/* 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; }