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ac253355ba
Provide a 64-bit-time version of __difftime (but do not assume __time64_t is a signed int so that Gnulib can reuse the code) and make the 32-bit version a wrapper of it. Current difftime expects two time_t arguments and returns a double. To preserve source-code compatibility, its 64-bit-time equivalent expects two __time64_t arguments but still returns a double. This patch was tested by running 'make check' on branch master then applying this patch and its two predecessors and running 'make check' again, and checking that both 'make check' yield identical results. This was done on x86_64-linux-gnu and i686-linux-gnu. This patch was also functionally tested with an ad hoc userland C program which checks the result of difftime for various pairs of 32-bit and, for 64-bit builds, of 64-bit time_t values too. The program was built and run against a glibc with and without the patch, and the results compared to ensure the patch does not change the behavior of difftime. * include/time.h (__difftime64): Add. * time/difftime.c (subtract): convert to 64-bit time. * time/difftime.c (__difftime64): Add. * time/difftime.c (__difftime): Wrap around __difftime64.
137 lines
4.1 KiB
C
137 lines
4.1 KiB
C
/* Copyright (C) 1991-2018 Free Software Foundation, Inc.
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This file is part of the GNU C Library.
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The GNU C Library is free software; you can redistribute it and/or
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modify it under the terms of the GNU Lesser General Public
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License as published by the Free Software Foundation; either
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version 2.1 of the License, or (at your option) any later version.
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The GNU C Library is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public
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License along with the GNU C Library; if not, see
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<http://www.gnu.org/licenses/>. */
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/* Written by Paul Eggert <eggert@cs.ucla.edu>. */
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#include <time.h>
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#include <limits.h>
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#include <float.h>
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#include <stdint.h>
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#define TYPE_BITS(type) (sizeof (type) * CHAR_BIT)
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#define TYPE_FLOATING(type) ((type) 0.5 == 0.5)
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#define TYPE_SIGNED(type) ((type) -1 < 0)
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/* Return the difference between TIME1 and TIME0, where TIME0 <= TIME1.
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time_t is known to be an integer type. */
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static double
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subtract (__time64_t time1, __time64_t time0)
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{
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if (! TYPE_SIGNED (__time64_t))
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return time1 - time0;
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else
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{
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/* Optimize the common special cases where time_t
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can be converted to uintmax_t without losing information. */
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uintmax_t dt = (uintmax_t) time1 - (uintmax_t) time0;
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double delta = dt;
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if (UINTMAX_MAX / 2 < INTMAX_MAX)
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{
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/* This is a rare host where uintmax_t has padding bits, and possibly
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information was lost when converting time_t to uintmax_t.
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Check for overflow by comparing dt/2 to (time1/2 - time0/2).
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Overflow occurred if they differ by more than a small slop.
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Thanks to Clive D.W. Feather for detailed technical advice about
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hosts with padding bits.
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In the following code the "h" prefix means half. By range
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analysis, we have:
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-0.5 <= ht1 - 0.5*time1 <= 0.5
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-0.5 <= ht0 - 0.5*time0 <= 0.5
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-1.0 <= dht - 0.5*(time1 - time0) <= 1.0
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If overflow has not occurred, we also have:
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-0.5 <= hdt - 0.5*(time1 - time0) <= 0
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-1.0 <= dht - hdt <= 1.5
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and since dht - hdt is an integer, we also have:
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-1 <= dht - hdt <= 1
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or equivalently:
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0 <= dht - hdt + 1 <= 2
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In the above analysis, all the operators have their exact
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mathematical semantics, not C semantics. However, dht - hdt +
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1 is unsigned in C, so it need not be compared to zero. */
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uintmax_t hdt = dt / 2;
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__time64_t ht1 = time1 / 2;
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__time64_t ht0 = time0 / 2;
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__time64_t dht = ht1 - ht0;
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if (2 < dht - hdt + 1)
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{
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/* Repair delta overflow.
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The following expression contains a second rounding,
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so the result may not be the closest to the true answer.
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This problem occurs only with very large differences.
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It's too painful to fix this portably. */
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delta = dt + 2.0L * (UINTMAX_MAX - UINTMAX_MAX / 2);
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}
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}
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return delta;
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}
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}
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/* Return the difference between TIME1 and TIME0. */
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double
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__difftime64 (__time64_t time1, __time64_t time0)
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{
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/* Convert to double and then subtract if no double-rounding error could
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result. */
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if (TYPE_BITS (__time64_t) <= DBL_MANT_DIG
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|| (TYPE_FLOATING (__time64_t) && sizeof (__time64_t) < sizeof (long double)))
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return (double) time1 - (double) time0;
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/* Likewise for long double. */
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if (TYPE_BITS (__time64_t) <= LDBL_MANT_DIG || TYPE_FLOATING (__time64_t))
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return (long double) time1 - (long double) time0;
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/* Subtract the smaller integer from the larger, convert the difference to
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double, and then negate if needed. */
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return time1 < time0 ? - subtract (time0, time1) : subtract (time1, time0);
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}
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/* Provide a 32-bit wrapper if needed. */
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#if __TIMESIZE != 64
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libc_hidden_def (__difftime64)
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double
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__difftime (time_t time1, time_t time0)
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{
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return __difftime64 (time1, time0);
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}
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#endif
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strong_alias (__difftime, difftime)
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