<var>const time_t *time, struct tm *resultp</var>)<var><a name="index-localtime_005fr-2641"></a></var><br> |
| <blockquote><p>The <code>localtime_r</code> function works just like the <code>localtime</code> |
| function. It takes a pointer to a variable containing a simple time |
| and converts it to the broken-down time format. |
| |
| <p>But the result is not placed in a static buffer. Instead it is placed |
| in the object of type <code>struct tm</code> to which the parameter |
| <var>resultp</var> points. |
| |
| <p>If the conversion is successful the function returns a pointer to the |
| object the result was written into, i.e., it returns <var>resultp</var>. |
| </p></blockquote></div> |
| |
| <!-- time.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: struct tm * <b>gmtime</b> (<var>const time_t *time</var>)<var><a name="index-gmtime-2642"></a></var><br> |
| <blockquote><p>This function is similar to <code>localtime</code>, except that the broken-down |
| time is expressed as Coordinated Universal Time (UTC) (formerly called |
| Greenwich Mean Time (GMT)) rather than relative to a local time zone. |
| |
| </blockquote></div> |
| |
| <p>As for the <code>localtime</code> function we have the problem that the result |
| is placed in a static variable. POSIX.1c also provides a replacement for |
| <code>gmtime</code>. |
| |
| <!-- time.h --> |
| <!-- POSIX.1c --> |
| <div class="defun"> |
| — Function: struct tm * <b>gmtime_r</b> (<var>const time_t *time, struct tm *resultp</var>)<var><a name="index-gmtime_005fr-2643"></a></var><br> |
| <blockquote><p>This function is similar to <code>localtime_r</code>, except that it converts |
| just like <code>gmtime</code> the given time as Coordinated Universal Time. |
| |
| <p>If the conversion is successful the function returns a pointer to the |
| object the result was written into, i.e., it returns <var>resultp</var>. |
| </p></blockquote></div> |
| |
| <!-- time.h --> |
| <!-- ISO --> |
| <div class="defun"> |
| — Function: time_t <b>mktime</b> (<var>struct tm *brokentime</var>)<var><a name="index-mktime-2644"></a></var><br> |
| <blockquote><p>The <code>mktime</code> function is used to convert a broken-down time structure |
| to a simple time representation. It also “normalizes” the contents of |
| the broken-down time structure, by filling in the day of week and day of |
| year based on the other date and time components. |
| |
| <p>The <code>mktime</code> function ignores the specified contents of the |
| <code>tm_wday</code> and <code>tm_yday</code> members of the broken-down time |
| structure. It uses the values of the other components to determine the |
| calendar time; it's permissible for these components to have |
| unnormalized values outside their normal ranges. The last thing that |
| <code>mktime</code> does is adjust the components of the <var>brokentime</var> |
| structure (including the <code>tm_wday</code> and <code>tm_yday</code>). |
| |
| <p>If the specified broken-down time cannot be represented as a simple time, |
| <code>mktime</code> returns a value of <code>(time_t)(-1)</code> and does not modify |
| the contents of <var>brokentime</var>. |
| |
| <p>Calling <code>mktime</code> also sets the variable <code>tzname</code> with |
| information about the current time zone. See <a href="Time-Zone-Functions.html#Time-Zone-Functions">Time Zone Functions</a>. |
| </p></blockquote></div> |
| |
| <!-- time.h --> |
| <!-- ??? --> |
| <div class="defun"> |
| — Function: time_t <b>timelocal</b> (<var>struct tm *brokentime</var>)<var><a name="index-timelocal-2645"></a></var><br> |
| <blockquote> |
| <p><code>timelocal</code> is functionally identical to <code>mktime</code>, but more |
| mnemonically named. Note that it is the inverse of the <code>localtime</code> |
| function. |
| |
| <p><strong>Portability note:</strong> <code>mktime</code> is essentially universally |
| available. <code>timelocal</code> is rather rare. |
| |
| </blockquote></div> |
| |
| <!-- time.h --> |
| <!-- ??? --> |
| <div class="defun"> |
| — Function: time_t <b>timegm</b> (<var>struct tm *brokentime</var>)<var><a name="index-timegm-2646"></a></var><br> |
| <blockquote> |
| <p><code>timegm</code> is functionally identical to <code>mktime</code> except it |
| always takes the input values to be Coordinated Universal Time (UTC) |
| regardless of any local time zone setting. |
| |
| <p>Note that <code>timegm</code> is the inverse of <code>gmtime</code>. |
| |
| <p><strong>Portability note:</strong> <code>mktime</code> is essentially universally |
| available. <code>timegm</code> is rather rare. For the most portable |
| conversion from a UTC broken-down time to a simple time, set |
| the <code>TZ</code> environment variable to UTC, call <code>mktime</code>, then set |
| <code>TZ</code> back. |
| |
| </blockquote></div> |
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