+ time_t t, dt, t0;
+ 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,
+ and leap seconds. POSIX.1 prohibits leap seconds, but some hosts
+ have them anyway. */
+ int remaining_probes = 4;
+
+ /* 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;
+
+ /* 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;
+ int year = year_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 yday = ((__mon_yday[__isleap (year + TM_YEAR_BASE)]
+ [mon_remainder + 12 * negative_mon_remainder])
+ + mday - 1);
+
+ 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;
+#endif
+
+ /* Invert CONVERT by probing. First assume the same offset as last time.
+ Then repeatedly use the error to improve the guess. */
+
+ tm.tm_year = EPOCH_YEAR - TM_YEAR_BASE;
+ tm.tm_yday = tm.tm_hour = tm.tm_min = tm.tm_sec = 0;
+ t0 = ydhms_tm_diff (year, yday, hour, min, sec, &tm);
+
+ for (t = t0 + *offset;
+ (dt = ydhms_tm_diff (year, yday, hour, min, sec,
+ ranged_convert (convert, &t, &tm)));
+ t += dt)
+ if (--remaining_probes == 0)
+ return -1;
+
+ /* Check whether tm.tm_isdst has the requested value, if any. */
+ if (0 <= isdst && 0 <= tm.tm_isdst)
+ {
+ 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,
+ ranged_convert (convert, &ot, &otm))))
+ {
+ t = ot;
+ tm = otm;
+ break;
+ }
+ if ((ot += dt) == t)
+ break; /* Avoid a redundant probe. */
+ }
+ }