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mktime: guess better near transitions where tm_isdst does not change

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* localtime.c (SMALLEST): New macro.
(time2sub) [TM_GMTOFF && !UNINIT_TRAP]:
If the UTC offset doesn't match the request, try the requested offset.
This catches a problem caught by -DTYPECHECK with a time stamp
near a transition from LMT to standard time, where both sides of
the transition have tm_isdst == 0.  If !defined TM_GMTOFF ||
UNINIT_TRAP you're out of luck: mktime will still conform
to its spec but it'll be more likely to guess wrong on these
ambiguous inputs.
* private.h (UNINIT_TRAP): New macro that defaults to 0.
* Makefile, NEWS: Document this.
This commit is contained in:
Paul Eggert
2014-08-24 11:14:18 -07:00
parent 7c681a8148
commit da184ab535
4 changed files with 44 additions and 0 deletions
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2
Makefile
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@ -134,6 +134,8 @@ LDLIBS=
# the default is system-supplied, typically "/usr/lib/locale"
# -DTZDEFRULESTRING=\",date/time,date/time\" to default to the specified
# DST transitions if the time zone files cannot be accessed
# -DUNINIT_TRAP=1 if reading uninitialized storage can cause problems
# other than simply getting garbage data
# -DZIC_MAX_ABBR_LEN_WO_WARN=3
# (or some other number) to set the maximum time zone abbreviation length
# that zic will accept without a warning (the default is 6)

8
NEWS
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@ -59,6 +59,14 @@ Unreleased, experimental changes
already defined, to make it easier to configure on common platforms.
Define NO_TM_GMTOFF and NO_TM_ZONE to suppress this.
Unless the new macro UNINIT_TRAP is defined to 0, the tz code now
assumes that reading uninitialized memory yields garbage values
but does not cause other problems such as traps.
If TM_GMTOFF is defined and UNINIT_TRAP is not 0, mktime is now
more likely to guess right for ambiguous time stamps near
transitions where tm_isdst does not change.
tzselect -c now uses a hybrid distance measure that works better
in Africa. (Thanks to Alan Barrett for noting the problem.)

30
localtime.c
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@ -103,6 +103,7 @@ struct lsinfo { /* leap second information */
int_fast64_t ls_corr; /* correction to apply */
};
#define SMALLEST(a, b) (((a) < (b)) ? (a) : (b))
#define BIGGEST(a, b) (((a) > (b)) ? (a) : (b))
#ifdef TZNAME_MAX
@ -1817,6 +1818,35 @@ time2sub(struct tm *const tmp,
else lo = t;
continue;
}
#if defined TM_GMTOFF && ! UNINIT_TRAP
if (mytm.TM_GMTOFF != yourtm.TM_GMTOFF
&& (yourtm.TM_GMTOFF < 0
? (-SECSPERDAY <= yourtm.TM_GMTOFF
&& (mytm.TM_GMTOFF <=
(SMALLEST (INT_FAST32_MAX, LONG_MAX)
+ yourtm.TM_GMTOFF)))
: (yourtm.TM_GMTOFF <= SECSPERDAY
&& ((BIGGEST (INT_FAST32_MIN, LONG_MIN)
+ yourtm.TM_GMTOFF)
<= mytm.TM_GMTOFF)))) {
/* MYTM matches YOURTM except with the wrong UTC offset.
YOURTM.TM_GMTOFF is plausible, so try it instead.
It's OK if YOURTM.TM_GMTOFF contains uninitialized data,
since the guess gets checked. */
time_t altt = t;
int_fast32_t diff = mytm.TM_GMTOFF - yourtm.TM_GMTOFF;
if (!increment_overflow_time(&altt, diff)) {
struct tm alttm;
if (funcp(&altt, offset, &alttm)
&& alttm.tm_isdst == mytm.tm_isdst
&& alttm.TM_GMTOFF == yourtm.TM_GMTOFF
&& tmcomp(&alttm, &yourtm) == 0) {
t = altt;
mytm = alttm;
}
}
}
#endif
if (yourtm.tm_isdst < 0 || mytm.tm_isdst == yourtm.tm_isdst)
break;
/*

4
private.h
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@ -413,6 +413,10 @@ static time_t const time_t_max =
# define INITIALIZE(x)
#endif
#ifndef UNINIT_TRAP
# define UNINIT_TRAP 0
#endif
/*
** For the benefit of GNU folk...
** '_(MSGID)' uses the current locale's message library string for MSGID.