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/* Copyright (C) 1991, 1994 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Library General Public License as
published by the Free Software Foundation; either version 2 of the
License, or (at your option) any later version.
The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Library General Public License for more details.
You should have received a copy of the GNU Library General Public
License along with the GNU C Library; see the file COPYING.LIB. If
not, write to the Free Software Foundation, Inc., 675 Mass Ave,
Cambridge, MA 02139, USA. */
#include <ansidecl.h>
#include <time.h>
/* Return the difference between TIME1 and TIME0. */
double
DEFUN(difftime, (time1, time0), time_t time1 AND time_t time0)
{
/* Algorithm courtesy Paul Eggert (eggert@twinsun.com). */
time_t delta, hibit;
if (sizeof (time_t) < sizeof (double))
return (double) time1 - (double) time0;
if (sizeof (time_t) < sizeof (LONG_DOUBLE))
return (LONG_DOUBLE) time1 - (LONG_DOUBLE) time0;
if (time1 < time0)
return - difftime (time0, time1);
/* As much as possible, avoid loss of precision by computing the
difference before converting to double. */
delta = time1 - time0;
if (delta >= 0)
return delta;
/* Repair delta overflow. */
hibit = 1;
while ((hibit <<= 1) > 0)
continue;
/* The following expression rounds twice, which means the result may not
be the closest to the true answer. For example, suppose time_t is
64-bit signed int, long_double is IEEE 754 double with default
rounding, time1 = 9223372036854775807 and time0 = -1536. Then the
true difference is 9223372036854777343, which rounds to
9223372036854777856 with a total error of 513. But delta overflows to
-9223372036854774273, which rounds to -9223372036854774784, and
correcting this by subtracting 2 * (long_double) hibit (i.e. by adding
2**64 = 18446744073709551616) yields 9223372036854776832, which rounds
to 9223372036854775808 with a total error of 1535 instead. This
problem occurs only with very large differences. It's too painful to
fix this portably. We are not alone in this problem; many C compilers
round twice when converting large unsigned types to small floating
types, so if time_t is unsigned the "return delta" above has the same
double-rounding problem. */
return delta - 2 * (LONG_DOUBLE) hibit;
}
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