4 * clock.c was written by Charles Hedrick, hedrick@cs.rutgers.edu, Apr 1992
5 * Modified for clock adjustments - Rob Hooft <hooft@chem.ruu.nl>, Nov 1992
6 * Improvements by Harald Koenig <koenig@nova.tat.physik.uni-tuebingen.de>
7 * and Alan Modra <alan@spri.levels.unisa.edu.au>.
9 * Major rewrite by Bryan Henderson <bryanh@giraffe-data.com>, 96.09.19.
10 * The new program is called hwclock. New features:
12 * - You can set the hardware clock without also modifying the system
14 * - You can read and set the clock with finer than 1 second precision.
15 * - When you set the clock, hwclock automatically refigures the drift
16 * rate, based on how far off the clock was before you set it.
18 * Reshuffled things, added sparc code, and re-added alpha stuff
19 * by David Mosberger <davidm@azstarnet.com>
20 * and Jay Estabrook <jestabro@amt.tay1.dec.com>
21 * and Martin Ostermann <ost@coments.rwth-aachen.de>, aeb@cwi.nl, 990212.
23 * Fix for Award 2094 bug, Dave Coffin (dcoffin@shore.net) 11/12/98
24 * Change of local time handling, Stefan Ring <e9725446@stud3.tuwien.ac.at>
25 * Change of adjtime handling, James P. Rutledge <ao112@rgfn.epcc.edu>.
27 * Distributed under GPL
30 * Explanation of `adjusting' (Rob Hooft):
32 * The problem with my machine is that its CMOS clock is 10 seconds
33 * per day slow. With this version of clock.c, and my '/etc/rc.local'
34 * reading '/etc/clock -au' instead of '/etc/clock -u -s', this error
35 * is automatically corrected at every boot.
37 * To do this job, the program reads and writes the file '/etc/adjtime'
38 * to determine the correction, and to save its data. In this file are
41 * 1) the correction in seconds per day. (So if your clock runs 5
42 * seconds per day fast, the first number should read -5.0)
43 * 2) the number of seconds since 1/1/1970 the last time the program
45 * 3) the remaining part of a second which was leftover after the last
48 * Installation and use of this program:
50 * a) create a file '/etc/adjtime' containing as the first and only
52 * b) run 'clock -au' or 'clock -a', depending on whether your cmos is
53 * in universal or local time. This updates the second number.
54 * c) set your system time using the 'date' command.
55 * d) update your cmos time using 'clock -wu' or 'clock -w'
56 * e) replace the first number in /etc/adjtime by your correction.
57 * f) put the command 'clock -au' or 'clock -a' in your '/etc/rc.local'
74 #define OPTUTILS_EXIT_CODE EX_USAGE
77 #include "closestream.h"
80 #include "pathnames.h"
83 #include "timeutils.h"
89 static int hwaudit_fd
= -1;
92 /* The struct that holds our hardware access routines */
93 static struct clock_ops
*ur
;
95 /* Maximal clock adjustment in seconds per day.
96 (adjtime() glibc call has 2145 seconds limit on i386, so it is good enough for us as well,
97 43219 is a maximal safe value preventing exact_adjustment overflow.) */
98 #define MAX_DRIFT 2145.0
102 * This is information we keep in the adjtime file that tells us how
103 * to do drift corrections. Elements are all straight from the
104 * adjtime file, so see documentation of that file for details.
105 * Exception is <dirty>, which is an indication that what's in this
106 * structure is not what's in the disk file (because it has been
107 * updated since read from the disk file).
112 time_t last_adj_time
;
115 time_t last_calib_time
;
117 * The most recent time that we set the clock from an external
118 * authority (as opposed to just doing a drift adjustment)
121 enum a_local_utc
{ UTC
= 0, LOCAL
, UNKNOWN
} local_utc
;
123 * To which time zone, local or UTC, we most recently set the
129 * time_t to timeval conversion.
131 static struct timeval
t2tv(time_t timet
)
133 struct timeval rettimeval
;
135 rettimeval
.tv_sec
= timet
;
136 rettimeval
.tv_usec
= 0;
141 * The difference in seconds between two times in "timeval" format.
143 double time_diff(struct timeval subtrahend
, struct timeval subtractor
)
145 return (subtrahend
.tv_sec
- subtractor
.tv_sec
)
146 + (subtrahend
.tv_usec
- subtractor
.tv_usec
) / 1E6
;
150 * The time, in "timeval" format, which is <increment> seconds after the
151 * time <addend>. Of course, <increment> may be negative.
153 static struct timeval
time_inc(struct timeval addend
, double increment
)
155 struct timeval newtime
;
157 newtime
.tv_sec
= addend
.tv_sec
+ (int)increment
;
158 newtime
.tv_usec
= addend
.tv_usec
+ (increment
- (int)increment
) * 1E6
;
161 * Now adjust it so that the microsecond value is between 0 and 1
164 if (newtime
.tv_usec
< 0) {
165 newtime
.tv_usec
+= 1E6
;
167 } else if (newtime
.tv_usec
>= 1E6
) {
168 newtime
.tv_usec
-= 1E6
;
175 hw_clock_is_utc(const struct hwclock_control
*ctl
,
176 const struct adjtime adjtime
)
181 ret
= TRUE
; /* --utc explicitly given on command line */
182 else if (ctl
->local_opt
)
183 ret
= FALSE
; /* --localtime explicitly given */
185 /* get info from adjtime file - default is UTC */
186 ret
= (adjtime
.local_utc
!= LOCAL
);
188 printf(_("Assuming hardware clock is kept in %s time.\n"),
189 ret
? _("UTC") : _("local"));
194 * Read the adjustment parameters out of the /etc/adjtime file.
196 * Return them as the adjtime structure <*adjtime_p>. If there is no
197 * /etc/adjtime file, return defaults. If values are missing from the file,
198 * return defaults for them.
200 * return value 0 if all OK, !=0 otherwise.
202 static int read_adjtime(const struct hwclock_control
*ctl
,
203 struct adjtime
*adjtime_p
)
206 char line1
[81]; /* String: first line of adjtime file */
207 char line2
[81]; /* String: second line of adjtime file */
208 char line3
[81]; /* String: third line of adjtime file */
210 if (access(ctl
->adj_file_name
, R_OK
) != 0)
213 adjfile
= fopen(ctl
->adj_file_name
, "r"); /* open file for reading */
214 if (adjfile
== NULL
) {
215 warn(_("cannot open %s"), ctl
->adj_file_name
);
219 if (!fgets(line1
, sizeof(line1
), adjfile
))
220 line1
[0] = '\0'; /* In case fgets fails */
221 if (!fgets(line2
, sizeof(line2
), adjfile
))
222 line2
[0] = '\0'; /* In case fgets fails */
223 if (!fgets(line3
, sizeof(line3
), adjfile
))
224 line3
[0] = '\0'; /* In case fgets fails */
228 sscanf(line1
, "%lf %ld %lf",
229 &adjtime_p
->drift_factor
,
230 &adjtime_p
->last_adj_time
,
231 &adjtime_p
->not_adjusted
);
233 sscanf(line2
, "%ld", &adjtime_p
->last_calib_time
);
235 if (!strcmp(line3
, "UTC\n")) {
236 adjtime_p
->local_utc
= UTC
;
237 } else if (!strcmp(line3
, "LOCAL\n")) {
238 adjtime_p
->local_utc
= LOCAL
;
240 adjtime_p
->local_utc
= UNKNOWN
;
242 warnx(_("Warning: unrecognized third line in adjtime file\n"
243 "(Expected: `UTC' or `LOCAL' or nothing.)"));
249 ("Last drift adjustment done at %ld seconds after 1969\n"),
250 (long)adjtime_p
->last_adj_time
);
251 printf(_("Last calibration done at %ld seconds after 1969\n"),
252 (long)adjtime_p
->last_calib_time
);
253 printf(_("Hardware clock is on %s time\n"),
254 (adjtime_p
->local_utc
==
255 LOCAL
) ? _("local") : (adjtime_p
->local_utc
==
256 UTC
) ? _("UTC") : _("unknown"));
263 * Wait until the falling edge of the Hardware Clock's update flag so that
264 * any time that is read from the clock immediately after we return will be
267 * The clock only has 1 second precision, so it gives the exact time only
268 * once per second, right on the falling edge of the update flag.
270 * We wait (up to one second) either blocked waiting for an rtc device or in
271 * a CPU spin loop. The former is probably not very accurate.
273 * Return 0 if it worked, nonzero if it didn't.
275 static int synchronize_to_clock_tick(const struct hwclock_control
*ctl
)
280 printf(_("Waiting for clock tick...\n"));
282 rc
= ur
->synchronize_to_clock_tick(ctl
);
286 printf(_("...synchronization failed\n"));
288 printf(_("...got clock tick\n"));
295 * Convert a time in broken down format (hours, minutes, etc.) into standard
296 * unix time (seconds into epoch). Return it as *systime_p.
298 * The broken down time is argument <tm>. This broken down time is either
299 * in local time zone or UTC, depending on value of logical argument
300 * "universal". True means it is in UTC.
302 * If the argument contains values that do not constitute a valid time, and
303 * mktime() recognizes this, return *valid_p == false and *systime_p
304 * undefined. However, mktime() sometimes goes ahead and computes a
305 * fictional time "as if" the input values were valid, e.g. if they indicate
306 * the 31st day of April, mktime() may compute the time of May 1. In such a
307 * case, we return the same fictional value mktime() does as *systime_p and
308 * return *valid_p == true.
311 mktime_tz(const struct hwclock_control
*ctl
, struct tm tm
,
312 bool *valid_p
, time_t *systime_p
)
315 *systime_p
= timegm(&tm
);
317 *systime_p
= mktime(&tm
);
318 if (*systime_p
== -1) {
320 * This apparently (not specified in mktime() documentation)
321 * means the 'tm' structure does not contain valid values
322 * (however, not containing valid values does _not_ imply
323 * mktime() returns -1).
327 printf(_("Invalid values in hardware clock: "
328 "%4d/%.2d/%.2d %.2d:%.2d:%.2d\n"),
329 tm
.tm_year
+ 1900, tm
.tm_mon
+ 1, tm
.tm_mday
,
330 tm
.tm_hour
, tm
.tm_min
, tm
.tm_sec
);
335 ("Hw clock time : %4d/%.2d/%.2d %.2d:%.2d:%.2d = "
336 "%ld seconds since 1969\n"), tm
.tm_year
+ 1900,
337 tm
.tm_mon
+ 1, tm
.tm_mday
, tm
.tm_hour
, tm
.tm_min
,
338 tm
.tm_sec
, (long)*systime_p
);
343 * Read the hardware clock and return the current time via <tm> argument.
345 * Use the method indicated by <method> argument to access the hardware
349 read_hardware_clock(const struct hwclock_control
*ctl
,
350 bool * valid_p
, time_t *systime_p
)
355 err
= ur
->read_hardware_clock(ctl
, &tm
);
361 ("Time read from Hardware Clock: %4d/%.2d/%.2d %02d:%02d:%02d\n"),
362 tm
.tm_year
+ 1900, tm
.tm_mon
+ 1, tm
.tm_mday
, tm
.tm_hour
,
363 tm
.tm_min
, tm
.tm_sec
);
364 mktime_tz(ctl
, tm
, valid_p
, systime_p
);
370 * Set the Hardware Clock to the time <newtime>, in local time zone or UTC,
371 * according to <universal>.
374 set_hardware_clock(const struct hwclock_control
*ctl
, const time_t newtime
)
376 struct tm new_broken_time
;
378 * Time to which we will set Hardware Clock, in broken down format,
379 * in the time zone of caller's choice
383 new_broken_time
= *gmtime(&newtime
);
385 new_broken_time
= *localtime(&newtime
);
388 printf(_("Setting Hardware Clock to %.2d:%.2d:%.2d "
389 "= %ld seconds since 1969\n"),
390 new_broken_time
.tm_hour
, new_broken_time
.tm_min
,
391 new_broken_time
.tm_sec
, (long)newtime
);
394 printf(_("Test mode: clock was not changed\n"));
396 ur
->set_hardware_clock(ctl
, &new_broken_time
);
400 * Set the Hardware Clock to the time "sethwtime", in local time zone or
401 * UTC, according to "universal".
403 * Wait for a fraction of a second so that "sethwtime" is the value of the
404 * Hardware Clock as of system time "refsystime", which is in the past. For
405 * example, if "sethwtime" is 14:03:05 and "refsystime" is 12:10:04.5 and
406 * the current system time is 12:10:06.0: Wait .5 seconds (to make exactly 2
407 * seconds since "refsystime") and then set the Hardware Clock to 14:03:07,
408 * thus getting a precise and retroactive setting of the clock.
410 * (Don't be confused by the fact that the system clock and the Hardware
411 * Clock differ by two hours in the above example. That's just to remind you
412 * that there are two independent time scales here).
414 * This function ought to be able to accept set times as fractional times.
415 * Idea for future enhancement.
418 set_hardware_clock_exact(const struct hwclock_control
*ctl
,
419 const time_t sethwtime
,
420 const struct timeval refsystime
)
423 * The Hardware Clock can only be set to any integer time plus one
424 * half second. The integer time is required because there is no
425 * interface to set or get a fractional second. The additional half
426 * second is because the Hardware Clock updates to the following
427 * second precisely 500 ms (not 1 second!) after you release the
428 * divider reset (after setting the new time) - see description of
429 * DV2, DV1, DV0 in Register A in the MC146818A data sheet (and note
430 * that although that document doesn't say so, real-world code seems
431 * to expect that the SET bit in Register B functions the same way).
432 * That means that, e.g., when you set the clock to 1:02:03, it
433 * effectively really sets it to 1:02:03.5, because it will update to
434 * 1:02:04 only half a second later. Our caller passes the desired
435 * integer Hardware Clock time in sethwtime, and the corresponding
436 * system time (which may have a fractional part, and which may or may
437 * not be the same!) in refsystime. In an ideal situation, we would
438 * then apply sethwtime to the Hardware Clock at refsystime+500ms, so
439 * that when the Hardware Clock ticks forward to sethwtime+1s half a
440 * second later at refsystime+1000ms, everything is in sync. So we
441 * spin, waiting for gettimeofday() to return a time at or after that
442 * time (refsystime+500ms) up to a tolerance value, initially 1ms. If
443 * we miss that time due to being preempted for some other process,
444 * then we increase the margin a little bit (initially 1ms, doubling
445 * each time), add 1 second (or more, if needed to get a time that is
446 * in the future) to both the time for which we are waiting and the
447 * time that we will apply to the Hardware Clock, and start waiting
450 * For example, the caller requests that we set the Hardware Clock to
451 * 1:02:03, with reference time (current system time) = 6:07:08.250.
452 * We want the Hardware Clock to update to 1:02:04 at 6:07:09.250 on
453 * the system clock, and the first such update will occur 0.500
454 * seconds after we write to the Hardware Clock, so we spin until the
455 * system clock reads 6:07:08.750. If we get there, great, but let's
456 * imagine the system is so heavily loaded that our process is
457 * preempted and by the time we get to run again, the system clock
458 * reads 6:07:11.990. We now want to wait until the next xx:xx:xx.750
459 * time, which is 6:07:12.750 (4.5 seconds after the reference time),
460 * at which point we will set the Hardware Clock to 1:02:07 (4 seconds
461 * after the originally requested time). If we do that successfully,
462 * then at 6:07:13.250 (5 seconds after the reference time), the
463 * Hardware Clock will update to 1:02:08 (5 seconds after the
464 * originally requested time), and all is well thereafter.
467 time_t newhwtime
= sethwtime
;
468 double target_time_tolerance_secs
= 0.001; /* initial value */
469 double tolerance_incr_secs
= 0.001; /* initial value */
470 const double RTC_SET_DELAY_SECS
= 0.5; /* 500 ms */
471 const struct timeval RTC_SET_DELAY_TV
= { 0, RTC_SET_DELAY_SECS
* 1E6
};
473 struct timeval targetsystime
;
474 struct timeval nowsystime
;
475 struct timeval prevsystime
= refsystime
;
476 double deltavstarget
;
478 timeradd(&refsystime
, &RTC_SET_DELAY_TV
, &targetsystime
);
483 /* FOR TESTING ONLY: inject random delays of up to 1000ms */
484 if (ctl
->debug
>= 10) {
485 int usec
= random() % 1000000;
486 printf(_("sleeping ~%d usec\n"), usec
);
490 gettimeofday(&nowsystime
, NULL
);
491 deltavstarget
= time_diff(nowsystime
, targetsystime
);
492 ticksize
= time_diff(nowsystime
, prevsystime
);
493 prevsystime
= nowsystime
;
497 printf(_("time jumped backward %.6f seconds "
498 "to %ld.%06ld - retargeting\n"),
499 ticksize
, nowsystime
.tv_sec
,
501 /* The retarget is handled at the end of the loop. */
502 } else if (deltavstarget
< 0) {
503 /* deltavstarget < 0 if current time < target time */
505 printf(_("%ld.%06ld < %ld.%06ld (%.6f)\n"),
508 targetsystime
.tv_sec
,
509 targetsystime
.tv_usec
,
511 continue; /* not there yet - keep spinning */
512 } else if (deltavstarget
<= target_time_tolerance_secs
) {
513 /* Close enough to the target time; done waiting. */
515 } else /* (deltavstarget > target_time_tolerance_secs) */ {
517 * We missed our window. Increase the tolerance and
518 * aim for the next opportunity.
521 printf(_("missed it - %ld.%06ld is too far "
522 "past %ld.%06ld (%.6f > %.6f)\n"),
525 targetsystime
.tv_sec
,
526 targetsystime
.tv_usec
,
528 target_time_tolerance_secs
);
529 target_time_tolerance_secs
+= tolerance_incr_secs
;
530 tolerance_incr_secs
*= 2;
534 * Aim for the same offset (tv_usec) within the second in
535 * either the current second (if that offset hasn't arrived
536 * yet), or the next second.
538 if (nowsystime
.tv_usec
< targetsystime
.tv_usec
)
539 targetsystime
.tv_sec
= nowsystime
.tv_sec
;
541 targetsystime
.tv_sec
= nowsystime
.tv_sec
+ 1;
544 newhwtime
= sethwtime
545 + (int)(time_diff(nowsystime
, refsystime
)
546 - RTC_SET_DELAY_SECS
/* don't count this */
547 + 0.5 /* for rounding */);
549 printf(_("%ld.%06ld is close enough to %ld.%06ld (%.6f < %.6f)\n"
550 "Set RTC to %ld (%ld + %d; refsystime = %ld.%06ld)\n"),
551 nowsystime
.tv_sec
, nowsystime
.tv_usec
,
552 targetsystime
.tv_sec
, targetsystime
.tv_usec
,
553 deltavstarget
, target_time_tolerance_secs
,
554 newhwtime
, sethwtime
,
555 (int)(newhwtime
- sethwtime
),
556 refsystime
.tv_sec
, refsystime
.tv_usec
);
558 set_hardware_clock(ctl
, newhwtime
);
562 * Put the time "hwctime" on standard output in display format. Except if
563 * hclock_valid == false, just tell standard output that we don't know what
567 display_time(const bool hclock_valid
, struct timeval hwctime
)
571 ("The Hardware Clock registers contain values that are "
572 "either invalid (e.g. 50th day of month) or beyond the range "
573 "we can handle (e.g. Year 2095)."));
575 char buf
[ISO_8601_BUFSIZ
];
577 strtimeval_iso(&hwctime
, ISO_8601_DATE
|ISO_8601_TIME
|ISO_8601_DOTUSEC
|
578 ISO_8601_TIMEZONE
|ISO_8601_SPACE
,
585 * Set the System Clock to time 'newtime'.
587 * Also set the kernel time zone value to the value indicated by the TZ
588 * environment variable and/or /usr/lib/zoneinfo/, interpreted as tzset()
589 * would interpret them.
591 * If this is the first call of settimeofday since boot, then this also sets
592 * the kernel variable persistent_clock_is_local so that NTP 11 minute mode
593 * will update the Hardware Clock with the proper timescale. If the Hardware
594 * Clock's timescale configuration is changed then a reboot is required for
595 * persistent_clock_is_local to be updated.
597 * EXCEPT: if hclock_valid is false, just issue an error message saying
598 * there is no valid time in the Hardware Clock to which to set the system
601 * If 'testing' is true, don't actually update anything -- just say we would
605 set_system_clock(const struct hwclock_control
*ctl
, const bool hclock_valid
,
606 const struct timeval newtime
)
612 ("The Hardware Clock does not contain a valid time, so "
613 "we cannot set the System Time from it."));
616 const struct timeval
*tv_null
= NULL
;
621 broken
= localtime(&newtime
.tv_sec
);
622 #ifdef HAVE_TM_GMTOFF
623 minuteswest
= -broken
->tm_gmtoff
/ 60; /* GNU extension */
625 minuteswest
= timezone
/ 60;
626 if (broken
->tm_isdst
)
631 printf(_("Calling settimeofday:\n"));
632 printf(_("\ttv.tv_sec = %ld, tv.tv_usec = %ld\n"),
633 newtime
.tv_sec
, newtime
.tv_usec
);
634 printf(_("\ttz.tz_minuteswest = %d\n"), minuteswest
);
638 ("Test mode: clock was not changed\n"));
641 const struct timezone tz
= { minuteswest
, 0 };
643 /* Set kernel persistent_clock_is_local so that 11 minute
644 * mode does not clobber the Hardware Clock with UTC. This
645 * is only available on first call of settimeofday after boot.
648 rc
= settimeofday(tv_null
, &tz
);
650 rc
= settimeofday(&newtime
, &tz
);
652 if (errno
== EPERM
) {
654 ("Must be superuser to set system clock."));
657 warn(_("settimeofday() failed"));
668 * Reset the System Clock from local time to UTC, based on its current value
669 * and the timezone unless universal is TRUE.
671 * Also set the kernel time zone value to the value indicated by the TZ
672 * environment variable and/or /usr/lib/zoneinfo/, interpreted as tzset()
673 * would interpret them.
675 * If 'testing' is true, don't actually update anything -- just say we would
678 static int set_system_clock_timezone(const struct hwclock_control
*ctl
)
685 gettimeofday(&tv
, NULL
);
687 struct tm broken_time
;
690 broken_time
= *gmtime(&tv
.tv_sec
);
691 strftime(ctime_now
, sizeof(ctime_now
), "%Y/%m/%d %H:%M:%S",
693 printf(_("Current system time: %ld = %s\n"), tv
.tv_sec
,
697 broken
= localtime(&tv
.tv_sec
);
698 #ifdef HAVE_TM_GMTOFF
699 minuteswest
= -broken
->tm_gmtoff
/ 60; /* GNU extension */
701 minuteswest
= timezone
/ 60;
702 if (broken
->tm_isdst
)
707 struct tm broken_time
;
710 gettimeofday(&tv
, NULL
);
712 tv
.tv_sec
+= minuteswest
* 60;
714 broken_time
= *gmtime(&tv
.tv_sec
);
715 strftime(ctime_now
, sizeof(ctime_now
), "%Y/%m/%d %H:%M:%S",
718 printf(_("Calling settimeofday:\n"));
719 printf(_("\tUTC: %s\n"), ctime_now
);
720 printf(_("\ttv.tv_sec = %ld, tv.tv_usec = %ld\n"),
721 tv
.tv_sec
, tv
.tv_usec
);
722 printf(_("\ttz.tz_minuteswest = %d\n"), minuteswest
);
726 ("Test mode: clock was not changed\n"));
729 const struct timezone tz_utc
= { 0, 0 };
730 const struct timezone tz
= { minuteswest
, 0 };
731 const struct timeval
*tv_null
= NULL
;
734 /* The first call to settimeofday after boot will assume the systemtime
735 * is in localtime, and adjust it according to the given timezone to
736 * compensate. If the systemtime is in fact in UTC, then this is wrong
737 * so we first do a dummy call to make sure the time is not shifted.
740 rc
= settimeofday(tv_null
, &tz_utc
);
742 /* Now we set the real timezone. Due to the above dummy call, this will
743 * only warp the systemtime if the RTC is not in UTC. */
745 rc
= settimeofday(tv_null
, &tz
);
748 if (errno
== EPERM
) {
750 ("Must be superuser to set system clock."));
753 warn(_("settimeofday() failed"));
763 * Refresh the last calibrated and last adjusted timestamps in <*adjtime_p>
764 * to facilitate future drift calculations based on this set point.
766 * With the --update-drift option:
767 * Update the drift factor in <*adjtime_p> based on the fact that the
768 * Hardware Clock was just calibrated to <nowtime> and before that was
769 * set to the <hclocktime> time scale.
771 * EXCEPT: if <hclock_valid> is false, assume Hardware Clock was not set
772 * before to anything meaningful and regular adjustments have not been done,
773 * so don't adjust the drift factor.
776 adjust_drift_factor(const struct hwclock_control
*ctl
,
777 struct adjtime
*adjtime_p
,
778 const struct timeval nowtime
,
779 const bool hclock_valid
,
780 const struct timeval hclocktime
)
784 printf(_("Not adjusting drift factor because the "
785 "--update-drift option was not used.\n"));
786 } else if (!hclock_valid
) {
788 printf(_("Not adjusting drift factor because the "
789 "Hardware Clock previously contained "
791 } else if (adjtime_p
->last_calib_time
== 0) {
793 printf(_("Not adjusting drift factor because last "
794 "calibration time is zero,\n"
795 "so history is bad and calibration startover "
797 } else if ((hclocktime
.tv_sec
- adjtime_p
->last_calib_time
) < 4 * 60 * 60) {
799 printf(_("Not adjusting drift factor because it has "
800 "been less than four hours since the last "
804 * At adjustment time we drift correct the hardware clock
805 * according to the contents of the adjtime file and refresh
806 * its last adjusted timestamp.
808 * At calibration time we set the Hardware Clock and refresh
809 * both timestamps in <*adjtime_p>.
811 * Here, with the --update-drift option, we also update the
812 * drift factor in <*adjtime_p>.
814 * Let us do computation in doubles. (Floats almost suffice,
815 * but 195 days + 1 second equals 195 days in floats.)
817 const double sec_per_day
= 24.0 * 60.0 * 60.0;
818 double factor_adjust
;
820 struct timeval last_calib
;
822 last_calib
= t2tv(adjtime_p
->last_calib_time
);
824 * Correction to apply to the current drift factor.
826 * Simplified: uncorrected_drift / days_since_calibration.
828 * hclocktime is fully corrected with the current drift factor.
829 * Its difference from nowtime is the missed drift correction.
831 factor_adjust
= time_diff(nowtime
, hclocktime
) /
832 (time_diff(nowtime
, last_calib
) / sec_per_day
);
834 drift_factor
= adjtime_p
->drift_factor
+ factor_adjust
;
835 if (fabs(drift_factor
) > MAX_DRIFT
) {
837 printf(_("Clock drift factor was calculated as "
839 "It is far too much. Resetting to zero.\n"),
844 printf(_("Clock drifted %f seconds in the past "
845 "%f seconds\nin spite of a drift factor of "
847 "Adjusting drift factor by %f seconds/day\n"),
848 time_diff(nowtime
, hclocktime
),
849 time_diff(nowtime
, last_calib
),
850 adjtime_p
->drift_factor
, factor_adjust
);
853 adjtime_p
->drift_factor
= drift_factor
;
855 adjtime_p
->last_calib_time
= nowtime
.tv_sec
;
857 adjtime_p
->last_adj_time
= nowtime
.tv_sec
;
859 adjtime_p
->not_adjusted
= 0;
861 adjtime_p
->dirty
= TRUE
;
865 * Calculate the drift correction currently needed for the
866 * Hardware Clock based on the last time it was adjusted,
867 * and the current drift factor, as stored in the adjtime file.
869 * The total drift adjustment needed is stored at tdrift_p.
873 calculate_adjustment(const struct hwclock_control
*ctl
,
875 const time_t last_time
,
876 const double not_adjusted
,
877 const time_t systime
, struct timeval
*tdrift_p
)
879 double exact_adjustment
;
882 ((double)(systime
- last_time
)) * factor
/ (24 * 60 * 60)
884 tdrift_p
->tv_sec
= (time_t) floor(exact_adjustment
);
885 tdrift_p
->tv_usec
= (exact_adjustment
-
886 (double)tdrift_p
->tv_sec
) * 1E6
;
888 printf(P_("Time since last adjustment is %ld second\n",
889 "Time since last adjustment is %ld seconds\n",
890 (systime
- last_time
)),
891 (systime
- last_time
));
892 printf(_("Calculated Hardware Clock drift is %ld.%06ld seconds\n"),
893 tdrift_p
->tv_sec
, tdrift_p
->tv_usec
);
898 * Write the contents of the <adjtime> structure to its disk file.
900 * But if the contents are clean (unchanged since read from disk), don't
903 static void save_adjtime(const struct hwclock_control
*ctl
,
904 const struct adjtime
*adjtime
)
906 char *content
; /* Stuff to write to disk file */
913 xasprintf(&content
, "%f %ld %f\n%ld\n%s\n",
914 adjtime
->drift_factor
,
915 adjtime
->last_adj_time
,
916 adjtime
->not_adjusted
,
917 adjtime
->last_calib_time
,
918 (adjtime
->local_utc
== LOCAL
) ? "LOCAL" : "UTC");
922 printf(_("Test mode: %s was not updated with:\n%s"),
923 ctl
->adj_file_name
, content
);
929 fp
= fopen(ctl
->adj_file_name
, "w");
931 warn(_("Could not open file with the clock adjustment parameters "
932 "in it (%s) for writing"), ctl
->adj_file_name
);
934 } else if (fputs(content
, fp
) < 0 || close_stream(fp
) != 0) {
935 warn(_("Could not update file with the clock adjustment "
936 "parameters (%s) in it"), ctl
->adj_file_name
);
941 warnx(_("Drift adjustment parameters not updated."));
945 * Do the adjustment requested, by 1) setting the Hardware Clock (if
946 * necessary), and 2) updating the last-adjusted time in the adjtime
949 * Do not update anything if the Hardware Clock does not currently present a
952 * <hclock_valid> means the Hardware Clock contains a valid time.
954 * <hclocktime> is the drift corrected time read from the Hardware Clock.
956 * <read_time> was the system time when the <hclocktime> was read, which due
957 * to computational delay could be a short time ago. It is used to define a
958 * trigger point for setting the Hardware Clock. The fractional part of the
959 * Hardware clock set time is subtracted from read_time to 'refer back', or
960 * delay, the trigger point. Fractional parts must be accounted for in this
961 * way, because the Hardware Clock can only be set to a whole second.
963 * <universal>: the Hardware Clock is kept in UTC.
965 * <testing>: We are running in test mode (no updating of clock).
969 do_adjustment(const struct hwclock_control
*ctl
, struct adjtime
*adjtime_p
,
970 const bool hclock_valid
, const struct timeval hclocktime
,
971 const struct timeval read_time
)
974 warnx(_("The Hardware Clock does not contain a valid time, "
975 "so we cannot adjust it."));
976 adjtime_p
->last_calib_time
= 0; /* calibration startover is required */
977 adjtime_p
->last_adj_time
= 0;
978 adjtime_p
->not_adjusted
= 0;
979 adjtime_p
->dirty
= TRUE
;
980 } else if (adjtime_p
->last_adj_time
== 0) {
982 printf(_("Not setting clock because last adjustment time is zero, "
983 "so history is bad.\n"));
984 } else if (fabs(adjtime_p
->drift_factor
) > MAX_DRIFT
) {
986 printf(_("Not setting clock because drift factor %f is far too high.\n"),
987 adjtime_p
->drift_factor
);
989 set_hardware_clock_exact(ctl
, hclocktime
.tv_sec
,
991 -(hclocktime
.tv_usec
/ 1E6
)));
992 adjtime_p
->last_adj_time
= hclocktime
.tv_sec
;
993 adjtime_p
->not_adjusted
= 0;
994 adjtime_p
->dirty
= TRUE
;
998 static void determine_clock_access_method(const struct hwclock_control
*ctl
)
1003 ur
= probe_for_cmos_clock();
1006 ur
= probe_for_rtc_clock(ctl
);
1010 puts(ur
->interface_name
);
1014 printf(_("No usable clock interface found.\n"));
1015 warnx(_("Cannot access the Hardware Clock via "
1016 "any known method."));
1018 warnx(_("Use the --debug option to see the "
1019 "details of our search for an access "
1021 hwclock_exit(ctl
, EX_SOFTWARE
);
1026 * Do all the normal work of hwclock - read, set clock, etc.
1028 * Issue output to stdout and error message to stderr where appropriate.
1030 * Return rc == 0 if everything went OK, rc != 0 if not.
1033 manipulate_clock(const struct hwclock_control
*ctl
, const time_t set_time
,
1034 const struct timeval startup_time
, struct adjtime
*adjtime
)
1036 /* The time at which we read the Hardware Clock */
1037 struct timeval read_time
;
1039 * The Hardware Clock gives us a valid time, or at
1040 * least something close enough to fool mktime().
1042 bool hclock_valid
= FALSE
;
1044 * Tick synchronized time read from the Hardware Clock and
1045 * then drift correct for all operations except --show.
1047 struct timeval hclocktime
= { 0, 0 };
1048 /* Total Hardware Clock drift correction needed. */
1049 struct timeval tdrift
;
1050 /* local return code */
1053 if (!ctl
->systz
&& !ctl
->predict
&& ur
->get_permissions())
1056 if ((ctl
->set
|| ctl
->systohc
|| ctl
->adjust
) &&
1057 (adjtime
->local_utc
== UTC
) != ctl
->universal
) {
1058 adjtime
->local_utc
= ctl
->universal
? UTC
: LOCAL
;
1059 adjtime
->dirty
= TRUE
;
1062 if (ctl
->show
|| ctl
->get
|| ctl
->adjust
|| ctl
->hctosys
1063 || (!ctl
->noadjfile
&& !ctl
->systz
&& !ctl
->predict
)) {
1064 /* data from HW-clock are required */
1065 rc
= synchronize_to_clock_tick(ctl
);
1068 * We don't error out if the user is attempting to set the
1069 * RTC and synchronization timeout happens - the RTC could
1070 * be functioning but contain invalid time data so we still
1071 * want to allow a user to set the RTC time.
1073 if (rc
== RTC_BUSYWAIT_FAILED
&& !ctl
->set
&& !ctl
->systohc
)
1075 gettimeofday(&read_time
, NULL
);
1078 * If we can't synchronize to a clock tick,
1079 * we likely can't read from the RTC so
1080 * don't bother reading it again.
1083 rc
= read_hardware_clock(ctl
, &hclock_valid
,
1084 &hclocktime
.tv_sec
);
1085 if (rc
&& !ctl
->set
&& !ctl
->systohc
)
1090 * Calculate Hardware Clock drift for --predict with the user
1091 * supplied --date option time, and with the time read from the
1092 * Hardware Clock for all other operations. Apply drift correction
1093 * to the Hardware Clock time for everything except --show and
1094 * --predict. For --predict negate the drift correction, because we
1095 * want to 'predict' a future Hardware Clock time that includes drift.
1097 hclocktime
= ctl
->predict
? t2tv(set_time
) : hclocktime
;
1098 calculate_adjustment(ctl
, adjtime
->drift_factor
,
1099 adjtime
->last_adj_time
,
1100 adjtime
->not_adjusted
,
1101 hclocktime
.tv_sec
, &tdrift
);
1102 if (!ctl
->show
&& !ctl
->predict
)
1103 hclocktime
= time_inc(tdrift
, hclocktime
.tv_sec
);
1104 if (ctl
->show
|| ctl
->get
) {
1105 display_time(hclock_valid
,
1106 time_inc(hclocktime
, -time_diff
1107 (read_time
, startup_time
)));
1108 } else if (ctl
->set
) {
1109 set_hardware_clock_exact(ctl
, set_time
, startup_time
);
1110 if (!ctl
->noadjfile
)
1111 adjust_drift_factor(ctl
, adjtime
,
1112 time_inc(t2tv(set_time
), time_diff
1113 (read_time
, startup_time
)),
1114 hclock_valid
, hclocktime
);
1115 } else if (ctl
->adjust
) {
1116 if (tdrift
.tv_sec
> 0 || tdrift
.tv_sec
< -1)
1117 do_adjustment(ctl
, adjtime
, hclock_valid
,
1118 hclocktime
, read_time
);
1120 printf(_("Needed adjustment is less than one second, "
1121 "so not setting clock.\n"));
1122 } else if (ctl
->systohc
) {
1123 struct timeval nowtime
, reftime
;
1125 * We can only set_hardware_clock_exact to a
1126 * whole seconds time, so we set it with
1127 * reference to the most recent whole
1130 gettimeofday(&nowtime
, NULL
);
1131 reftime
.tv_sec
= nowtime
.tv_sec
;
1132 reftime
.tv_usec
= 0;
1133 set_hardware_clock_exact(ctl
, (time_t) reftime
.tv_sec
, reftime
);
1134 if (!ctl
->noadjfile
)
1135 adjust_drift_factor(ctl
, adjtime
, nowtime
,
1136 hclock_valid
, hclocktime
);
1137 } else if (ctl
->hctosys
) {
1138 rc
= set_system_clock(ctl
, hclock_valid
, hclocktime
);
1140 printf(_("Unable to set system clock.\n"));
1143 } else if (ctl
->systz
) {
1144 rc
= set_system_clock_timezone(ctl
);
1146 printf(_("Unable to set system clock.\n"));
1149 } else if (ctl
->predict
) {
1150 hclocktime
= time_inc(hclocktime
, (double)
1151 -(tdrift
.tv_sec
+ tdrift
.tv_usec
/ 1E6
));
1154 ("At %ld seconds after 1969, RTC is predicted to read %ld seconds after 1969.\n"),
1155 set_time
, hclocktime
.tv_sec
);
1157 display_time(TRUE
, hclocktime
);
1159 if (!ctl
->noadjfile
)
1160 save_adjtime(ctl
, adjtime
);
1165 * Get or set the kernel RTC driver's epoch on Alpha machines.
1166 * ISA machines are hard coded for 1900.
1168 #if defined(__linux__) && defined(__alpha__)
1170 manipulate_epoch(const struct hwclock_control
*ctl
)
1172 if (ctl
->getepoch
) {
1173 unsigned long epoch
;
1175 if (get_epoch_rtc(ctl
, &epoch
))
1177 ("Unable to get the epoch value from the kernel."));
1179 printf(_("Kernel is assuming an epoch value of %lu\n"),
1181 } else if (ctl
->setepoch
) {
1182 if (ctl
->epoch_option
== 0)
1184 ("To set the epoch value, you must use the 'epoch' "
1185 "option to tell to what value to set it."));
1186 else if (ctl
->testing
)
1188 ("Not setting the epoch to %lu - testing only.\n"),
1190 else if (set_epoch_rtc(ctl
))
1192 ("Unable to set the epoch value in the kernel.\n"));
1195 #endif /* __linux__ __alpha__ */
1197 static void out_version(void)
1199 printf(UTIL_LINUX_VERSION
);
1203 * usage - Output (error and) usage information
1205 * This function is called both directly from main to show usage information
1206 * and as fatal function from shhopt if some argument is not understood. In
1207 * case of normal usage info FMT should be NULL. In that case the info is
1208 * printed to stdout. If FMT is given usage will act like fprintf( stderr,
1209 * fmt, ... ), show a usage information and terminate the program
1212 static void __attribute__((__noreturn__
))
1213 usage(const struct hwclock_control
*ctl
, const char *fmt
, ...)
1218 usageto
= fmt
? stderr
: stdout
;
1220 fputs(USAGE_HEADER
, usageto
);
1221 fputs(_(" hwclock [function] [option...]\n"), usageto
);
1223 fputs(USAGE_SEPARATOR
, usageto
);
1224 fputs(_("Query or set the hardware clock.\n"), usageto
);
1226 fputs(_("\nFunctions:\n"), usageto
);
1227 fputs(_(" -h, --help show this help text and exit\n"
1228 " -r, --show read hardware clock and print result\n"
1229 " --get read hardware clock and print drift corrected result\n"
1230 " --set set the RTC to the time given with --date\n"), usageto
);
1231 fputs(_(" -s, --hctosys set the system time from the hardware clock\n"
1232 " -w, --systohc set the hardware clock from the current system time\n"
1233 " --systz set the system time based on the current timezone\n"
1234 " --adjust adjust the RTC to account for systematic drift since\n"
1235 " the clock was last set or adjusted\n"), usageto
);
1236 #if defined(__linux__) && defined(__alpha__)
1237 fputs(_(" --getepoch print out the kernel's hardware clock epoch value\n"
1238 " --setepoch set the kernel's hardware clock epoch value to the \n"
1239 " value given with --epoch\n"), usageto
);
1241 fputs(_(" --predict predict RTC reading at time given with --date\n"
1242 " -V, --version display version information and exit\n"), usageto
);
1244 fputs(USAGE_OPTIONS
, usageto
);
1245 fputs(_(" -u, --utc the hardware clock is kept in UTC\n"
1246 " -l, --localtime the hardware clock is kept in local time\n"), usageto
);
1248 fputs(_(" -f, --rtc <file> special /dev/... file to use instead of default\n"), usageto
);
1251 " --directisa access the ISA bus directly instead of %s\n"
1252 " --date <time> specifies the time to which to set the hardware clock\n"), _PATH_RTC_DEV
);
1253 #if defined(__linux__) && defined(__alpha__)
1254 fputs(_(" --epoch <year> specifies the hardware clock's epoch value\n"), usageto
);
1257 " --update-drift update drift factor in %1$s (requires\n"
1258 " --set or --systohc)\n"
1259 " --noadjfile do not access %1$s; this requires the use of\n"
1260 " either --utc or --localtime\n"
1261 " --adjfile <file> specifies the path to the adjust file;\n"
1262 " the default is %1$s\n"), _PATH_ADJTIME
);
1263 fputs(_(" --test do not update anything, just show what would happen\n"
1264 " -D, --debug debugging mode\n" "\n"), usageto
);
1268 vfprintf(usageto
, fmt
, ap
);
1273 hwclock_exit(ctl
, fmt
? EX_USAGE
: EX_OK
);
1278 * EX_USAGE: bad invocation
1279 * EX_NOPERM: no permission
1280 * EX_OSFILE: cannot open /dev/rtc or /etc/adjtime
1281 * EX_IOERR: ioctl error getting or setting the time
1285 int main(int argc
, char **argv
)
1287 struct hwclock_control ctl
= { .show
= 1 }; /* default op is show */
1288 struct timeval startup_time
;
1289 struct adjtime adjtime
= { 0 };
1290 struct timespec when
= { 0 };
1292 * The time we started up, in seconds into the epoch, including
1295 time_t set_time
= 0; /* Time to which user said to set Hardware Clock */
1298 /* Long only options. */
1300 OPT_ADJFILE
= CHAR_MAX
+ 1,
1315 static const struct option longopts
[] = {
1316 { "adjust", no_argument
, NULL
, 'a' },
1317 { "help", no_argument
, NULL
, 'h' },
1318 { "localtime", no_argument
, NULL
, 'l' },
1319 { "show", no_argument
, NULL
, 'r' },
1320 { "hctosys", no_argument
, NULL
, 's' },
1321 { "utc", no_argument
, NULL
, 'u' },
1322 { "version", no_argument
, NULL
, 'v' },
1323 { "systohc", no_argument
, NULL
, 'w' },
1324 { "debug", no_argument
, NULL
, 'D' },
1325 { "set", no_argument
, NULL
, OPT_SET
},
1326 #if defined(__linux__) && defined(__alpha__)
1327 { "getepoch", no_argument
, NULL
, OPT_GETEPOCH
},
1328 { "setepoch", no_argument
, NULL
, OPT_SETEPOCH
},
1329 { "epoch", required_argument
, NULL
, OPT_EPOCH
},
1331 { "noadjfile", no_argument
, NULL
, OPT_NOADJFILE
},
1332 { "directisa", no_argument
, NULL
, OPT_DIRECTISA
},
1333 { "test", no_argument
, NULL
, OPT_TEST
},
1334 { "date", required_argument
, NULL
, OPT_DATE
},
1336 { "rtc", required_argument
, NULL
, 'f' },
1338 { "adjfile", required_argument
, NULL
, OPT_ADJFILE
},
1339 { "systz", no_argument
, NULL
, OPT_SYSTZ
},
1340 { "predict", no_argument
, NULL
, OPT_PREDICT
},
1341 { "get", no_argument
, NULL
, OPT_GET
},
1342 { "update-drift", no_argument
, NULL
, OPT_UPDATE
},
1343 { NULL
, 0, NULL
, 0 }
1346 static const ul_excl_t excl
[] = { /* rows and cols in ASCII order */
1348 OPT_GET
, OPT_GETEPOCH
, OPT_PREDICT
,
1349 OPT_SET
, OPT_SETEPOCH
, OPT_SYSTZ
},
1351 { OPT_ADJFILE
, OPT_NOADJFILE
},
1352 { OPT_NOADJFILE
, OPT_UPDATE
},
1355 int excl_st
[ARRAY_SIZE(excl
)] = UL_EXCL_STATUS_INIT
;
1357 /* Remember what time we were invoked */
1358 gettimeofday(&startup_time
, NULL
);
1360 #ifdef HAVE_LIBAUDIT
1361 hwaudit_fd
= audit_open();
1362 if (hwaudit_fd
< 0 && !(errno
== EINVAL
|| errno
== EPROTONOSUPPORT
||
1363 errno
== EAFNOSUPPORT
)) {
1365 * You get these error codes only when the kernel doesn't
1366 * have audit compiled in.
1368 warnx(_("Unable to connect to audit system"));
1372 setlocale(LC_ALL
, "");
1375 * We need LC_CTYPE and LC_TIME and LC_MESSAGES, but must avoid
1376 * LC_NUMERIC since it gives problems when we write to /etc/adjtime.
1377 * - gqueri@mail.dotcom.fr
1379 setlocale(LC_NUMERIC
, "C");
1381 bindtextdomain(PACKAGE
, LOCALEDIR
);
1382 textdomain(PACKAGE
);
1383 atexit(close_stdout
);
1385 while ((c
= getopt_long(argc
, argv
,
1386 "?hvVDalrsuwAJSFf:", longopts
, NULL
)) != -1) {
1388 err_exclusive_options(c
, longopts
, excl
, excl_st
);
1400 ctl
.local_opt
= 1; /* --localtime */
1423 #if defined(__linux__) && defined(__alpha__)
1434 ctl
.epoch_option
= /* --epoch */
1435 strtoul_or_err(optarg
, _("invalid epoch argument"));
1445 ctl
.testing
= 1; /* --test */
1448 ctl
.date_opt
= optarg
; /* --date */
1451 ctl
.adj_file_name
= optarg
; /* --adjfile */
1454 ctl
.systz
= 1; /* --systz */
1458 ctl
.predict
= 1; /* --predict */
1462 ctl
.get
= 1; /* --get */
1466 ctl
.update
= 1; /* --update-drift */
1470 ctl
.rtc_dev_name
= optarg
; /* --rtc */
1473 case 'v': /* --version */
1477 case 'h': /* --help */
1480 errtryhelp(EXIT_FAILURE
);
1488 warnx(_("%d too many arguments given"), argc
);
1489 errtryhelp(EXIT_FAILURE
);
1492 if (!ctl
.adj_file_name
)
1493 ctl
.adj_file_name
= _PATH_ADJTIME
;
1495 if (ctl
.noadjfile
&& !ctl
.utc
&& !ctl
.local_opt
) {
1496 warnx(_("With --noadjfile, you must specify "
1497 "either --utc or --localtime"));
1498 hwclock_exit(&ctl
, EX_USAGE
);
1501 if (ctl
.set
|| ctl
.predict
) {
1503 warnx(_("--date is required for --set or --predict"));
1504 hwclock_exit(&ctl
, EX_USAGE
);
1506 if (parse_date(&when
, ctl
.date_opt
, NULL
))
1507 set_time
= when
.tv_sec
;
1509 warnx(_("invalid date '%s'"), ctl
.date_opt
);
1510 hwclock_exit(&ctl
, EX_USAGE
);
1514 #if defined(__linux__) && defined(__alpha__)
1515 if (ctl
.getepoch
|| ctl
.setepoch
) {
1516 manipulate_epoch(&ctl
);
1517 hwclock_exit(&ctl
, EX_OK
);
1524 if (!ctl
.systz
&& !ctl
.predict
)
1525 determine_clock_access_method(&ctl
);
1527 if (!ctl
.noadjfile
&& !(ctl
.systz
&& (ctl
.utc
|| ctl
.local_opt
))) {
1528 if ((rc
= read_adjtime(&ctl
, &adjtime
)) != 0)
1529 hwclock_exit(&ctl
, rc
);
1531 /* Avoid writing adjtime file if we don't have to. */
1532 adjtime
.dirty
= FALSE
;
1533 ctl
.universal
= hw_clock_is_utc(&ctl
, adjtime
);
1534 rc
= manipulate_clock(&ctl
, set_time
, startup_time
, &adjtime
);
1535 hwclock_exit(&ctl
, rc
);
1536 return rc
; /* Not reached */
1539 void __attribute__((__noreturn__
))
1540 hwclock_exit(const struct hwclock_control
*ctl
1541 #ifndef HAVE_LIBAUDIT
1542 __attribute__((__unused__
))
1546 #ifdef HAVE_LIBAUDIT
1547 if (ctl
->hwaudit_on
&& !ctl
->testing
) {
1548 audit_log_user_message(hwaudit_fd
, AUDIT_USYS_CONFIG
,
1549 "op=change-system-time", NULL
, NULL
, NULL
,
1558 * History of this program:
1560 * 98.08.12 BJH Version 2.4
1562 * Don't use century byte from Hardware Clock. Add comments telling why.
1564 * 98.06.20 BJH Version 2.3.
1566 * Make --hctosys set the kernel timezone from TZ environment variable
1567 * and/or /usr/lib/zoneinfo. From Klaus Ripke (klaus@ripke.com).
1569 * 98.03.05 BJH. Version 2.2.
1571 * Add --getepoch and --setepoch.
1573 * Fix some word length things so it works on Alpha.
1575 * Make it work when /dev/rtc doesn't have the interrupt functions. In this
1576 * case, busywait for the top of a second instead of blocking and waiting
1577 * for the update complete interrupt.
1579 * Fix a bunch of bugs too numerous to mention.
1581 * 97.06.01: BJH. Version 2.1. Read and write the century byte (Byte 50) of
1582 * the ISA Hardware Clock when using direct ISA I/O. Problem discovered by
1583 * job (jei@iclnl.icl.nl).
1585 * Use the rtc clock access method in preference to the KDGHWCLK method.
1586 * Problem discovered by Andreas Schwab <schwab@LS5.informatik.uni-dortmund.de>.
1588 * November 1996: Version 2.0.1. Modifications by Nicolai Langfeldt
1589 * (janl@math.uio.no) to make it compile on linux 1.2 machines as well as
1590 * more recent versions of the kernel. Introduced the NO_CLOCK access method
1591 * and wrote feature test code to detect absence of rtc headers.
1593 ***************************************************************************
1596 * To compile this, you must use GNU compiler optimization (-O option) in
1597 * order to make the "extern inline" functions from asm/io.h (inb(), etc.)
1598 * compile. If you don't optimize, which means the compiler will generate no
1599 * inline functions, the references to these functions in this program will
1600 * be compiled as external references. Since you probably won't be linking
1601 * with any functions by these names, you will have unresolved external
1602 * references when you link.
1604 * Here's some info on how we must deal with the time that elapses while
1605 * this program runs: There are two major delays as we run:
1607 * 1) Waiting up to 1 second for a transition of the Hardware Clock so
1608 * we are synchronized to the Hardware Clock.
1609 * 2) Running the "date" program to interpret the value of our --date
1612 * Reading the /etc/adjtime file is the next biggest source of delay and
1615 * The user wants to know what time it was at the moment he invoked us, not
1616 * some arbitrary time later. And in setting the clock, he is giving us the
1617 * time at the moment we are invoked, so if we set the clock some time
1618 * later, we have to add some time to that.
1620 * So we check the system time as soon as we start up, then run "date" and
1621 * do file I/O if necessary, then wait to synchronize with a Hardware Clock
1622 * edge, then check the system time again to see how much time we spent. We
1623 * immediately read the clock then and (if appropriate) report that time,
1624 * and additionally, the delay we measured.
1626 * If we're setting the clock to a time given by the user, we wait some more
1627 * so that the total delay is an integral number of seconds, then set the
1628 * Hardware Clock to the time the user requested plus that integral number
1629 * of seconds. N.B. The Hardware Clock can only be set in integral seconds.
1631 * If we're setting the clock to the system clock value, we wait for the
1632 * system clock to reach the top of a second, and then set the Hardware
1633 * Clock to the system clock's value.
1635 * Here's an interesting point about setting the Hardware Clock: On my
1636 * machine, when you set it, it sets to that precise time. But one can
1637 * imagine another clock whose update oscillator marches on a steady one
1638 * second period, so updating the clock between any two oscillator ticks is
1639 * the same as updating it right at the earlier tick. To avoid any
1640 * complications that might cause, we set the clock as soon as possible
1641 * after an oscillator tick.
1643 * About synchronizing to the Hardware Clock when reading the time: The
1644 * precision of the Hardware Clock counters themselves is one second. You
1645 * can't read the counters and find out that is 12:01:02.5. But if you
1646 * consider the location in time of the counter's ticks as part of its
1647 * value, then its precision is as infinite as time is continuous! What I'm
1648 * saying is this: To find out the _exact_ time in the hardware clock, we
1649 * wait until the next clock tick (the next time the second counter changes)
1650 * and measure how long we had to wait. We then read the value of the clock
1651 * counters and subtract the wait time and we know precisely what time it
1652 * was when we set out to query the time.
1654 * hwclock uses this method, and considers the Hardware Clock to have
1655 * infinite precision.