From 28f540f45bbacd939bfd07f213bcad2bf730b1bf Mon Sep 17 00:00:00 2001 From: Roland McGrath Date: Sat, 18 Feb 1995 01:27:10 +0000 Subject: initial import --- time/difftime.c | 66 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 66 insertions(+) create mode 100644 time/difftime.c (limited to 'time/difftime.c') diff --git a/time/difftime.c b/time/difftime.c new file mode 100644 index 0000000000..49c5bfbc5c --- /dev/null +++ b/time/difftime.c @@ -0,0 +1,66 @@ +/* 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 +#include + + +/* 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; +} -- cgit v1.2.3