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'
73 #define OPTUTILS_EXIT_CODE EX_USAGE
76 #include "closestream.h"
79 #include "pathnames.h"
82 #include "timeutils.h"
88 static int hwaudit_fd
= -1;
91 /* The struct that holds our hardware access routines */
92 static struct clock_ops
*ur
;
94 /* Maximal clock adjustment in seconds per day.
95 (adjtime() glibc call has 2145 seconds limit on i386, so it is good enough for us as well,
96 43219 is a maximal safe value preventing exact_adjustment overflow.) */
97 #define MAX_DRIFT 2145.0
101 * This is information we keep in the adjtime file that tells us how
102 * to do drift corrections. Elements are all straight from the
103 * adjtime file, so see documentation of that file for details.
104 * Exception is <dirty>, which is an indication that what's in this
105 * structure is not what's in the disk file (because it has been
106 * updated since read from the disk file).
111 time_t last_adj_time
;
114 time_t last_calib_time
;
116 * The most recent time that we set the clock from an external
117 * authority (as opposed to just doing a drift adjustment)
120 enum a_local_utc
{ UTC
= 0, LOCAL
, UNKNOWN
} local_utc
;
122 * To which time zone, local or UTC, we most recently set the
128 * time_t to timeval conversion.
130 static struct timeval
t2tv(time_t timet
)
132 struct timeval rettimeval
;
134 rettimeval
.tv_sec
= timet
;
135 rettimeval
.tv_usec
= 0;
140 * The difference in seconds between two times in "timeval" format.
142 double time_diff(struct timeval subtrahend
, struct timeval subtractor
)
144 return (subtrahend
.tv_sec
- subtractor
.tv_sec
)
145 + (subtrahend
.tv_usec
- subtractor
.tv_usec
) / 1E6
;
149 * The time, in "timeval" format, which is <increment> seconds after the
150 * time <addend>. Of course, <increment> may be negative.
152 static struct timeval
time_inc(struct timeval addend
, double increment
)
154 struct timeval newtime
;
156 newtime
.tv_sec
= addend
.tv_sec
+ (int)increment
;
157 newtime
.tv_usec
= addend
.tv_usec
+ (increment
- (int)increment
) * 1E6
;
160 * Now adjust it so that the microsecond value is between 0 and 1
163 if (newtime
.tv_usec
< 0) {
164 newtime
.tv_usec
+= 1E6
;
166 } else if (newtime
.tv_usec
>= 1E6
) {
167 newtime
.tv_usec
-= 1E6
;
174 hw_clock_is_utc(const struct hwclock_control
*ctl
,
175 const struct adjtime adjtime
)
180 ret
= TRUE
; /* --utc explicitly given on command line */
181 else if (ctl
->local_opt
)
182 ret
= FALSE
; /* --localtime explicitly given */
184 /* get info from adjtime file - default is UTC */
185 ret
= (adjtime
.local_utc
!= LOCAL
);
187 printf(_("Assuming hardware clock is kept in %s time.\n"),
188 ret
? _("UTC") : _("local"));
193 * Read the adjustment parameters out of the /etc/adjtime file.
195 * Return them as the adjtime structure <*adjtime_p>. If there is no
196 * /etc/adjtime file, return defaults. If values are missing from the file,
197 * return defaults for them.
199 * return value 0 if all OK, !=0 otherwise.
201 static int read_adjtime(const struct hwclock_control
*ctl
,
202 struct adjtime
*adjtime_p
)
205 char line1
[81]; /* String: first line of adjtime file */
206 char line2
[81]; /* String: second line of adjtime file */
207 char line3
[81]; /* String: third line of adjtime file */
209 if (access(ctl
->adj_file_name
, R_OK
) != 0)
212 adjfile
= fopen(ctl
->adj_file_name
, "r"); /* open file for reading */
213 if (adjfile
== NULL
) {
214 warn(_("cannot open %s"), ctl
->adj_file_name
);
218 if (!fgets(line1
, sizeof(line1
), adjfile
))
219 line1
[0] = '\0'; /* In case fgets fails */
220 if (!fgets(line2
, sizeof(line2
), adjfile
))
221 line2
[0] = '\0'; /* In case fgets fails */
222 if (!fgets(line3
, sizeof(line3
), adjfile
))
223 line3
[0] = '\0'; /* In case fgets fails */
227 sscanf(line1
, "%lf %ld %lf",
228 &adjtime_p
->drift_factor
,
229 &adjtime_p
->last_adj_time
,
230 &adjtime_p
->not_adjusted
);
232 sscanf(line2
, "%ld", &adjtime_p
->last_calib_time
);
234 if (!strcmp(line3
, "UTC\n")) {
235 adjtime_p
->local_utc
= UTC
;
236 } else if (!strcmp(line3
, "LOCAL\n")) {
237 adjtime_p
->local_utc
= LOCAL
;
239 adjtime_p
->local_utc
= UNKNOWN
;
241 warnx(_("Warning: unrecognized third line in adjtime file\n"
242 "(Expected: `UTC' or `LOCAL' or nothing.)"));
248 ("Last drift adjustment done at %ld seconds after 1969\n"),
249 (long)adjtime_p
->last_adj_time
);
250 printf(_("Last calibration done at %ld seconds after 1969\n"),
251 (long)adjtime_p
->last_calib_time
);
252 printf(_("Hardware clock is on %s time\n"),
253 (adjtime_p
->local_utc
==
254 LOCAL
) ? _("local") : (adjtime_p
->local_utc
==
255 UTC
) ? _("UTC") : _("unknown"));
262 * Wait until the falling edge of the Hardware Clock's update flag so that
263 * any time that is read from the clock immediately after we return will be
266 * The clock only has 1 second precision, so it gives the exact time only
267 * once per second, right on the falling edge of the update flag.
269 * We wait (up to one second) either blocked waiting for an rtc device or in
270 * a CPU spin loop. The former is probably not very accurate.
272 * Return 0 if it worked, nonzero if it didn't.
274 static int synchronize_to_clock_tick(const struct hwclock_control
*ctl
)
279 printf(_("Waiting for clock tick...\n"));
281 rc
= ur
->synchronize_to_clock_tick(ctl
);
285 printf(_("...synchronization failed\n"));
287 printf(_("...got clock tick\n"));
294 * Convert a time in broken down format (hours, minutes, etc.) into standard
295 * unix time (seconds into epoch). Return it as *systime_p.
297 * The broken down time is argument <tm>. This broken down time is either
298 * in local time zone or UTC, depending on value of logical argument
299 * "universal". True means it is in UTC.
301 * If the argument contains values that do not constitute a valid time, and
302 * mktime() recognizes this, return *valid_p == false and *systime_p
303 * undefined. However, mktime() sometimes goes ahead and computes a
304 * fictional time "as if" the input values were valid, e.g. if they indicate
305 * the 31st day of April, mktime() may compute the time of May 1. In such a
306 * case, we return the same fictional value mktime() does as *systime_p and
307 * return *valid_p == true.
310 mktime_tz(const struct hwclock_control
*ctl
, struct tm tm
,
311 bool *valid_p
, time_t *systime_p
)
314 *systime_p
= timegm(&tm
);
316 *systime_p
= mktime(&tm
);
317 if (*systime_p
== -1) {
319 * This apparently (not specified in mktime() documentation)
320 * means the 'tm' structure does not contain valid values
321 * (however, not containing valid values does _not_ imply
322 * mktime() returns -1).
326 printf(_("Invalid values in hardware clock: "
327 "%4d/%.2d/%.2d %.2d:%.2d:%.2d\n"),
328 tm
.tm_year
+ 1900, tm
.tm_mon
+ 1, tm
.tm_mday
,
329 tm
.tm_hour
, tm
.tm_min
, tm
.tm_sec
);
334 ("Hw clock time : %4d/%.2d/%.2d %.2d:%.2d:%.2d = "
335 "%ld seconds since 1969\n"), tm
.tm_year
+ 1900,
336 tm
.tm_mon
+ 1, tm
.tm_mday
, tm
.tm_hour
, tm
.tm_min
,
337 tm
.tm_sec
, (long)*systime_p
);
342 * Read the hardware clock and return the current time via <tm> argument.
344 * Use the method indicated by <method> argument to access the hardware
348 read_hardware_clock(const struct hwclock_control
*ctl
,
349 bool * valid_p
, time_t *systime_p
)
354 err
= ur
->read_hardware_clock(ctl
, &tm
);
360 ("Time read from Hardware Clock: %4d/%.2d/%.2d %02d:%02d:%02d\n"),
361 tm
.tm_year
+ 1900, tm
.tm_mon
+ 1, tm
.tm_mday
, tm
.tm_hour
,
362 tm
.tm_min
, tm
.tm_sec
);
363 mktime_tz(ctl
, tm
, valid_p
, systime_p
);
369 * Set the Hardware Clock to the time <newtime>, in local time zone or UTC,
370 * according to <universal>.
373 set_hardware_clock(const struct hwclock_control
*ctl
, const time_t newtime
)
375 struct tm new_broken_time
;
377 * Time to which we will set Hardware Clock, in broken down format,
378 * in the time zone of caller's choice
382 new_broken_time
= *gmtime(&newtime
);
384 new_broken_time
= *localtime(&newtime
);
387 printf(_("Setting Hardware Clock to %.2d:%.2d:%.2d "
388 "= %ld seconds since 1969\n"),
389 new_broken_time
.tm_hour
, new_broken_time
.tm_min
,
390 new_broken_time
.tm_sec
, (long)newtime
);
393 printf(_("Test mode: clock was not changed\n"));
395 ur
->set_hardware_clock(ctl
, &new_broken_time
);
399 * Set the Hardware Clock to the time "sethwtime", in local time zone or
400 * UTC, according to "universal".
402 * Wait for a fraction of a second so that "sethwtime" is the value of the
403 * Hardware Clock as of system time "refsystime", which is in the past. For
404 * example, if "sethwtime" is 14:03:05 and "refsystime" is 12:10:04.5 and
405 * the current system time is 12:10:06.0: Wait .5 seconds (to make exactly 2
406 * seconds since "refsystime") and then set the Hardware Clock to 14:03:07,
407 * thus getting a precise and retroactive setting of the clock.
409 * (Don't be confused by the fact that the system clock and the Hardware
410 * Clock differ by two hours in the above example. That's just to remind you
411 * that there are two independent time scales here).
413 * This function ought to be able to accept set times as fractional times.
414 * Idea for future enhancement.
417 set_hardware_clock_exact(const struct hwclock_control
*ctl
,
418 const time_t sethwtime
,
419 const struct timeval refsystime
)
422 * The Hardware Clock can only be set to any integer time plus one
423 * half second. The integer time is required because there is no
424 * interface to set or get a fractional second. The additional half
425 * second is because the Hardware Clock updates to the following
426 * second precisely 500 ms (not 1 second!) after you release the
427 * divider reset (after setting the new time) - see description of
428 * DV2, DV1, DV0 in Register A in the MC146818A data sheet (and note
429 * that although that document doesn't say so, real-world code seems
430 * to expect that the SET bit in Register B functions the same way).
431 * That means that, e.g., when you set the clock to 1:02:03, it
432 * effectively really sets it to 1:02:03.5, because it will update to
433 * 1:02:04 only half a second later. Our caller passes the desired
434 * integer Hardware Clock time in sethwtime, and the corresponding
435 * system time (which may have a fractional part, and which may or may
436 * not be the same!) in refsystime. In an ideal situation, we would
437 * then apply sethwtime to the Hardware Clock at refsystime+500ms, so
438 * that when the Hardware Clock ticks forward to sethwtime+1s half a
439 * second later at refsystime+1000ms, everything is in sync. So we
440 * spin, waiting for gettimeofday() to return a time at or after that
441 * time (refsystime+500ms) up to a tolerance value, initially 1ms. If
442 * we miss that time due to being preempted for some other process,
443 * then we increase the margin a little bit (initially 1ms, doubling
444 * each time), add 1 second (or more, if needed to get a time that is
445 * in the future) to both the time for which we are waiting and the
446 * time that we will apply to the Hardware Clock, and start waiting
449 * For example, the caller requests that we set the Hardware Clock to
450 * 1:02:03, with reference time (current system time) = 6:07:08.250.
451 * We want the Hardware Clock to update to 1:02:04 at 6:07:09.250 on
452 * the system clock, and the first such update will occur 0.500
453 * seconds after we write to the Hardware Clock, so we spin until the
454 * system clock reads 6:07:08.750. If we get there, great, but let's
455 * imagine the system is so heavily loaded that our process is
456 * preempted and by the time we get to run again, the system clock
457 * reads 6:07:11.990. We now want to wait until the next xx:xx:xx.750
458 * time, which is 6:07:12.750 (4.5 seconds after the reference time),
459 * at which point we will set the Hardware Clock to 1:02:07 (4 seconds
460 * after the originally requested time). If we do that successfully,
461 * then at 6:07:13.250 (5 seconds after the reference time), the
462 * Hardware Clock will update to 1:02:08 (5 seconds after the
463 * originally requested time), and all is well thereafter.
466 time_t newhwtime
= sethwtime
;
467 double target_time_tolerance_secs
= 0.001; /* initial value */
468 double tolerance_incr_secs
= 0.001; /* initial value */
469 const double RTC_SET_DELAY_SECS
= 0.5; /* 500 ms */
470 const struct timeval RTC_SET_DELAY_TV
= { 0, RTC_SET_DELAY_SECS
* 1E6
};
472 struct timeval targetsystime
;
473 struct timeval nowsystime
;
474 struct timeval prevsystime
= refsystime
;
475 double deltavstarget
;
477 timeradd(&refsystime
, &RTC_SET_DELAY_TV
, &targetsystime
);
482 /* FOR TESTING ONLY: inject random delays of up to 1000ms */
483 if (ctl
->debug
>= 10) {
484 int usec
= random() % 1000000;
485 printf(_("sleeping ~%d usec\n"), usec
);
489 gettimeofday(&nowsystime
, NULL
);
490 deltavstarget
= time_diff(nowsystime
, targetsystime
);
491 ticksize
= time_diff(nowsystime
, prevsystime
);
492 prevsystime
= nowsystime
;
496 printf(_("time jumped backward %.6f seconds "
497 "to %ld.%06ld - retargeting\n"),
498 ticksize
, nowsystime
.tv_sec
,
500 /* The retarget is handled at the end of the loop. */
501 } else if (deltavstarget
< 0) {
502 /* deltavstarget < 0 if current time < target time */
504 printf(_("%ld.%06ld < %ld.%06ld (%.6f)\n"),
507 targetsystime
.tv_sec
,
508 targetsystime
.tv_usec
,
510 continue; /* not there yet - keep spinning */
511 } else if (deltavstarget
<= target_time_tolerance_secs
) {
512 /* Close enough to the target time; done waiting. */
514 } else /* (deltavstarget > target_time_tolerance_secs) */ {
516 * We missed our window. Increase the tolerance and
517 * aim for the next opportunity.
520 printf(_("missed it - %ld.%06ld is too far "
521 "past %ld.%06ld (%.6f > %.6f)\n"),
524 targetsystime
.tv_sec
,
525 targetsystime
.tv_usec
,
527 target_time_tolerance_secs
);
528 target_time_tolerance_secs
+= tolerance_incr_secs
;
529 tolerance_incr_secs
*= 2;
533 * Aim for the same offset (tv_usec) within the second in
534 * either the current second (if that offset hasn't arrived
535 * yet), or the next second.
537 if (nowsystime
.tv_usec
< targetsystime
.tv_usec
)
538 targetsystime
.tv_sec
= nowsystime
.tv_sec
;
540 targetsystime
.tv_sec
= nowsystime
.tv_sec
+ 1;
543 newhwtime
= sethwtime
544 + (int)(time_diff(nowsystime
, refsystime
)
545 - RTC_SET_DELAY_SECS
/* don't count this */
546 + 0.5 /* for rounding */);
548 printf(_("%ld.%06ld is close enough to %ld.%06ld (%.6f < %.6f)\n"
549 "Set RTC to %ld (%ld + %d; refsystime = %ld.%06ld)\n"),
550 nowsystime
.tv_sec
, nowsystime
.tv_usec
,
551 targetsystime
.tv_sec
, targetsystime
.tv_usec
,
552 deltavstarget
, target_time_tolerance_secs
,
553 newhwtime
, sethwtime
,
554 (int)(newhwtime
- sethwtime
),
555 refsystime
.tv_sec
, refsystime
.tv_usec
);
557 set_hardware_clock(ctl
, newhwtime
);
561 * Put the time "hwctime" on standard output in display format. Except if
562 * hclock_valid == false, just tell standard output that we don't know what
566 display_time(const bool hclock_valid
, struct timeval hwctime
)
570 ("The Hardware Clock registers contain values that are "
571 "either invalid (e.g. 50th day of month) or beyond the range "
572 "we can handle (e.g. Year 2095)."));
574 char buf
[ISO_8601_BUFSIZ
];
576 strtimeval_iso(&hwctime
, ISO_8601_DATE
|ISO_8601_TIME
|ISO_8601_DOTUSEC
|
577 ISO_8601_TIMEZONE
|ISO_8601_SPACE
,
584 * Set the System Clock to time 'newtime'.
586 * Also set the kernel time zone value to the value indicated by the TZ
587 * environment variable and/or /usr/lib/zoneinfo/, interpreted as tzset()
588 * would interpret them.
590 * If this is the first call of settimeofday since boot, then this also sets
591 * the kernel variable persistent_clock_is_local so that NTP 11 minute mode
592 * will update the Hardware Clock with the proper timescale. If the Hardware
593 * Clock's timescale configuration is changed then a reboot is required for
594 * persistent_clock_is_local to be updated.
596 * EXCEPT: if hclock_valid is false, just issue an error message saying
597 * there is no valid time in the Hardware Clock to which to set the system
600 * If 'testing' is true, don't actually update anything -- just say we would
604 set_system_clock(const struct hwclock_control
*ctl
, const bool hclock_valid
,
605 const struct timeval newtime
)
611 ("The Hardware Clock does not contain a valid time, so "
612 "we cannot set the System Time from it."));
615 const struct timeval
*tv_null
= NULL
;
620 broken
= localtime(&newtime
.tv_sec
);
621 #ifdef HAVE_TM_GMTOFF
622 minuteswest
= -broken
->tm_gmtoff
/ 60; /* GNU extension */
624 minuteswest
= timezone
/ 60;
625 if (broken
->tm_isdst
)
630 printf(_("Calling settimeofday:\n"));
631 printf(_("\ttv.tv_sec = %ld, tv.tv_usec = %ld\n"),
632 newtime
.tv_sec
, newtime
.tv_usec
);
633 printf(_("\ttz.tz_minuteswest = %d\n"), minuteswest
);
637 ("Test mode: clock was not changed\n"));
640 const struct timezone tz
= { minuteswest
, 0 };
642 /* Set kernel persistent_clock_is_local so that 11 minute
643 * mode does not clobber the Hardware Clock with UTC. This
644 * is only available on first call of settimeofday after boot.
647 rc
= settimeofday(tv_null
, &tz
);
649 rc
= settimeofday(&newtime
, &tz
);
651 if (errno
== EPERM
) {
653 ("Must be superuser to set system clock."));
656 warn(_("settimeofday() failed"));
667 * Reset the System Clock from local time to UTC, based on its current value
668 * and the timezone unless universal is TRUE.
670 * Also set the kernel time zone value to the value indicated by the TZ
671 * environment variable and/or /usr/lib/zoneinfo/, interpreted as tzset()
672 * would interpret them.
674 * If 'testing' is true, don't actually update anything -- just say we would
677 static int set_system_clock_timezone(const struct hwclock_control
*ctl
)
684 gettimeofday(&tv
, NULL
);
686 struct tm broken_time
;
689 broken_time
= *gmtime(&tv
.tv_sec
);
690 strftime(ctime_now
, sizeof(ctime_now
), "%Y/%m/%d %H:%M:%S",
692 printf(_("Current system time: %ld = %s\n"), tv
.tv_sec
,
696 broken
= localtime(&tv
.tv_sec
);
697 #ifdef HAVE_TM_GMTOFF
698 minuteswest
= -broken
->tm_gmtoff
/ 60; /* GNU extension */
700 minuteswest
= timezone
/ 60;
701 if (broken
->tm_isdst
)
706 struct tm broken_time
;
709 gettimeofday(&tv
, NULL
);
711 tv
.tv_sec
+= minuteswest
* 60;
713 broken_time
= *gmtime(&tv
.tv_sec
);
714 strftime(ctime_now
, sizeof(ctime_now
), "%Y/%m/%d %H:%M:%S",
717 printf(_("Calling settimeofday:\n"));
718 printf(_("\tUTC: %s\n"), ctime_now
);
719 printf(_("\ttv.tv_sec = %ld, tv.tv_usec = %ld\n"),
720 tv
.tv_sec
, tv
.tv_usec
);
721 printf(_("\ttz.tz_minuteswest = %d\n"), minuteswest
);
725 ("Test mode: clock was not changed\n"));
728 const struct timezone tz_utc
= { 0, 0 };
729 const struct timezone tz
= { minuteswest
, 0 };
730 const struct timeval
*tv_null
= NULL
;
733 /* The first call to settimeofday after boot will assume the systemtime
734 * is in localtime, and adjust it according to the given timezone to
735 * compensate. If the systemtime is in fact in UTC, then this is wrong
736 * so we first do a dummy call to make sure the time is not shifted.
739 rc
= settimeofday(tv_null
, &tz_utc
);
741 /* Now we set the real timezone. Due to the above dummy call, this will
742 * only warp the systemtime if the RTC is not in UTC. */
744 rc
= settimeofday(tv_null
, &tz
);
747 if (errno
== EPERM
) {
749 ("Must be superuser to set system clock."));
752 warn(_("settimeofday() failed"));
762 * Refresh the last calibrated and last adjusted timestamps in <*adjtime_p>
763 * to facilitate future drift calculations based on this set point.
765 * With the --update-drift option:
766 * Update the drift factor in <*adjtime_p> based on the fact that the
767 * Hardware Clock was just calibrated to <nowtime> and before that was
768 * set to the <hclocktime> time scale.
770 * EXCEPT: if <hclock_valid> is false, assume Hardware Clock was not set
771 * before to anything meaningful and regular adjustments have not been done,
772 * so don't adjust the drift factor.
775 adjust_drift_factor(const struct hwclock_control
*ctl
,
776 struct adjtime
*adjtime_p
,
777 const struct timeval nowtime
,
778 const bool hclock_valid
,
779 const struct timeval hclocktime
)
783 printf(_("Not adjusting drift factor because the "
784 "--update-drift option was not used.\n"));
785 } else if (!hclock_valid
) {
787 printf(_("Not adjusting drift factor because the "
788 "Hardware Clock previously contained "
790 } else if (adjtime_p
->last_calib_time
== 0) {
792 printf(_("Not adjusting drift factor because last "
793 "calibration time is zero,\n"
794 "so history is bad and calibration startover "
796 } else if ((hclocktime
.tv_sec
- adjtime_p
->last_calib_time
) < 4 * 60 * 60) {
798 printf(_("Not adjusting drift factor because it has "
799 "been less than four hours since the last "
803 * At adjustment time we drift correct the hardware clock
804 * according to the contents of the adjtime file and refresh
805 * its last adjusted timestamp.
807 * At calibration time we set the Hardware Clock and refresh
808 * both timestamps in <*adjtime_p>.
810 * Here, with the --update-drift option, we also update the
811 * drift factor in <*adjtime_p>.
813 * Let us do computation in doubles. (Floats almost suffice,
814 * but 195 days + 1 second equals 195 days in floats.)
816 const double sec_per_day
= 24.0 * 60.0 * 60.0;
817 double factor_adjust
;
819 struct timeval last_calib
;
821 last_calib
= t2tv(adjtime_p
->last_calib_time
);
823 * Correction to apply to the current drift factor.
825 * Simplified: uncorrected_drift / days_since_calibration.
827 * hclocktime is fully corrected with the current drift factor.
828 * Its difference from nowtime is the missed drift correction.
830 factor_adjust
= time_diff(nowtime
, hclocktime
) /
831 (time_diff(nowtime
, last_calib
) / sec_per_day
);
833 drift_factor
= adjtime_p
->drift_factor
+ factor_adjust
;
834 if (fabs(drift_factor
) > MAX_DRIFT
) {
836 printf(_("Clock drift factor was calculated as "
838 "It is far too much. Resetting to zero.\n"),
843 printf(_("Clock drifted %f seconds in the past "
844 "%f seconds\nin spite of a drift factor of "
846 "Adjusting drift factor by %f seconds/day\n"),
847 time_diff(nowtime
, hclocktime
),
848 time_diff(nowtime
, last_calib
),
849 adjtime_p
->drift_factor
, factor_adjust
);
852 adjtime_p
->drift_factor
= drift_factor
;
854 adjtime_p
->last_calib_time
= nowtime
.tv_sec
;
856 adjtime_p
->last_adj_time
= nowtime
.tv_sec
;
858 adjtime_p
->not_adjusted
= 0;
860 adjtime_p
->dirty
= TRUE
;
864 * Calculate the drift correction currently needed for the
865 * Hardware Clock based on the last time it was adjusted,
866 * and the current drift factor, as stored in the adjtime file.
868 * The total drift adjustment needed is stored at tdrift_p.
872 calculate_adjustment(const struct hwclock_control
*ctl
,
874 const time_t last_time
,
875 const double not_adjusted
,
876 const time_t systime
, struct timeval
*tdrift_p
)
878 double exact_adjustment
;
881 ((double)(systime
- last_time
)) * factor
/ (24 * 60 * 60)
883 tdrift_p
->tv_sec
= (time_t) floor(exact_adjustment
);
884 tdrift_p
->tv_usec
= (exact_adjustment
-
885 (double)tdrift_p
->tv_sec
) * 1E6
;
887 printf(P_("Time since last adjustment is %ld second\n",
888 "Time since last adjustment is %ld seconds\n",
889 (systime
- last_time
)),
890 (systime
- last_time
));
891 printf(_("Calculated Hardware Clock drift is %ld.%06ld seconds\n"),
892 tdrift_p
->tv_sec
, tdrift_p
->tv_usec
);
897 * Write the contents of the <adjtime> structure to its disk file.
899 * But if the contents are clean (unchanged since read from disk), don't
902 static void save_adjtime(const struct hwclock_control
*ctl
,
903 const struct adjtime
*adjtime
)
905 char *content
; /* Stuff to write to disk file */
912 xasprintf(&content
, "%f %ld %f\n%ld\n%s\n",
913 adjtime
->drift_factor
,
914 adjtime
->last_adj_time
,
915 adjtime
->not_adjusted
,
916 adjtime
->last_calib_time
,
917 (adjtime
->local_utc
== LOCAL
) ? "LOCAL" : "UTC");
921 printf(_("Test mode: %s was not updated with:\n%s"),
922 ctl
->adj_file_name
, content
);
928 fp
= fopen(ctl
->adj_file_name
, "w");
930 warn(_("Could not open file with the clock adjustment parameters "
931 "in it (%s) for writing"), ctl
->adj_file_name
);
933 } else if (fputs(content
, fp
) < 0 || close_stream(fp
) != 0) {
934 warn(_("Could not update file with the clock adjustment "
935 "parameters (%s) in it"), ctl
->adj_file_name
);
940 warnx(_("Drift adjustment parameters not updated."));
944 * Do the adjustment requested, by 1) setting the Hardware Clock (if
945 * necessary), and 2) updating the last-adjusted time in the adjtime
948 * Do not update anything if the Hardware Clock does not currently present a
951 * <hclock_valid> means the Hardware Clock contains a valid time.
953 * <hclocktime> is the drift corrected time read from the Hardware Clock.
955 * <read_time> was the system time when the <hclocktime> was read, which due
956 * to computational delay could be a short time ago. It is used to define a
957 * trigger point for setting the Hardware Clock. The fractional part of the
958 * Hardware clock set time is subtracted from read_time to 'refer back', or
959 * delay, the trigger point. Fractional parts must be accounted for in this
960 * way, because the Hardware Clock can only be set to a whole second.
962 * <universal>: the Hardware Clock is kept in UTC.
964 * <testing>: We are running in test mode (no updating of clock).
968 do_adjustment(const struct hwclock_control
*ctl
, struct adjtime
*adjtime_p
,
969 const bool hclock_valid
, const struct timeval hclocktime
,
970 const struct timeval read_time
)
973 warnx(_("The Hardware Clock does not contain a valid time, "
974 "so we cannot adjust it."));
975 adjtime_p
->last_calib_time
= 0; /* calibration startover is required */
976 adjtime_p
->last_adj_time
= 0;
977 adjtime_p
->not_adjusted
= 0;
978 adjtime_p
->dirty
= TRUE
;
979 } else if (adjtime_p
->last_adj_time
== 0) {
981 printf(_("Not setting clock because last adjustment time is zero, "
982 "so history is bad.\n"));
983 } else if (fabs(adjtime_p
->drift_factor
) > MAX_DRIFT
) {
985 printf(_("Not setting clock because drift factor %f is far too high.\n"),
986 adjtime_p
->drift_factor
);
988 set_hardware_clock_exact(ctl
, hclocktime
.tv_sec
,
990 -(hclocktime
.tv_usec
/ 1E6
)));
991 adjtime_p
->last_adj_time
= hclocktime
.tv_sec
;
992 adjtime_p
->not_adjusted
= 0;
993 adjtime_p
->dirty
= TRUE
;
997 static void determine_clock_access_method(const struct hwclock_control
*ctl
)
1002 ur
= probe_for_cmos_clock();
1005 ur
= probe_for_rtc_clock(ctl
);
1009 puts(ur
->interface_name
);
1013 printf(_("No usable clock interface found.\n"));
1014 warnx(_("Cannot access the Hardware Clock via "
1015 "any known method."));
1017 warnx(_("Use the --debug option to see the "
1018 "details of our search for an access "
1020 hwclock_exit(ctl
, EX_SOFTWARE
);
1025 * Do all the normal work of hwclock - read, set clock, etc.
1027 * Issue output to stdout and error message to stderr where appropriate.
1029 * Return rc == 0 if everything went OK, rc != 0 if not.
1032 manipulate_clock(const struct hwclock_control
*ctl
, const time_t set_time
,
1033 const struct timeval startup_time
, struct adjtime
*adjtime
)
1035 /* The time at which we read the Hardware Clock */
1036 struct timeval read_time
;
1038 * The Hardware Clock gives us a valid time, or at
1039 * least something close enough to fool mktime().
1041 bool hclock_valid
= FALSE
;
1043 * Tick synchronized time read from the Hardware Clock and
1044 * then drift correct for all operations except --show.
1046 struct timeval hclocktime
= { 0, 0 };
1047 /* Total Hardware Clock drift correction needed. */
1048 struct timeval tdrift
;
1049 /* local return code */
1052 if (!ctl
->systz
&& !ctl
->predict
&& ur
->get_permissions())
1055 if ((ctl
->set
|| ctl
->systohc
|| ctl
->adjust
) &&
1056 (adjtime
->local_utc
== UTC
) != ctl
->universal
) {
1057 adjtime
->local_utc
= ctl
->universal
? UTC
: LOCAL
;
1058 adjtime
->dirty
= TRUE
;
1061 if (ctl
->show
|| ctl
->get
|| ctl
->adjust
|| ctl
->hctosys
1062 || (!ctl
->noadjfile
&& !ctl
->systz
&& !ctl
->predict
)) {
1063 /* data from HW-clock are required */
1064 rc
= synchronize_to_clock_tick(ctl
);
1067 * We don't error out if the user is attempting to set the
1068 * RTC and synchronization timeout happens - the RTC could
1069 * be functioning but contain invalid time data so we still
1070 * want to allow a user to set the RTC time.
1072 if (rc
== RTC_BUSYWAIT_FAILED
&& !ctl
->set
&& !ctl
->systohc
)
1074 gettimeofday(&read_time
, NULL
);
1077 * If we can't synchronize to a clock tick,
1078 * we likely can't read from the RTC so
1079 * don't bother reading it again.
1082 rc
= read_hardware_clock(ctl
, &hclock_valid
,
1083 &hclocktime
.tv_sec
);
1084 if (rc
&& !ctl
->set
&& !ctl
->systohc
)
1089 * Calculate Hardware Clock drift for --predict with the user
1090 * supplied --date option time, and with the time read from the
1091 * Hardware Clock for all other operations. Apply drift correction
1092 * to the Hardware Clock time for everything except --show and
1093 * --predict. For --predict negate the drift correction, because we
1094 * want to 'predict' a future Hardware Clock time that includes drift.
1096 hclocktime
= ctl
->predict
? t2tv(set_time
) : hclocktime
;
1097 calculate_adjustment(ctl
, adjtime
->drift_factor
,
1098 adjtime
->last_adj_time
,
1099 adjtime
->not_adjusted
,
1100 hclocktime
.tv_sec
, &tdrift
);
1101 if (!ctl
->show
&& !ctl
->predict
)
1102 hclocktime
= time_inc(tdrift
, hclocktime
.tv_sec
);
1103 if (ctl
->show
|| ctl
->get
) {
1104 display_time(hclock_valid
,
1105 time_inc(hclocktime
, -time_diff
1106 (read_time
, startup_time
)));
1107 } else if (ctl
->set
) {
1108 set_hardware_clock_exact(ctl
, set_time
, startup_time
);
1109 if (!ctl
->noadjfile
)
1110 adjust_drift_factor(ctl
, adjtime
,
1111 time_inc(t2tv(set_time
), time_diff
1112 (read_time
, startup_time
)),
1113 hclock_valid
, hclocktime
);
1114 } else if (ctl
->adjust
) {
1115 if (tdrift
.tv_sec
> 0 || tdrift
.tv_sec
< -1)
1116 do_adjustment(ctl
, adjtime
, hclock_valid
,
1117 hclocktime
, read_time
);
1119 printf(_("Needed adjustment is less than one second, "
1120 "so not setting clock.\n"));
1121 } else if (ctl
->systohc
) {
1122 struct timeval nowtime
, reftime
;
1124 * We can only set_hardware_clock_exact to a
1125 * whole seconds time, so we set it with
1126 * reference to the most recent whole
1129 gettimeofday(&nowtime
, NULL
);
1130 reftime
.tv_sec
= nowtime
.tv_sec
;
1131 reftime
.tv_usec
= 0;
1132 set_hardware_clock_exact(ctl
, (time_t) reftime
.tv_sec
, reftime
);
1133 if (!ctl
->noadjfile
)
1134 adjust_drift_factor(ctl
, adjtime
, nowtime
,
1135 hclock_valid
, hclocktime
);
1136 } else if (ctl
->hctosys
) {
1137 rc
= set_system_clock(ctl
, hclock_valid
, hclocktime
);
1139 printf(_("Unable to set system clock.\n"));
1142 } else if (ctl
->systz
) {
1143 rc
= set_system_clock_timezone(ctl
);
1145 printf(_("Unable to set system clock.\n"));
1148 } else if (ctl
->predict
) {
1149 hclocktime
= time_inc(hclocktime
, (double)
1150 -(tdrift
.tv_sec
+ tdrift
.tv_usec
/ 1E6
));
1153 ("At %ld seconds after 1969, RTC is predicted to read %ld seconds after 1969.\n"),
1154 set_time
, hclocktime
.tv_sec
);
1156 display_time(TRUE
, hclocktime
);
1158 if (!ctl
->noadjfile
)
1159 save_adjtime(ctl
, adjtime
);
1164 * Get or set the kernel RTC driver's epoch on Alpha machines.
1165 * ISA machines are hard coded for 1900.
1167 #if defined(__linux__) && defined(__alpha__)
1169 manipulate_epoch(const struct hwclock_control
*ctl
)
1171 if (ctl
->getepoch
) {
1172 unsigned long epoch
;
1174 if (get_epoch_rtc(ctl
, &epoch
))
1176 ("Unable to get the epoch value from the kernel."));
1178 printf(_("Kernel is assuming an epoch value of %lu\n"),
1180 } else if (ctl
->setepoch
) {
1181 if (ctl
->epoch_option
== 0)
1183 ("To set the epoch value, you must use the 'epoch' "
1184 "option to tell to what value to set it."));
1185 else if (ctl
->testing
)
1187 ("Not setting the epoch to %lu - testing only.\n"),
1189 else if (set_epoch_rtc(ctl
))
1191 ("Unable to set the epoch value in the kernel.\n"));
1194 #endif /* __linux__ __alpha__ */
1196 static void out_version(void)
1198 printf(UTIL_LINUX_VERSION
);
1201 static void __attribute__((__noreturn__
))
1202 usage(const struct hwclock_control
*ctl
, FILE *out
)
1204 fputs(USAGE_HEADER
, out
);
1205 fputs(_(" hwclock [function] [option...]\n"), out
);
1207 fputs(USAGE_SEPARATOR
, out
);
1208 fputs(_("Query or set the RTC (Real Time Clock / Hardware Clock)\n"), out
);
1210 fputs(USAGE_FUNCTIONS
, out
);
1211 fputs(_(" -r, --show display the RTC time\n"), out
);
1212 fputs(_(" --get display drift corrected RTC time\n"), out
);
1213 fputs(_(" --set set the RTC according to --date\n"), out
);
1214 fputs(_(" -s, --hctosys set the system time from the RTC\n"), out
);
1215 fputs(_(" -w, --systohc set the RTC from the system time\n"), out
);
1216 fputs(_(" --systz send timescale configurations to the kernel\n"), out
);
1217 fputs(_(" --adjust adjust the RTC to account for systematic drift\n"), out
);
1218 #if defined(__linux__) && defined(__alpha__)
1219 fputs(_(" --getepoch display the RTC epoch\n"), out
);
1220 fputs(_(" --setepoch set the RTC epoch according to --epoch\n"), out
);
1222 fputs(_(" --predict predict the drifted RTC time according to --date\n"), out
);
1223 fputs(USAGE_OPTIONS
, out
);
1224 fputs(_(" -u, --utc inform hwclock the RTC timescale is UTC\n"), out
);
1225 fputs(_(" -l, --localtime inform hwclock the RTC timescale is Local\n"), out
);
1228 " -f, --rtc <file> use an alternate file to %1$s\n"
1230 " --directisa use the ISA bus instead of %1$s access\n"), _PATH_RTC_DEV
);
1231 fputs(_(" --date <time> date/time input for --set and --predict\n"), out
);
1232 #if defined(__linux__) && defined(__alpha__)
1233 fputs(_(" --epoch <year> epoch input for --setepoch\n"), out
);
1235 fputs(_(" --update-drift update the RTC drift factor\n"), out
);
1237 " --noadjfile do not use %1$s\n"
1238 " --adjfile <file> use an alternate file to %1$s\n"), _PATH_ADJTIME
);
1239 fputs(_(" --test dry run; use -D to view what would have happened\n"), out
);
1240 fputs(_(" -D, --debug use debug mode\n"), out
);
1241 fputs(USAGE_SEPARATOR
, out
);
1242 fputs(USAGE_HELP
, out
);
1243 fputs(USAGE_VERSION
, out
);
1244 fprintf(out
, USAGE_MAN_TAIL("hwclock(8)"));
1245 hwclock_exit(ctl
, EXIT_SUCCESS
);
1250 * EX_USAGE: bad invocation
1251 * EX_NOPERM: no permission
1252 * EX_OSFILE: cannot open /dev/rtc or /etc/adjtime
1253 * EX_IOERR: ioctl error getting or setting the time
1257 int main(int argc
, char **argv
)
1259 struct hwclock_control ctl
= { .show
= 1 }; /* default op is show */
1260 struct timeval startup_time
;
1261 struct adjtime adjtime
= { 0 };
1262 struct timespec when
= { 0 };
1264 * The time we started up, in seconds into the epoch, including
1267 time_t set_time
= 0; /* Time to which user said to set Hardware Clock */
1270 /* Long only options. */
1272 OPT_ADJFILE
= CHAR_MAX
+ 1,
1287 static const struct option longopts
[] = {
1288 { "adjust", no_argument
, NULL
, 'a' },
1289 { "help", no_argument
, NULL
, 'h' },
1290 { "localtime", no_argument
, NULL
, 'l' },
1291 { "show", no_argument
, NULL
, 'r' },
1292 { "hctosys", no_argument
, NULL
, 's' },
1293 { "utc", no_argument
, NULL
, 'u' },
1294 { "version", no_argument
, NULL
, 'v' },
1295 { "systohc", no_argument
, NULL
, 'w' },
1296 { "debug", no_argument
, NULL
, 'D' },
1297 { "set", no_argument
, NULL
, OPT_SET
},
1298 #if defined(__linux__) && defined(__alpha__)
1299 { "getepoch", no_argument
, NULL
, OPT_GETEPOCH
},
1300 { "setepoch", no_argument
, NULL
, OPT_SETEPOCH
},
1301 { "epoch", required_argument
, NULL
, OPT_EPOCH
},
1303 { "noadjfile", no_argument
, NULL
, OPT_NOADJFILE
},
1304 { "directisa", no_argument
, NULL
, OPT_DIRECTISA
},
1305 { "test", no_argument
, NULL
, OPT_TEST
},
1306 { "date", required_argument
, NULL
, OPT_DATE
},
1308 { "rtc", required_argument
, NULL
, 'f' },
1310 { "adjfile", required_argument
, NULL
, OPT_ADJFILE
},
1311 { "systz", no_argument
, NULL
, OPT_SYSTZ
},
1312 { "predict", no_argument
, NULL
, OPT_PREDICT
},
1313 { "get", no_argument
, NULL
, OPT_GET
},
1314 { "update-drift", no_argument
, NULL
, OPT_UPDATE
},
1315 { NULL
, 0, NULL
, 0 }
1318 static const ul_excl_t excl
[] = { /* rows and cols in ASCII order */
1320 OPT_GET
, OPT_GETEPOCH
, OPT_PREDICT
,
1321 OPT_SET
, OPT_SETEPOCH
, OPT_SYSTZ
},
1323 { OPT_ADJFILE
, OPT_NOADJFILE
},
1324 { OPT_NOADJFILE
, OPT_UPDATE
},
1327 int excl_st
[ARRAY_SIZE(excl
)] = UL_EXCL_STATUS_INIT
;
1329 /* Remember what time we were invoked */
1330 gettimeofday(&startup_time
, NULL
);
1332 #ifdef HAVE_LIBAUDIT
1333 hwaudit_fd
= audit_open();
1334 if (hwaudit_fd
< 0 && !(errno
== EINVAL
|| errno
== EPROTONOSUPPORT
||
1335 errno
== EAFNOSUPPORT
)) {
1337 * You get these error codes only when the kernel doesn't
1338 * have audit compiled in.
1340 warnx(_("Unable to connect to audit system"));
1344 setlocale(LC_ALL
, "");
1347 * We need LC_CTYPE and LC_TIME and LC_MESSAGES, but must avoid
1348 * LC_NUMERIC since it gives problems when we write to /etc/adjtime.
1349 * - gqueri@mail.dotcom.fr
1351 setlocale(LC_NUMERIC
, "C");
1353 bindtextdomain(PACKAGE
, LOCALEDIR
);
1354 textdomain(PACKAGE
);
1355 atexit(close_stdout
);
1357 while ((c
= getopt_long(argc
, argv
,
1358 "hvVDalrsuwf:", longopts
, NULL
)) != -1) {
1360 err_exclusive_options(c
, longopts
, excl
, excl_st
);
1372 ctl
.local_opt
= 1; /* --localtime */
1395 #if defined(__linux__) && defined(__alpha__)
1406 ctl
.epoch_option
= /* --epoch */
1407 strtoul_or_err(optarg
, _("invalid epoch argument"));
1417 ctl
.testing
= 1; /* --test */
1420 ctl
.date_opt
= optarg
; /* --date */
1423 ctl
.adj_file_name
= optarg
; /* --adjfile */
1426 ctl
.systz
= 1; /* --systz */
1431 ctl
.predict
= 1; /* --predict */
1435 ctl
.get
= 1; /* --get */
1439 ctl
.update
= 1; /* --update-drift */
1443 ctl
.rtc_dev_name
= optarg
; /* --rtc */
1446 case 'v': /* --version */
1450 case 'h': /* --help */
1451 usage(&ctl
, stdout
);
1453 errtryhelp(EXIT_FAILURE
);
1461 warnx(_("%d too many arguments given"), argc
);
1462 errtryhelp(EXIT_FAILURE
);
1465 if (!ctl
.adj_file_name
)
1466 ctl
.adj_file_name
= _PATH_ADJTIME
;
1468 if (ctl
.update
&& !ctl
.set
&& !ctl
.systohc
) {
1469 warnx(_("--update-drift requires --set or --systohc"));
1470 hwclock_exit(&ctl
, EX_USAGE
);
1473 if (ctl
.noadjfile
&& !ctl
.utc
&& !ctl
.local_opt
) {
1474 warnx(_("With --noadjfile, you must specify "
1475 "either --utc or --localtime"));
1476 hwclock_exit(&ctl
, EX_USAGE
);
1479 if (ctl
.set
|| ctl
.predict
) {
1481 warnx(_("--date is required for --set or --predict"));
1482 hwclock_exit(&ctl
, EX_USAGE
);
1484 if (parse_date(&when
, ctl
.date_opt
, NULL
))
1485 set_time
= when
.tv_sec
;
1487 warnx(_("invalid date '%s'"), ctl
.date_opt
);
1488 hwclock_exit(&ctl
, EX_USAGE
);
1492 #if defined(__linux__) && defined(__alpha__)
1493 if (ctl
.getepoch
|| ctl
.setepoch
) {
1494 manipulate_epoch(&ctl
);
1495 hwclock_exit(&ctl
, EX_OK
);
1502 if (!ctl
.systz
&& !ctl
.predict
)
1503 determine_clock_access_method(&ctl
);
1505 if (!ctl
.noadjfile
&& !(ctl
.systz
&& (ctl
.utc
|| ctl
.local_opt
))) {
1506 if ((rc
= read_adjtime(&ctl
, &adjtime
)) != 0)
1507 hwclock_exit(&ctl
, rc
);
1509 /* Avoid writing adjtime file if we don't have to. */
1510 adjtime
.dirty
= FALSE
;
1511 ctl
.universal
= hw_clock_is_utc(&ctl
, adjtime
);
1512 rc
= manipulate_clock(&ctl
, set_time
, startup_time
, &adjtime
);
1513 hwclock_exit(&ctl
, rc
);
1514 return rc
; /* Not reached */
1518 hwclock_exit(const struct hwclock_control
*ctl
1519 #ifndef HAVE_LIBAUDIT
1520 __attribute__((__unused__
))
1524 #ifdef HAVE_LIBAUDIT
1525 if (ctl
->hwaudit_on
&& !ctl
->testing
) {
1526 audit_log_user_message(hwaudit_fd
, AUDIT_USYS_CONFIG
,
1527 "op=change-system-time", NULL
, NULL
, NULL
,
1536 * History of this program:
1538 * 98.08.12 BJH Version 2.4
1540 * Don't use century byte from Hardware Clock. Add comments telling why.
1542 * 98.06.20 BJH Version 2.3.
1544 * Make --hctosys set the kernel timezone from TZ environment variable
1545 * and/or /usr/lib/zoneinfo. From Klaus Ripke (klaus@ripke.com).
1547 * 98.03.05 BJH. Version 2.2.
1549 * Add --getepoch and --setepoch.
1551 * Fix some word length things so it works on Alpha.
1553 * Make it work when /dev/rtc doesn't have the interrupt functions. In this
1554 * case, busywait for the top of a second instead of blocking and waiting
1555 * for the update complete interrupt.
1557 * Fix a bunch of bugs too numerous to mention.
1559 * 97.06.01: BJH. Version 2.1. Read and write the century byte (Byte 50) of
1560 * the ISA Hardware Clock when using direct ISA I/O. Problem discovered by
1561 * job (jei@iclnl.icl.nl).
1563 * Use the rtc clock access method in preference to the KDGHWCLK method.
1564 * Problem discovered by Andreas Schwab <schwab@LS5.informatik.uni-dortmund.de>.
1566 * November 1996: Version 2.0.1. Modifications by Nicolai Langfeldt
1567 * (janl@math.uio.no) to make it compile on linux 1.2 machines as well as
1568 * more recent versions of the kernel. Introduced the NO_CLOCK access method
1569 * and wrote feature test code to detect absence of rtc headers.
1571 ***************************************************************************
1574 * To compile this, you must use GNU compiler optimization (-O option) in
1575 * order to make the "extern inline" functions from asm/io.h (inb(), etc.)
1576 * compile. If you don't optimize, which means the compiler will generate no
1577 * inline functions, the references to these functions in this program will
1578 * be compiled as external references. Since you probably won't be linking
1579 * with any functions by these names, you will have unresolved external
1580 * references when you link.
1582 * Here's some info on how we must deal with the time that elapses while
1583 * this program runs: There are two major delays as we run:
1585 * 1) Waiting up to 1 second for a transition of the Hardware Clock so
1586 * we are synchronized to the Hardware Clock.
1587 * 2) Running the "date" program to interpret the value of our --date
1590 * Reading the /etc/adjtime file is the next biggest source of delay and
1593 * The user wants to know what time it was at the moment he invoked us, not
1594 * some arbitrary time later. And in setting the clock, he is giving us the
1595 * time at the moment we are invoked, so if we set the clock some time
1596 * later, we have to add some time to that.
1598 * So we check the system time as soon as we start up, then run "date" and
1599 * do file I/O if necessary, then wait to synchronize with a Hardware Clock
1600 * edge, then check the system time again to see how much time we spent. We
1601 * immediately read the clock then and (if appropriate) report that time,
1602 * and additionally, the delay we measured.
1604 * If we're setting the clock to a time given by the user, we wait some more
1605 * so that the total delay is an integral number of seconds, then set the
1606 * Hardware Clock to the time the user requested plus that integral number
1607 * of seconds. N.B. The Hardware Clock can only be set in integral seconds.
1609 * If we're setting the clock to the system clock value, we wait for the
1610 * system clock to reach the top of a second, and then set the Hardware
1611 * Clock to the system clock's value.
1613 * Here's an interesting point about setting the Hardware Clock: On my
1614 * machine, when you set it, it sets to that precise time. But one can
1615 * imagine another clock whose update oscillator marches on a steady one
1616 * second period, so updating the clock between any two oscillator ticks is
1617 * the same as updating it right at the earlier tick. To avoid any
1618 * complications that might cause, we set the clock as soon as possible
1619 * after an oscillator tick.
1621 * About synchronizing to the Hardware Clock when reading the time: The
1622 * precision of the Hardware Clock counters themselves is one second. You
1623 * can't read the counters and find out that is 12:01:02.5. But if you
1624 * consider the location in time of the counter's ticks as part of its
1625 * value, then its precision is as infinite as time is continuous! What I'm
1626 * saying is this: To find out the _exact_ time in the hardware clock, we
1627 * wait until the next clock tick (the next time the second counter changes)
1628 * and measure how long we had to wait. We then read the value of the clock
1629 * counters and subtract the wait time and we know precisely what time it
1630 * was when we set out to query the time.
1632 * hwclock uses this method, and considers the Hardware Clock to have
1633 * infinite precision.