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glibc/time/difftime.c
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/* Copyright (C) 1991-2016 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 Lesser General Public | |
License as published by the Free Software Foundation; either | |
version 2.1 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 | |
Lesser General Public License for more details. | |
You should have received a copy of the GNU Lesser General Public | |
License along with the GNU C Library; if not, see | |
<http://www.gnu.org/licenses/>. */ | |
/* Written by Paul Eggert <eggert@cs.ucla.edu>. */ | |
#include <time.h> | |
#include <limits.h> | |
#include <float.h> | |
#include <stdint.h> | |
#define TYPE_BITS(type) (sizeof (type) * CHAR_BIT) | |
#define TYPE_FLOATING(type) ((type) 0.5 == 0.5) | |
#define TYPE_SIGNED(type) ((type) -1 < 0) | |
/* Return the difference between TIME1 and TIME0, where TIME0 <= TIME1. | |
time_t is known to be an integer type. */ | |
static double | |
subtract (time_t time1, time_t time0) | |
{ | |
if (! TYPE_SIGNED (time_t)) | |
return time1 - time0; | |
else | |
{ | |
/* Optimize the common special cases where time_t | |
can be converted to uintmax_t without losing information. */ | |
uintmax_t dt = (uintmax_t) time1 - (uintmax_t) time0; | |
double delta = dt; | |
if (UINTMAX_MAX / 2 < INTMAX_MAX) | |
{ | |
/* This is a rare host where uintmax_t has padding bits, and possibly | |
information was lost when converting time_t to uintmax_t. | |
Check for overflow by comparing dt/2 to (time1/2 - time0/2). | |
Overflow occurred if they differ by more than a small slop. | |
Thanks to Clive D.W. Feather for detailed technical advice about | |
hosts with padding bits. | |
In the following code the "h" prefix means half. By range | |
analysis, we have: | |
-0.5 <= ht1 - 0.5*time1 <= 0.5 | |
-0.5 <= ht0 - 0.5*time0 <= 0.5 | |
-1.0 <= dht - 0.5*(time1 - time0) <= 1.0 | |
If overflow has not occurred, we also have: | |
-0.5 <= hdt - 0.5*(time1 - time0) <= 0 | |
-1.0 <= dht - hdt <= 1.5 | |
and since dht - hdt is an integer, we also have: | |
-1 <= dht - hdt <= 1 | |
or equivalently: | |
0 <= dht - hdt + 1 <= 2 | |
In the above analysis, all the operators have their exact | |
mathematical semantics, not C semantics. However, dht - hdt + | |
1 is unsigned in C, so it need not be compared to zero. */ | |
uintmax_t hdt = dt / 2; | |
time_t ht1 = time1 / 2; | |
time_t ht0 = time0 / 2; | |
time_t dht = ht1 - ht0; | |
if (2 < dht - hdt + 1) | |
{ | |
/* Repair delta overflow. | |
The following expression contains a second rounding, | |
so the result may not be the closest to the true answer. | |
This problem occurs only with very large differences. | |
It's too painful to fix this portably. */ | |
delta = dt + 2.0L * (UINTMAX_MAX - UINTMAX_MAX / 2); | |
} | |
} | |
return delta; | |
} | |
} | |
/* Return the difference between TIME1 and TIME0. */ | |
double | |
__difftime (time_t time1, time_t time0) | |
{ | |
/* Convert to double and then subtract if no double-rounding error could | |
result. */ | |
if (TYPE_BITS (time_t) <= DBL_MANT_DIG | |
|| (TYPE_FLOATING (time_t) && sizeof (time_t) < sizeof (long double))) | |
return (double) time1 - (double) time0; | |
/* Likewise for long double. */ | |
if (TYPE_BITS (time_t) <= LDBL_MANT_DIG || TYPE_FLOATING (time_t)) | |
return (long double) time1 - (long double) time0; | |
/* Subtract the smaller integer from the larger, convert the difference to | |
double, and then negate if needed. */ | |
return time1 < time0 ? - subtract (time0, time1) : subtract (time1, time0); | |
} | |
strong_alias (__difftime, difftime) |