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107 lines
3.3 KiB
C
107 lines
3.3 KiB
C
/* Safe automatic memory allocation.
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Copyright (C) 2003, 2006-2007, 2009-2021 Free Software Foundation,
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Inc.
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Written by Bruno Haible <bruno@clisp.org>, 2003, 2018.
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This program is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3, or (at your option)
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any later version.
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This program 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
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, see <https://www.gnu.org/licenses/>. */
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#define _GL_USE_STDLIB_ALLOC 1
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#include <config.h>
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/* Specification. */
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#include "malloca.h"
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#include "verify.h"
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/* The speed critical point in this file is freea() applied to an alloca()
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result: it must be fast, to match the speed of alloca(). The speed of
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mmalloca() and freea() in the other case are not critical, because they
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are only invoked for big memory sizes.
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Here we use a bit in the address as an indicator, an idea by Ondřej Bílka.
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malloca() can return three types of pointers:
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- Pointers ≡ 0 mod 2*sa_alignment_max come from stack allocation.
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- Pointers ≡ sa_alignment_max mod 2*sa_alignment_max come from heap
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allocation.
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- NULL comes from a failed heap allocation. */
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/* Type for holding very small pointer differences. */
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typedef unsigned char small_t;
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/* Verify that it is wide enough. */
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verify (2 * sa_alignment_max - 1 <= (small_t) -1);
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void *
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mmalloca (size_t n)
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{
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#if HAVE_ALLOCA
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/* Allocate one more word, used to determine the address to pass to freea(),
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and room for the alignment ≡ sa_alignment_max mod 2*sa_alignment_max. */
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size_t nplus = n + sizeof (small_t) + 2 * sa_alignment_max - 1;
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if (nplus >= n)
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{
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char *mem = (char *) malloc (nplus);
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if (mem != NULL)
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{
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char *p =
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(char *)((((uintptr_t)mem + sizeof (small_t) + sa_alignment_max - 1)
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& ~(uintptr_t)(2 * sa_alignment_max - 1))
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+ sa_alignment_max);
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/* Here p >= mem + sizeof (small_t),
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and p <= mem + sizeof (small_t) + 2 * sa_alignment_max - 1
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hence p + n <= mem + nplus.
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So, the memory range [p, p+n) lies in the allocated memory range
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[mem, mem + nplus). */
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((small_t *) p)[-1] = p - mem;
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/* p ≡ sa_alignment_max mod 2*sa_alignment_max. */
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return p;
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}
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}
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/* Out of memory. */
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return NULL;
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#else
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# if !MALLOC_0_IS_NONNULL
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if (n == 0)
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n = 1;
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# endif
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return malloc (n);
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#endif
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}
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#if HAVE_ALLOCA
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void
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freea (void *p)
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{
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/* Check argument. */
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if ((uintptr_t) p & (sa_alignment_max - 1))
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{
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/* p was not the result of a malloca() call. Invalid argument. */
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abort ();
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}
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/* Determine whether p was a non-NULL pointer returned by mmalloca(). */
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if ((uintptr_t) p & sa_alignment_max)
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{
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void *mem = (char *) p - ((small_t *) p)[-1];
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free (mem);
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}
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}
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#endif
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/*
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* Hey Emacs!
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* Local Variables:
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* coding: utf-8
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* End:
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*/
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