+ time_t t, gt, t0, t1, t2;
+ struct tm tm;
+
+ /* The maximum number of probes (calls to CONVERT) should be enough
+ to handle any combinations of time zone rule changes, solar time,
+ leap seconds, and oscillations around a spring-forward gap.
+ POSIX.1 prohibits leap seconds, but some hosts have them anyway. */
+ int remaining_probes = 6;
+
+ /* Time requested. Copy it in case CONVERT modifies *TP; this can
+ occur if TP is localtime's returned value and CONVERT is localtime. */
+ int sec = tp->tm_sec;
+ int min = tp->tm_min;
+ int hour = tp->tm_hour;
+ int mday = tp->tm_mday;
+ int mon = tp->tm_mon;
+ int year_requested = tp->tm_year;
+ int isdst = tp->tm_isdst;
+
+ /* 1 if the previous probe was DST. */
+ int dst2;
+
+ /* Ensure that mon is in range, and set year accordingly. */
+ int mon_remainder = mon % 12;
+ int negative_mon_remainder = mon_remainder < 0;
+ int mon_years = mon / 12 - negative_mon_remainder;
+ long_int lyear_requested = year_requested;
+ long_int year = lyear_requested + mon_years;
+
+ /* The other values need not be in range:
+ the remaining code handles minor overflows correctly,
+ assuming int and time_t arithmetic wraps around.
+ Major overflows are caught at the end. */
+
+ /* Calculate day of year from year, month, and day of month.
+ The result need not be in range. */
+ int mon_yday = ((__mon_yday[leapyear (year)]
+ [mon_remainder + 12 * negative_mon_remainder])
+ - 1);
+ long_int lmday = mday;
+ long_int yday = mon_yday + lmday;
+
+ time_t guessed_offset = *offset;
+
+ int sec_requested = sec;
+
+ if (LEAP_SECONDS_POSSIBLE)
+ {
+ /* Handle out-of-range seconds specially,
+ since ydhms_tm_diff assumes every minute has 60 seconds. */
+ if (sec < 0)
+ sec = 0;
+ if (59 < sec)
+ sec = 59;
+ }
+
+ /* Invert CONVERT by probing. First assume the same offset as last
+ time. */
+
+ t0 = ydhms_diff (year, yday, hour, min, sec,
+ EPOCH_YEAR - TM_YEAR_BASE, 0, 0, 0, - guessed_offset);
+
+ 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 =
+ (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;
+ }