2 * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
4 * Copyright (c) 2006 Poul-Henning Kamp
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8 * modification, are permitted provided that the following conditions
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25 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
30 * Convert MS-DOS FAT format timestamps to and from unix timespecs
32 * FAT filestamps originally consisted of two 16 bit integers, encoded like
35 * yyyyyyymmmmddddd (year - 1980, month, day)
37 * hhhhhmmmmmmsssss (hour, minutes, seconds divided by two)
39 * Subsequently even Microsoft realized that files could be accessed in less
40 * than two seconds and a byte was added containing:
42 * sfffffff (second mod two, 100ths of second)
44 * FAT timestamps are in the local timezone, with no indication of which
45 * timezone much less if daylight savings time applies.
47 * Later on again, in Windows NT, timestamps were defined relative to GMT.
49 * Purists will point out that UTC replaced GMT for such uses around
50 * half a century ago, already then. Ironically "NT" was an abbreviation of
51 * "New Technology". Anyway...
53 * The 'utc' argument determines if the resulting FATTIME timestamp
54 * should be on the UTC or local timezone calendar.
56 * The conversion functions below cut time into four-year leap-year
57 * cycles rather than single years and uses table lookups inside those
58 * cycles to get the months and years sorted out.
60 * Obviously we cannot calculate the correct table index going from
61 * a posix seconds count to Y/M/D, but we can get pretty close by
62 * dividing the daycount by 32 (giving a too low index), and then
63 * adjusting upwards a couple of steps if necessary.
65 * FAT timestamps have 7 bits for the year and starts at 1980, so
66 * they can represent up to 2107 which means that the non-leap-year
67 * 2100 must be handled.
69 * XXX: As long as time_t is 32 bits this is not relevant or easily
70 * XXX: testable. Revisit when time_t grows bigger.
71 * XXX: grepfodder: 64 bit time_t, y2100, y2.1k, 2100, leap year
75 #include <sys/param.h>
76 #include <sys/types.h>
78 #include <sys/clock.h>
80 #define DAY (24 * 60 * 60) /* Length of day in seconds */
81 #define YEAR 365 /* Length of normal year */
82 #define LYC (4 * YEAR + 1) /* Length of 4 year leap-year cycle */
83 #define T1980 (10 * 365 + 2) /* Days from 1970 to 1980 */
85 /* End of month is N days from start of (normal) year */
87 #define FEB (JAN + 28)
88 #define MAR (FEB + 31)
89 #define APR (MAR + 30)
90 #define MAY (APR + 31)
91 #define JUN (MAY + 30)
92 #define JUL (JUN + 31)
93 #define AUG (JUL + 31)
94 #define SEP (AUG + 30)
95 #define OCT (SEP + 31)
96 #define NOV (OCT + 30)
97 #define DEC (NOV + 31)
99 /* Table of months in a 4 year leap-year cycle */
101 #define ENC(y,m) (((y) << 9) | ((m) << 5))
103 static const struct {
104 uint16_t days; /* month start in days relative to cycle */
105 uint16_t coded; /* encoded year + month information */
107 { 0 + 0 * YEAR, ENC(0, 1) },
109 { JAN + 0 * YEAR, ENC(0, 2) }, { FEB + 0 * YEAR + 1, ENC(0, 3) },
110 { MAR + 0 * YEAR + 1, ENC(0, 4) }, { APR + 0 * YEAR + 1, ENC(0, 5) },
111 { MAY + 0 * YEAR + 1, ENC(0, 6) }, { JUN + 0 * YEAR + 1, ENC(0, 7) },
112 { JUL + 0 * YEAR + 1, ENC(0, 8) }, { AUG + 0 * YEAR + 1, ENC(0, 9) },
113 { SEP + 0 * YEAR + 1, ENC(0, 10) }, { OCT + 0 * YEAR + 1, ENC(0, 11) },
114 { NOV + 0 * YEAR + 1, ENC(0, 12) }, { DEC + 0 * YEAR + 1, ENC(1, 1) },
116 { JAN + 1 * YEAR + 1, ENC(1, 2) }, { FEB + 1 * YEAR + 1, ENC(1, 3) },
117 { MAR + 1 * YEAR + 1, ENC(1, 4) }, { APR + 1 * YEAR + 1, ENC(1, 5) },
118 { MAY + 1 * YEAR + 1, ENC(1, 6) }, { JUN + 1 * YEAR + 1, ENC(1, 7) },
119 { JUL + 1 * YEAR + 1, ENC(1, 8) }, { AUG + 1 * YEAR + 1, ENC(1, 9) },
120 { SEP + 1 * YEAR + 1, ENC(1, 10) }, { OCT + 1 * YEAR + 1, ENC(1, 11) },
121 { NOV + 1 * YEAR + 1, ENC(1, 12) }, { DEC + 1 * YEAR + 1, ENC(2, 1) },
123 { JAN + 2 * YEAR + 1, ENC(2, 2) }, { FEB + 2 * YEAR + 1, ENC(2, 3) },
124 { MAR + 2 * YEAR + 1, ENC(2, 4) }, { APR + 2 * YEAR + 1, ENC(2, 5) },
125 { MAY + 2 * YEAR + 1, ENC(2, 6) }, { JUN + 2 * YEAR + 1, ENC(2, 7) },
126 { JUL + 2 * YEAR + 1, ENC(2, 8) }, { AUG + 2 * YEAR + 1, ENC(2, 9) },
127 { SEP + 2 * YEAR + 1, ENC(2, 10) }, { OCT + 2 * YEAR + 1, ENC(2, 11) },
128 { NOV + 2 * YEAR + 1, ENC(2, 12) }, { DEC + 2 * YEAR + 1, ENC(3, 1) },
130 { JAN + 3 * YEAR + 1, ENC(3, 2) }, { FEB + 3 * YEAR + 1, ENC(3, 3) },
131 { MAR + 3 * YEAR + 1, ENC(3, 4) }, { APR + 3 * YEAR + 1, ENC(3, 5) },
132 { MAY + 3 * YEAR + 1, ENC(3, 6) }, { JUN + 3 * YEAR + 1, ENC(3, 7) },
133 { JUL + 3 * YEAR + 1, ENC(3, 8) }, { AUG + 3 * YEAR + 1, ENC(3, 9) },
134 { SEP + 3 * YEAR + 1, ENC(3, 10) }, { OCT + 3 * YEAR + 1, ENC(3, 11) },
135 { NOV + 3 * YEAR + 1, ENC(3, 12) }
140 timespec2fattime(const struct timespec *tsp, int utc, uint16_t *ddp,
141 uint16_t *dtp, uint8_t *dhp)
151 *dhp = (tsp->tv_sec & 1) * 100 + tsp->tv_nsec / 10000000;
153 *dtp = (t1 / 2) % 30;
154 *dtp |= ((t1 / 60) % 60) << 5;
155 *dtp |= ((t1 / 3600) % 24) << 11;
160 /* Impossible date, truncate to 1980-01-01 */
166 * 2100 is not a leap year.
167 * XXX: a 32 bit time_t can not get us here.
169 if (t2 >= ((2100 - 1980) / 4 * LYC + FEB))
172 /* Account for full leapyear cycles */
177 /* Find approximate table entry */
180 /* Find correct table entry */
181 while (m < 47 && mtab[m + 1].days <= t2)
184 /* Get year + month from the table */
185 *ddp += mtab[m].coded;
187 /* And apply the day in the month */
188 t2 -= mtab[m].days - 1;
195 * Table indexed by the bottom two bits of year + four bits of the month
196 * from the FAT timestamp, returning number of days into 4 year long
200 #define DCOD(m, y, l) ((m) + YEAR * (y) + (l))
201 static const uint16_t daytab[64] = {
202 0, DCOD( 0, 0, 0), DCOD(JAN, 0, 0), DCOD(FEB, 0, 1),
203 DCOD(MAR, 0, 1), DCOD(APR, 0, 1), DCOD(MAY, 0, 1), DCOD(JUN, 0, 1),
204 DCOD(JUL, 0, 1), DCOD(AUG, 0, 1), DCOD(SEP, 0, 1), DCOD(OCT, 0, 1),
205 DCOD(NOV, 0, 1), DCOD(DEC, 0, 1), 0, 0,
206 0, DCOD( 0, 1, 1), DCOD(JAN, 1, 1), DCOD(FEB, 1, 1),
207 DCOD(MAR, 1, 1), DCOD(APR, 1, 1), DCOD(MAY, 1, 1), DCOD(JUN, 1, 1),
208 DCOD(JUL, 1, 1), DCOD(AUG, 1, 1), DCOD(SEP, 1, 1), DCOD(OCT, 1, 1),
209 DCOD(NOV, 1, 1), DCOD(DEC, 1, 1), 0, 0,
210 0, DCOD( 0, 2, 1), DCOD(JAN, 2, 1), DCOD(FEB, 2, 1),
211 DCOD(MAR, 2, 1), DCOD(APR, 2, 1), DCOD(MAY, 2, 1), DCOD(JUN, 2, 1),
212 DCOD(JUL, 2, 1), DCOD(AUG, 2, 1), DCOD(SEP, 2, 1), DCOD(OCT, 2, 1),
213 DCOD(NOV, 2, 1), DCOD(DEC, 2, 1), 0, 0,
214 0, DCOD( 0, 3, 1), DCOD(JAN, 3, 1), DCOD(FEB, 3, 1),
215 DCOD(MAR, 3, 1), DCOD(APR, 3, 1), DCOD(MAY, 3, 1), DCOD(JUN, 3, 1),
216 DCOD(JUL, 3, 1), DCOD(AUG, 3, 1), DCOD(SEP, 3, 1), DCOD(OCT, 3, 1),
217 DCOD(NOV, 3, 1), DCOD(DEC, 3, 1), 0, 0
221 fattime2timespec(unsigned dd, unsigned dt, unsigned dh, int utc,
222 struct timespec *tsp)
226 /* Unpack time fields */
227 tsp->tv_sec = (dt & 0x1f) << 1;
228 tsp->tv_sec += ((dt & 0x7e0) >> 5) * 60;
229 tsp->tv_sec += ((dt & 0xf800) >> 11) * 3600;
230 tsp->tv_sec += dh / 100;
231 tsp->tv_nsec = (dh % 100) * 10000000;
234 day = (dd & 0x1f) - 1;
236 /* Full leap-year cycles */
237 day += LYC * ((dd >> 11) & 0x1f);
239 /* Month offset from leap-year cycle */
240 day += daytab[(dd >> 5) & 0x3f];
243 * 2100 is not a leap year.
244 * XXX: a 32 bit time_t can not get us here.
246 if (day >= ((2100 - 1980) / 4 * LYC + FEB))
249 /* Align with time_t epoch */
252 tsp->tv_sec += DAY * day;
254 tsp->tv_sec += utc_offset();
264 main(int argc __unused, char **argv __unused)
274 for (i = 0; i < 10000; i++) {
276 ts.tv_sec = random();
277 } while (ts.tv_sec < T1980 * 86400);
278 ts.tv_nsec = random() % 1000000000;
280 printf("%10d.%03ld -- ", ts.tv_sec, ts.tv_nsec / 1000000);
282 gmtime_r(&ts.tv_sec, &tm);
283 strftime(buf, sizeof buf, "%Y %m %d %H %M %S", &tm);
284 printf("%s -- ", buf);
286 a = ts.tv_sec + ts.tv_nsec * 1e-9;
288 timet2fattime(&ts, &d, &t, &p);
289 printf("%04x %04x %02x -- ", d, t, p);
290 printf("%3d %02d %02d %02d %02d %02d -- ",
291 ((d >> 9) & 0x7f) + 1980,
296 ((t >> 0) & 0x1f) * 2);
298 ts.tv_sec = ts.tv_nsec = 0;
299 fattime2timet(d, t, p, &ts);
300 printf("%10d.%03ld == ", ts.tv_sec, ts.tv_nsec / 1000000);
301 gmtime_r(&ts.tv_sec, &tm);
302 strftime(buf, sizeof buf, "%Y %m %d %H %M %S", &tm);
303 printf("%s -- ", buf);
304 a -= ts.tv_sec + ts.tv_nsec * 1e-9;
311 #endif /* TEST_DRIVER */