time/difftime.c

   1 /* Copyright (C) 1991, 1994 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 Library General Public License as
   6 published by the Free Software Foundation; either version 2 of the
   7 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 Library General Public License for more details.
  13
  14 You should have received a copy of the GNU Library General Public
  15 License along with the GNU C Library; see the file COPYING.LIB.  If
  16 not, write to the Free Software Foundation, Inc., 675 Mass Ave,
  17 Cambridge, MA 02139, USA.  */
  18
  19 #include <ansidecl.h>
  20 #include <time.h>
  21
  22
  23 /* Return the difference between TIME1 and TIME0.  */
  24 double
  25 DEFUN(difftime, (time1, time0), time_t time1 AND time_t time0)
  26 {
  27   /* Algorithm courtesy Paul Eggert (eggert@twinsun.com).  */
  28
  29   time_t delta, hibit;
  30
  31   if (sizeof (time_t) < sizeof (double))
  32     return (double) time1 - (double) time0;
  33   if (sizeof (time_t) < sizeof (LONG_DOUBLE))
  34     return (LONG_DOUBLE) time1 - (LONG_DOUBLE) time0;
  35
  36   if (time1 < time0)
  37     return - difftime (time0, time1);
  38
  39   /* As much as possible, avoid loss of precision by computing the
  40     difference before converting to double.  */
  41   delta = time1 - time0;
  42   if (delta >= 0)
  43     return delta;
  44
  45   /* Repair delta overflow.  */
  46   hibit = 1;
  47   while ((hibit <<= 1) > 0)
  48     continue;
  49
  50   /* The following expression rounds twice, which means the result may not
  51      be the closest to the true answer.  For example, suppose time_t is
  52      64-bit signed int, long_double is IEEE 754 double with default
  53      rounding, time1 = 9223372036854775807 and time0 = -1536.  Then the
  54      true difference is 9223372036854777343, which rounds to
  55      9223372036854777856 with a total error of 513.  But delta overflows to
  56      -9223372036854774273, which rounds to -9223372036854774784, and
  57      correcting this by subtracting 2 * (long_double) hibit (i.e. by adding
  58      2**64 = 18446744073709551616) yields 9223372036854776832, which rounds
  59      to 9223372036854775808 with a total error of 1535 instead.  This
  60      problem occurs only with very large differences.  It's too painful to
  61      fix this portably.  We are not alone in this problem; many C compilers
  62      round twice when converting large unsigned types to small floating
  63      types, so if time_t is unsigned the "return delta" above has the same
  64      double-rounding problem.  */
  65   return delta - 2 * (LONG_DOUBLE) hibit;
  66 }