+ if (TIME_T_MAX / INT_MAX / 366 / 24 / 60 / 60 < 3)
+ {
+ /* 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 =
+ ((year_requested >> LOG2_YEARS_PER_BIENNIUM)
+ - ((EPOCH_YEAR - TM_YEAR_BASE) >> LOG2_YEARS_PER_BIENNIUM)
+ + (mday >> ALOG2_DAYS_PER_BIENNIUM)
+ + (hour >> ALOG2_HOURS_PER_BIENNIUM)
+ + (min >> ALOG2_MINUTES_PER_BIENNIUM)
+ + (LEAP_SECONDS_POSSIBLE ? 0 : sec >> ALOG2_SECONDS_PER_BIENNIUM));
+
+ int approx_biennia = t0 >> ALOG2_SECONDS_PER_BIENNIUM;
+ int diff = approx_biennia - approx_requested_biennia;
+ int abs_diff = diff < 0 ? - diff : diff;
+
+ /* IRIX 4.0.5 cc miscaculates 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 = repaired_t0 >> ALOG2_SECONDS_PER_BIENNIUM;
+ diff = approx_biennia - approx_requested_biennia;
+ abs_diff = diff < 0 ? - diff : diff;
+ if (overflow_threshold < abs_diff)
+ return -1;
+ guessed_offset += repaired_t0 - t0;
+ t0 = repaired_t0;
+ }
+ }