time/difftime.c

   1 /* Copyright (C) 1991-2018 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <http://www.gnu.org/licenses/>.  */
  17
  18 /* Written by Paul Eggert <eggert@cs.ucla.edu>.  */
  19
  20 #include <time.h>
  21
  22 #include <limits.h>
  23 #include <float.h>
  24 #include <stdint.h>
  25
  26 #define TYPE_BITS(type) (sizeof (type) * CHAR_BIT)
  27 #define TYPE_FLOATING(type) ((type) 0.5 == 0.5)
  28 #define TYPE_SIGNED(type) ((type) -1 < 0)
  29
  30 /* Return the difference between TIME1 and TIME0, where TIME0 <= TIME1.
  31    time_t is known to be an integer type.  */
  32
  33 static double
  34 subtract (time_t time1, time_t time0)
  35 {
  36   if (! TYPE_SIGNED (time_t))
  37     return time1 - time0;
  38   else
  39     {
  40       /* Optimize the common special cases where time_t
  41          can be converted to uintmax_t without losing information.  */
  42       uintmax_t dt = (uintmax_t) time1 - (uintmax_t) time0;
  43       double delta = dt;
  44
  45       if (UINTMAX_MAX / 2 < INTMAX_MAX)
  46         {
  47           /* This is a rare host where uintmax_t has padding bits, and possibly
  48              information was lost when converting time_t to uintmax_t.
  49              Check for overflow by comparing dt/2 to (time1/2 - time0/2).
  50              Overflow occurred if they differ by more than a small slop.
  51              Thanks to Clive D.W. Feather for detailed technical advice about
  52              hosts with padding bits.
  53
  54              In the following code the "h" prefix means half.  By range
  55              analysis, we have:
  56
  57                   -0.5 <= ht1 - 0.5*time1 <= 0.5
  58                   -0.5 <= ht0 - 0.5*time0 <= 0.5
  59                   -1.0 <= dht - 0.5*(time1 - time0) <= 1.0
  60
  61              If overflow has not occurred, we also have:
  62
  63                   -0.5 <= hdt - 0.5*(time1 - time0) <= 0
  64                   -1.0 <= dht - hdt <= 1.5
  65
  66              and since dht - hdt is an integer, we also have:
  67
  68                   -1 <= dht - hdt <= 1
  69
  70              or equivalently:
  71
  72                   0 <= dht - hdt + 1 <= 2
  73
  74              In the above analysis, all the operators have their exact
  75              mathematical semantics, not C semantics.  However, dht - hdt +
  76              1 is unsigned in C, so it need not be compared to zero.  */
  77
  78           uintmax_t hdt = dt / 2;
  79           time_t ht1 = time1 / 2;
  80           time_t ht0 = time0 / 2;
  81           time_t dht = ht1 - ht0;
  82
  83           if (2 < dht - hdt + 1)
  84             {
  85               /* Repair delta overflow.
  86
  87                  The following expression contains a second rounding,
  88                  so the result may not be the closest to the true answer.
  89                  This problem occurs only with very large differences.
  90                  It's too painful to fix this portably.  */
  91
  92               delta = dt + 2.0L * (UINTMAX_MAX - UINTMAX_MAX / 2);
  93             }
  94         }
  95
  96       return delta;
  97     }
  98 }
  99
 100 /* Return the difference between TIME1 and TIME0.  */
 101 double
 102 __difftime (time_t time1, time_t time0)
 103 {
 104   /* Convert to double and then subtract if no double-rounding error could
 105      result.  */
 106
 107   if (TYPE_BITS (time_t) <= DBL_MANT_DIG
 108       || (TYPE_FLOATING (time_t) && sizeof (time_t) < sizeof (long double)))
 109     return (double) time1 - (double) time0;
 110
 111   /* Likewise for long double.  */
 112
 113   if (TYPE_BITS (time_t) <= LDBL_MANT_DIG || TYPE_FLOATING (time_t))
 114     return (long double) time1 - (long double) time0;
 115
 116   /* Subtract the smaller integer from the larger, convert the difference to
 117      double, and then negate if needed.  */
 118
 119   return time1 < time0 ? - subtract (time0, time1) : subtract (time1, time0);
 120 }
 121 strong_alias (__difftime, difftime)