gno/lib/libc/stdtime/difftime.c
1997-09-21 06:22:10 +00:00

94 lines
2.5 KiB
C

#line 1 ":src:gno:lib:libc:stdtime:difftime.c"
/*
* $Id: difftime.c,v 1.1 1997/09/21 06:22:10 gdr Exp $
*
* This file is formatted for tab stops every 8 columns.
*/
#ifdef __ORCAC__
segment "libc_stdtm";
#define NOID
#define CONST /* ORCA/C v2.1.1b2 chokes on the const as used here */
#else
#define CONST const
#endif
#ifndef lint
#ifndef NOID
static char elsieid[] = "@(#)difftime.c 7.4";
#endif /* !defined NOID */
#endif /* !defined lint */
/*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
#ifdef USE_PROTOS
difftime(CONST time_t time1, CONST time_t time0)
#else
difftime(time1, time0)
CONST time_t time1;
CONST time_t time0;
#endif
{
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 = 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;
}