time/difftime.c

   1 /* Copyright (C) 1991-2021 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    <https://www.gnu.org/licenses/>.  */
  17
  18
  19 #include <time.h>
  20
  21 #include <limits.h>
  22 #include <float.h>
  23 #include <stdint.h>
  24
  25 #define TYPE_BITS(type) (sizeof (type) * CHAR_BIT)
  26 #define TYPE_FLOATING(type) ((type) 0.5 == 0.5)
  27 #define TYPE_SIGNED(type) ((type) -1 < 0)
  28
  29 /* Return the difference between TIME1 and TIME0, where TIME0 <= TIME1.
  30    time_t is known to be an integer type.  */
  31
  32 static double
  33 subtract (__time64_t time1, __time64_t time0)
  34 {
  35   if (! TYPE_SIGNED (__time64_t))
  36     return time1 - time0;
  37   else
  38     {
  39       /* Optimize the common special cases where time_t
  40          can be converted to uintmax_t without losing information.  */
  41       uintmax_t dt = (uintmax_t) time1 - (uintmax_t) time0;
  42       double delta = dt;
  43
  44       if (UINTMAX_MAX / 2 < INTMAX_MAX)
  45         {
  46           /* This is a rare host where uintmax_t has padding bits, and possibly
  47              information was lost when converting time_t to uintmax_t.
  48              Check for overflow by comparing dt/2 to (time1/2 - time0/2).
  49              Overflow occurred if they differ by more than a small slop.
  50              Thanks to Clive D.W. Feather for detailed technical advice about
  51              hosts with padding bits.
  52
  53              In the following code the "h" prefix means half.  By range
  54              analysis, we have:
  55
  56                   -0.5 <= ht1 - 0.5*time1 <= 0.5
  57                   -0.5 <= ht0 - 0.5*time0 <= 0.5
  58                   -1.0 <= dht - 0.5*(time1 - time0) <= 1.0
  59
  60              If overflow has not occurred, we also have:
  61
  62                   -0.5 <= hdt - 0.5*(time1 - time0) <= 0
  63                   -1.0 <= dht - hdt <= 1.5
  64
  65              and since dht - hdt is an integer, we also have:
  66
  67                   -1 <= dht - hdt <= 1
  68
  69              or equivalently:
  70
  71                   0 <= dht - hdt + 1 <= 2
  72
  73              In the above analysis, all the operators have their exact
  74              mathematical semantics, not C semantics.  However, dht - hdt +
  75              1 is unsigned in C, so it need not be compared to zero.  */
  76
  77           uintmax_t hdt = dt / 2;
  78           __time64_t ht1 = time1 / 2;
  79           __time64_t ht0 = time0 / 2;
  80           __time64_t dht = ht1 - ht0;
  81
  82           if (2 < dht - hdt + 1)
  83             {
  84               /* Repair delta overflow.
  85
  86                  The following expression contains a second rounding,
  87                  so the result may not be the closest to the true answer.
  88                  This problem occurs only with very large differences.
  89                  It's too painful to fix this portably.  */
  90
  91               delta = dt + 2.0L * (UINTMAX_MAX - UINTMAX_MAX / 2);
  92             }
  93         }
  94
  95       return delta;
  96     }
  97 }
  98
  99 /* Return the difference between TIME1 and TIME0.  */
 100 double
 101 __difftime64 (__time64_t time1, __time64_t time0)
 102 {
 103   /* Convert to double and then subtract if no double-rounding error could
 104      result.  */
 105
 106   if (TYPE_BITS (__time64_t) <= DBL_MANT_DIG
 107       || (TYPE_FLOATING (__time64_t) && sizeof (__time64_t) < sizeof (long double)))
 108     return (double) time1 - (double) time0;
 109
 110   /* Likewise for long double.  */
 111
 112   if (TYPE_BITS (__time64_t) <= LDBL_MANT_DIG || TYPE_FLOATING (__time64_t))
 113     return (long double) time1 - (long double) time0;
 114
 115   /* Subtract the smaller integer from the larger, convert the difference to
 116      double, and then negate if needed.  */
 117
 118   return time1 < time0 ? - subtract (time0, time1) : subtract (time1, time0);
 119 }
 120
 121 /* Provide a 32-bit wrapper if needed.  */
 122
 123 #if __TIMESIZE != 64
 124
 125 libc_hidden_def (__difftime64)
 126
 127 double
 128 __difftime (time_t time1, time_t time0)
 129 {
 130   return __difftime64 (time1, time0);
 131 }
 132
 133 #endif
 134
 135 strong_alias (__difftime, difftime)