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| <h4 class="subsection">21.4.2 High-Resolution Calendar</h4> |
| |
| <p>The <code>time_t</code> data type used to represent simple times has a |
| resolution of only one second. Some applications need more precision. |
| |
| <p>So, the GNU C library also contains functions which are capable of |
| representing calendar times to a higher resolution than one second. The |
| functions and the associated data types described in this section are |
| declared in <samp><span class="file">sys/time.h</span></samp>. |
| <a name="index-sys_002ftime_002eh-2628"></a> |
| <!-- sys/time.h --> |
| <!-- BSD --> |
| |
| <div class="defun"> |
| — Data Type: <b>struct timezone</b><var><a name="index-struct-timezone-2629"></a></var><br> |
| <blockquote><p>The <code>struct timezone</code> structure is used to hold minimal information |
| about the local time zone. It has the following members: |
| |
| <dl> |
| <dt><code>int tz_minuteswest</code><dd>This is the number of minutes west of UTC. |
| |
| <br><dt><code>int tz_dsttime</code><dd>If nonzero, Daylight Saving Time applies during some part of the year. |
| </dl> |
| |
| <p>The <code>struct timezone</code> type is obsolete and should never be used. |
| Instead, use the facilities described in <a href="Time-Zone-Functions.html#Time-Zone-Functions">Time Zone Functions</a>. |
| </p></blockquote></div> |
| |
| <!-- sys/time.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: int <b>gettimeofday</b> (<var>struct timeval *tp, struct timezone *tzp</var>)<var><a name="index-gettimeofday-2630"></a></var><br> |
| <blockquote><p>The <code>gettimeofday</code> function returns the current calendar time as |
| the elapsed time since the epoch in the <code>struct timeval</code> structure |
| indicated by <var>tp</var>. (see <a href="Elapsed-Time.html#Elapsed-Time">Elapsed Time</a> for a description of |
| <code>struct timeval</code>). Information about the time zone is returned in |
| the structure pointed at <var>tzp</var>. If the <var>tzp</var> argument is a null |
| pointer, time zone information is ignored. |
| |
| <p>The return value is <code>0</code> on success and <code>-1</code> on failure. The |
| following <code>errno</code> error condition is defined for this function: |
| |
| <dl> |
| <dt><code>ENOSYS</code><dd>The operating system does not support getting time zone information, and |
| <var>tzp</var> is not a null pointer. The GNU operating system does not |
| support using <code>struct timezone</code><!-- /@w --> to represent time zone |
| information; that is an obsolete feature of 4.3 BSD. |
| Instead, use the facilities described in <a href="Time-Zone-Functions.html#Time-Zone-Functions">Time Zone Functions</a>. |
| </dl> |
| </p></blockquote></div> |
| |
| <!-- sys/time.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: int <b>settimeofday</b> (<var>const struct timeval *tp, const struct timezone *tzp</var>)<var><a name="index-settimeofday-2631"></a></var><br> |
| <blockquote><p>The <code>settimeofday</code> function sets the current calendar time in the |
| system clock according to the arguments. As for <code>gettimeofday</code>, |
| the calendar time is represented as the elapsed time since the epoch. |
| As for <code>gettimeofday</code>, time zone information is ignored if |
| <var>tzp</var> is a null pointer. |
| |
| <p>You must be a privileged user in order to use <code>settimeofday</code>. |
| |
| <p>Some kernels automatically set the system clock from some source such as |
| a hardware clock when they start up. Others, including Linux, place the |
| system clock in an “invalid” state (in which attempts to read the clock |
| fail). A call of <code>stime</code> removes the system clock from an invalid |
| state, and system startup scripts typically run a program that calls |
| <code>stime</code>. |
| |
| <p><code>settimeofday</code> causes a sudden jump forwards or backwards, which |
| can cause a variety of problems in a system. Use <code>adjtime</code> (below) |
| to make a smooth transition from one time to another by temporarily |
| speeding up or slowing down the clock. |
| |
| <p>With a Linux kernel, <code>adjtimex</code> does the same thing and can also |
| make permanent changes to the speed of the system clock so it doesn't |
| need to be corrected as often. |
| |
| <p>The return value is <code>0</code> on success and <code>-1</code> on failure. The |
| following <code>errno</code> error conditions are defined for this function: |
| |
| <dl> |
| <dt><code>EPERM</code><dd>This process cannot set the clock because it is not privileged. |
| |
| <br><dt><code>ENOSYS</code><dd>The operating system does not support setting time zone information, and |
| <var>tzp</var> is not a null pointer. |
| </dl> |
| </p></blockquote></div> |
| |
| <!-- On Linux, GNU libc implements adjtime() as a call to adjtimex(). --> |
| <!-- sys/time.h --> |
| <!-- BSD --> |
| <div class="defun"> |
| — Function: int <b>adjtime</b> (<var>const struct timeval *delta, struct timeval *olddelta</var>)<var><a name="index-adjtime-2632"></a></var><br> |
| <blockquote><p>This function speeds up or slows down the system clock in order to make |
| a gradual adjustment. This ensures that the calendar time reported by |
| the system clock is always monotonically increasing, which might not |
| happen if you simply set the clock. |
| |
| <p>The <var>delta</var> argument specifies a relative adjustment to be made to |
| the clock time. If negative, the system clock is slowed down for a |
| while until it has lost this much elapsed time. If positive, the system |
| clock is speeded up for a while. |
| |
| <p>If the <var>olddelta</var> argument is not a null pointer, the <code>adjtime</code> |
| function returns information about any previous time adjustment that |
| has not yet completed. |
| |
| <p>This function is typically used to synchronize the clocks of computers |
| in a local network. You must be a privileged user to use it. |
| |
| <p>With a Linux kernel, you can use the <code>adjtimex</code> function to |
| permanently change the clock speed. |
| |
| <p>The return value is <code>0</code> on success and <code>-1</code> on failure. The |
| following <code>errno</code> error condition is defined for this function: |
| |
| <dl> |
| <dt><code>EPERM</code><dd>You do not have privilege to set the time. |
| </dl> |
| </p></blockquote></div> |
| |
| <p><strong>Portability Note:</strong> The <code>gettimeofday</code>, <code>settimeofday</code>, |
| and <code>adjtime</code> functions are derived from BSD. |
| |
| <p>Symbols for the following function are declared in <samp><span class="file">sys/timex.h</span></samp>. |
| |
| <!-- sys/timex.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Function: int <b>adjtimex</b> (<var>struct timex *timex</var>)<var><a name="index-adjtimex-2633"></a></var><br> |
| <blockquote> |
| <p><code>adjtimex</code> is functionally identical to <code>ntp_adjtime</code>. |
| See <a href="High-Accuracy-Clock.html#High-Accuracy-Clock">High Accuracy Clock</a>. |
| |
| <p>This function is present only with a Linux kernel. |
| |
| </blockquote></div> |
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