FreeBSD/Linux Kernel Cross Reference
sys/kern/subr_fattime.c

     /*-
      * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
      *
      * Copyright (c) 2006 Poul-Henning Kamp
      * All rights reserved.
      *
      * Redistribution and use in source and binary forms, with or without
      * modification, are permitted provided that the following conditions
      * are met:
     * 1. Redistributions of source code must retain the above copyright
     *    notice, this list of conditions and the following disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in the
     *    documentation and/or other materials provided with the distribution.
     *
     * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
     * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
     * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
     * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
     * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
     * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
     * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
     * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
     * SUCH DAMAGE.
     *
     * $FreeBSD$
     *
     * Convert MS-DOS FAT format timestamps to and from unix timespecs
     *
     * FAT filestamps originally consisted of two 16 bit integers, encoded like
     * this:
     *
     *      yyyyyyymmmmddddd (year - 1980, month, day)
     *
     *      hhhhhmmmmmmsssss (hour, minutes, seconds divided by two)
     *
     * Subsequently even Microsoft realized that files could be accessed in less
     * than two seconds and a byte was added containing:
     *
     *      sfffffff         (second mod two, 100ths of second)
     *
     * FAT timestamps are in the local timezone, with no indication of which
     * timezone much less if daylight savings time applies.
     *
     * Later on again, in Windows NT, timestamps were defined relative to GMT.
     *
     * Purists will point out that UTC replaced GMT for such uses around
     * half a century ago, already then.  Ironically "NT" was an abbreviation of 
     * "New Technology".  Anyway...
     *
     * The 'utc' argument determines if the resulting FATTIME timestamp
     * should be on the UTC or local timezone calendar.
     *
     * The conversion functions below cut time into four-year leap-year
     * cycles rather than single years and uses table lookups inside those
     * cycles to get the months and years sorted out.
     *
     * Obviously we cannot calculate the correct table index going from
     * a posix seconds count to Y/M/D, but we can get pretty close by
     * dividing the daycount by 32 (giving a too low index), and then
     * adjusting upwards a couple of steps if necessary.
     *
     * FAT timestamps have 7 bits for the year and starts at 1980, so
     * they can represent up to 2107 which means that the non-leap-year
     * 2100 must be handled.
     *
     * XXX: As long as time_t is 32 bits this is not relevant or easily
     * XXX: testable.  Revisit when time_t grows bigger.
     * XXX: grepfodder: 64 bit time_t, y2100, y2.1k, 2100, leap year
     *
     */
    
    #include <sys/param.h>
    #include <sys/types.h>
    #include <sys/time.h>
    #include <sys/clock.h>
    
    #define DAY     (24 * 60 * 60)  /* Length of day in seconds */
    #define YEAR    365             /* Length of normal year */
    #define LYC     (4 * YEAR + 1)  /* Length of 4 year leap-year cycle */
    #define T1980   (10 * 365 + 2)  /* Days from 1970 to 1980 */
    
    /* End of month is N days from start of (normal) year */
    #define JAN     31
    #define FEB     (JAN + 28)
    #define MAR     (FEB + 31)
    #define APR     (MAR + 30)
    #define MAY     (APR + 31)
    #define JUN     (MAY + 30)
    #define JUL     (JUN + 31)
    #define AUG     (JUL + 31)
    #define SEP     (AUG + 30)
    #define OCT     (SEP + 31)
    #define NOV     (OCT + 30)
    #define DEC     (NOV + 31)
    
    /* Table of months in a 4 year leap-year cycle */
   
   #define ENC(y,m)        (((y) << 9) | ((m) << 5))
   
   static const struct {
           uint16_t        days;   /* month start in days relative to cycle */
           uint16_t        coded;  /* encoded year + month information */
   } mtab[48] = {
           {   0 + 0 * YEAR,     ENC(0, 1)  },
   
           { JAN + 0 * YEAR,     ENC(0, 2)  }, { FEB + 0 * YEAR + 1, ENC(0, 3)  },
           { MAR + 0 * YEAR + 1, ENC(0, 4)  }, { APR + 0 * YEAR + 1, ENC(0, 5)  },
           { MAY + 0 * YEAR + 1, ENC(0, 6)  }, { JUN + 0 * YEAR + 1, ENC(0, 7)  },
           { JUL + 0 * YEAR + 1, ENC(0, 8)  }, { AUG + 0 * YEAR + 1, ENC(0, 9)  },
           { SEP + 0 * YEAR + 1, ENC(0, 10) }, { OCT + 0 * YEAR + 1, ENC(0, 11) },
           { NOV + 0 * YEAR + 1, ENC(0, 12) }, { DEC + 0 * YEAR + 1, ENC(1, 1)  },
   
           { JAN + 1 * YEAR + 1, ENC(1, 2)  }, { FEB + 1 * YEAR + 1, ENC(1, 3)  },
           { MAR + 1 * YEAR + 1, ENC(1, 4)  }, { APR + 1 * YEAR + 1, ENC(1, 5)  },
           { MAY + 1 * YEAR + 1, ENC(1, 6)  }, { JUN + 1 * YEAR + 1, ENC(1, 7)  },
           { JUL + 1 * YEAR + 1, ENC(1, 8)  }, { AUG + 1 * YEAR + 1, ENC(1, 9)  },
           { SEP + 1 * YEAR + 1, ENC(1, 10) }, { OCT + 1 * YEAR + 1, ENC(1, 11) },
           { NOV + 1 * YEAR + 1, ENC(1, 12) }, { DEC + 1 * YEAR + 1, ENC(2, 1)  },
   
           { JAN + 2 * YEAR + 1, ENC(2, 2)  }, { FEB + 2 * YEAR + 1, ENC(2, 3)  },
           { MAR + 2 * YEAR + 1, ENC(2, 4)  }, { APR + 2 * YEAR + 1, ENC(2, 5)  },
           { MAY + 2 * YEAR + 1, ENC(2, 6)  }, { JUN + 2 * YEAR + 1, ENC(2, 7)  },
           { JUL + 2 * YEAR + 1, ENC(2, 8)  }, { AUG + 2 * YEAR + 1, ENC(2, 9)  },
           { SEP + 2 * YEAR + 1, ENC(2, 10) }, { OCT + 2 * YEAR + 1, ENC(2, 11) },
           { NOV + 2 * YEAR + 1, ENC(2, 12) }, { DEC + 2 * YEAR + 1, ENC(3, 1)  },
   
           { JAN + 3 * YEAR + 1, ENC(3, 2)  }, { FEB + 3 * YEAR + 1, ENC(3, 3)  },
           { MAR + 3 * YEAR + 1, ENC(3, 4)  }, { APR + 3 * YEAR + 1, ENC(3, 5)  },
           { MAY + 3 * YEAR + 1, ENC(3, 6)  }, { JUN + 3 * YEAR + 1, ENC(3, 7)  },
           { JUL + 3 * YEAR + 1, ENC(3, 8)  }, { AUG + 3 * YEAR + 1, ENC(3, 9)  },
           { SEP + 3 * YEAR + 1, ENC(3, 10) }, { OCT + 3 * YEAR + 1, ENC(3, 11) },
           { NOV + 3 * YEAR + 1, ENC(3, 12) }
   };
   
   void
   timespec2fattime(const struct timespec *tsp, int utc, uint16_t *ddp,
       uint16_t *dtp, uint8_t *dhp)
   {
           time_t t1;
           unsigned t2, l, m;
   
           t1 = tsp->tv_sec;
           if (!utc)
                   t1 -= utc_offset();
   
           if (dhp != NULL)
                   *dhp = (tsp->tv_sec & 1) * 100 + tsp->tv_nsec / 10000000;
           if (dtp != NULL) {
                   *dtp = (t1 / 2) % 30;
                   *dtp |= ((t1 / 60) % 60) << 5;
                   *dtp |= ((t1 / 3600) % 24) << 11;
           }
           if (ddp != NULL) {
                   t2 = t1 / DAY;
                   if (t2 < T1980) {
                           /* Impossible date, truncate to 1980-01-01 */
                           *ddp = 0x0021;
                   } else {
                           t2 -= T1980;
   
                           /*
                            * 2100 is not a leap year.
                            * XXX: a 32 bit time_t can not get us here.
                            */
                           if (t2 >= ((2100 - 1980) / 4 * LYC + FEB))
                                   t2++;
   
                           /* Account for full leapyear cycles */
                           l = t2 / LYC;
                           *ddp = (l * 4) << 9;
                           t2 -= l * LYC;
   
                           /* Find approximate table entry */
                           m = t2 / 32;
   
                           /* Find correct table entry */
                           while (m < 47 && mtab[m + 1].days <= t2)
                                   m++;
   
                           /* Get year + month from the table */
                           *ddp += mtab[m].coded;
   
                           /* And apply the day in the month */
                           t2 -= mtab[m].days - 1;
                           *ddp |= t2;
                   }
           }
   }
   
   /*
    * Table indexed by the bottom two bits of year + four bits of the month
    * from the FAT timestamp, returning number of days into 4 year long
    * leap-year cycle
    */
   
   #define DCOD(m, y, l)   ((m) + YEAR * (y) + (l))
   static const uint16_t daytab[64] = {
           0,               DCOD(  0, 0, 0), DCOD(JAN, 0, 0), DCOD(FEB, 0, 1),
           DCOD(MAR, 0, 1), DCOD(APR, 0, 1), DCOD(MAY, 0, 1), DCOD(JUN, 0, 1),
           DCOD(JUL, 0, 1), DCOD(AUG, 0, 1), DCOD(SEP, 0, 1), DCOD(OCT, 0, 1),
           DCOD(NOV, 0, 1), DCOD(DEC, 0, 1), 0,               0,
           0,               DCOD(  0, 1, 1), DCOD(JAN, 1, 1), DCOD(FEB, 1, 1),
           DCOD(MAR, 1, 1), DCOD(APR, 1, 1), DCOD(MAY, 1, 1), DCOD(JUN, 1, 1),
           DCOD(JUL, 1, 1), DCOD(AUG, 1, 1), DCOD(SEP, 1, 1), DCOD(OCT, 1, 1),
           DCOD(NOV, 1, 1), DCOD(DEC, 1, 1), 0,               0,
           0,               DCOD(  0, 2, 1), DCOD(JAN, 2, 1), DCOD(FEB, 2, 1),
           DCOD(MAR, 2, 1), DCOD(APR, 2, 1), DCOD(MAY, 2, 1), DCOD(JUN, 2, 1),
           DCOD(JUL, 2, 1), DCOD(AUG, 2, 1), DCOD(SEP, 2, 1), DCOD(OCT, 2, 1),
           DCOD(NOV, 2, 1), DCOD(DEC, 2, 1), 0,               0,
           0,               DCOD(  0, 3, 1), DCOD(JAN, 3, 1), DCOD(FEB, 3, 1),
           DCOD(MAR, 3, 1), DCOD(APR, 3, 1), DCOD(MAY, 3, 1), DCOD(JUN, 3, 1),
           DCOD(JUL, 3, 1), DCOD(AUG, 3, 1), DCOD(SEP, 3, 1), DCOD(OCT, 3, 1),
           DCOD(NOV, 3, 1), DCOD(DEC, 3, 1), 0,               0
   };
   
   void
   fattime2timespec(unsigned dd, unsigned dt, unsigned dh, int utc,
       struct timespec *tsp)
   {
           unsigned day;
   
           /* Unpack time fields */
           tsp->tv_sec = (dt & 0x1f) << 1;
           tsp->tv_sec += ((dt & 0x7e0) >> 5) * 60;
           tsp->tv_sec += ((dt & 0xf800) >> 11) * 3600;
           tsp->tv_sec += dh / 100;
           tsp->tv_nsec = (dh % 100) * 10000000;
   
           /* Day of month */
           day = (dd & 0x1f) - 1;
   
           /* Full leap-year cycles */
           day += LYC * ((dd >> 11) & 0x1f);
   
           /* Month offset from leap-year cycle */
           day += daytab[(dd >> 5) & 0x3f];
   
           /*
            * 2100 is not a leap year.
            * XXX: a 32 bit time_t can not get us here.
            */
           if (day >= ((2100 - 1980) / 4 * LYC + FEB))
                   day--;
   
           /* Align with time_t epoch */
           day += T1980;
   
           tsp->tv_sec += DAY * day;
           if (!utc)
                   tsp->tv_sec += utc_offset();
   }
   
   #ifdef TEST_DRIVER
   
   #include <stdio.h>
   #include <unistd.h>
   #include <stdlib.h>
   
   int
   main(int argc __unused, char **argv __unused)
   {
           int i;
           struct timespec ts;
           struct tm tm;
           double a;
           uint16_t d, t;
           uint8_t p;
           char buf[100];
   
           for (i = 0; i < 10000; i++) {
                   do {
                           ts.tv_sec = random();
                   } while (ts.tv_sec < T1980 * 86400);
                   ts.tv_nsec = random() % 1000000000;
   
                   printf("%10d.%03ld -- ", ts.tv_sec, ts.tv_nsec / 1000000);
   
                   gmtime_r(&ts.tv_sec, &tm);
                   strftime(buf, sizeof buf, "%Y %m %d %H %M %S", &tm);
                   printf("%s -- ", buf);
   
                   a = ts.tv_sec + ts.tv_nsec * 1e-9;
                   d = t = p = 0;
                   timet2fattime(&ts, &d, &t, &p);
                   printf("%04x %04x %02x -- ", d, t, p);
                   printf("%3d %02d %02d %02d %02d %02d -- ",
                       ((d >> 9)  & 0x7f) + 1980,
                       (d >> 5)  & 0x0f,
                       (d >> 0)  & 0x1f,
                       (t >> 11) & 0x1f,
                       (t >> 5)  & 0x3f,
                       ((t >> 0)  & 0x1f) * 2);
   
                   ts.tv_sec = ts.tv_nsec = 0;
                   fattime2timet(d, t, p, &ts);
                   printf("%10d.%03ld == ", ts.tv_sec, ts.tv_nsec / 1000000);
                   gmtime_r(&ts.tv_sec, &tm);
                   strftime(buf, sizeof buf, "%Y %m %d %H %M %S", &tm);
                   printf("%s -- ", buf);
                   a -= ts.tv_sec + ts.tv_nsec * 1e-9;
                   printf("%.3f", a);
                   printf("\n");
           }
           return (0);
   }
   
   #endif /* TEST_DRIVER */

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