- int dst_diff = (isdst != 0) - (tm.tm_isdst != 0);
- if (dst_diff)
- {
- /* Move two hours in the direction indicated by the disagreement,
- probe some more, and switch to a new time if found.
- The largest known fallback due to daylight savings is two hours:
- once, in Newfoundland, 1988-10-30 02:00 -> 00:00. */
- time_t ot = t - 2 * 60 * 60 * dst_diff;
- while (--remaining_probes != 0)
- {
- struct tm otm;
- if (! (dt = ydhms_tm_diff (year, yday, hour, min, sec,
- (*convert) (&ot, &otm))))
- {
- t = ot;
- tm = otm;
- break;
- }
- if ((ot += dt) == t)
- break; /* Avoid a redundant probe. */
- }
- }
+ /* time_t isn't large enough to rule out overflows, so check
+ for major overflows. A gross check suffices, since if t0
+ has overflowed, it is off by a multiple of TIME_T_MAX -
+ TIME_T_MIN + 1. So ignore any component of the difference
+ that is bounded by a small value. */
+
+ /* Approximate log base 2 of the number of time units per
+ biennium. A biennium is 2 years; use this unit instead of
+ years to avoid integer overflow. For example, 2 average
+ Gregorian years are 2 * 365.2425 * 24 * 60 * 60 seconds,
+ which is 63113904 seconds, and rint (log2 (63113904)) is
+ 26. */
+ int ALOG2_SECONDS_PER_BIENNIUM = 26;
+ int ALOG2_MINUTES_PER_BIENNIUM = 20;
+ int ALOG2_HOURS_PER_BIENNIUM = 14;
+ int ALOG2_DAYS_PER_BIENNIUM = 10;
+ int LOG2_YEARS_PER_BIENNIUM = 1;
+
+ int approx_requested_biennia =
+ (SHR (year_requested, LOG2_YEARS_PER_BIENNIUM)
+ - SHR (EPOCH_YEAR - TM_YEAR_BASE, LOG2_YEARS_PER_BIENNIUM)
+ + SHR (mday, ALOG2_DAYS_PER_BIENNIUM)
+ + SHR (hour, ALOG2_HOURS_PER_BIENNIUM)
+ + SHR (min, ALOG2_MINUTES_PER_BIENNIUM)
+ + (LEAP_SECONDS_POSSIBLE
+ ? 0
+ : SHR (sec, ALOG2_SECONDS_PER_BIENNIUM)));
+
+ int approx_biennia = SHR (t0, ALOG2_SECONDS_PER_BIENNIUM);
+ int diff = approx_biennia - approx_requested_biennia;
+ int abs_diff = diff < 0 ? -1 - diff : diff;
+
+ /* IRIX 4.0.5 cc miscalculates TIME_T_MIN / 3: it erroneously
+ gives a positive value of 715827882. Setting a variable
+ first then doing math on it seems to work.
+ (ghazi@caip.rutgers.edu) */
+ time_t time_t_max = TIME_T_MAX;
+ time_t time_t_min = TIME_T_MIN;
+ time_t overflow_threshold =
+ (time_t_max / 3 - time_t_min / 3) >> ALOG2_SECONDS_PER_BIENNIUM;
+
+ if (overflow_threshold < abs_diff)
+ {
+ /* Overflow occurred. Try repairing it; this might work if
+ the time zone offset is enough to undo the overflow. */
+ time_t repaired_t0 = -1 - t0;
+ approx_biennia = SHR (repaired_t0, ALOG2_SECONDS_PER_BIENNIUM);
+ diff = approx_biennia - approx_requested_biennia;
+ abs_diff = diff < 0 ? -1 - diff : diff;
+ if (overflow_threshold < abs_diff)
+ return -1;
+ guessed_offset += repaired_t0 - t0;
+ t0 = repaired_t0;
+ }