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authorRoland McGrath <roland@gnu.org>1995-02-18 01:27:10 +0000
committerRoland McGrath <roland@gnu.org>1995-02-18 01:27:10 +0000
commit28f540f45bbacd939bfd07f213bcad2bf730b1bf (patch)
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diff --git a/time/difftime.c b/time/difftime.c
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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;
+}