source: vendor/FreeBSD-libc/5.3/stdtime/difftime.c

/*
** This file is in the public domain, so clarified as of
** June 5, 1996 by Arthur David Olson (arthur_david_olson@nih.gov).
*/

#include <sys/cdefs.h>
#ifndef lint
#ifndef NOID
static char     elsieid[] __unused = "@(#)difftime.c    7.9";
#endif /* !defined NOID */
#endif /* !defined lint */
__FBSDID("$FreeBSD: src/lib/libc/stdtime/difftime.c,v 1.8 2004/06/14 10:31:52 stefanf Exp $");

/*LINTLIBRARY*/

#include "namespace.h"
#include "private.h"
#include "un-namespace.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;

        {
                time_t          tt;
                double          d;
                long_double     ld;

                if (sizeof tt < sizeof d)
                        return (double) time1 - (double) time0;
                if (sizeof tt < sizeof ld)
                        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;
}