/*
** This file is in the public domain, so clarified as of
** June 5, 1996 by Arthur David Olson (arthur_david_olson@nih.gov).
**
** $FreeBSD: src/lib/libc/stdtime/difftime.c,v 1.4.8.1 2001/03/05 11:37:21 obrien Exp $
** $DragonFly: src/lib/libcr/stdtime/Attic/difftime.c,v 1.2 2003/06/17 04:26:46 dillon Exp $
*/
/*
* @(#)difftime.c 7.7
*/
/*LINTLIBRARY*/
#include "private.h"
/*
** Algorithm courtesy Paul Eggert (eggert@twinsun.com).
*/
#ifdef HAVE_LONG_DOUBLE
#define long_double long double
#endif /* defined HAVE_LONG_DOUBLE */
#ifndef HAVE_LONG_DOUBLE
#define long_double double
#endif /* !defined HAVE_LONG_DOUBLE */
double
difftime(time1, time0)
const time_t time1;
const time_t time0;
{
time_t delta;
time_t 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 = (~ (time_t) 0) << (TYPE_BIT(time_t) - 1);
/*
** 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;
** some 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 with those compilers.
*/
return delta - 2 * (long_double) hibit;
}