| 1 | /* Safe automatic memory allocation.
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| 2 | Copyright (C) 2003, 2006-2007, 2009-2012 Free Software Foundation, Inc.
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| 3 | Written by Bruno Haible <bruno@clisp.org>, 2003.
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| 4 |
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| 5 | This program is free software; you can redistribute it and/or modify
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| 6 | it under the terms of the GNU General Public License as published by
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| 7 | the Free Software Foundation; either version 3, or (at your option)
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| 8 | any later version.
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| 9 |
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| 10 | This program is distributed in the hope that it will be useful,
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| 11 | but WITHOUT ANY WARRANTY; without even the implied warranty of
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| 12 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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| 13 | GNU General Public License for more details.
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| 14 |
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| 15 | You should have received a copy of the GNU General Public License
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| 16 | along with this program; if not, see <http://www.gnu.org/licenses/>. */
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| 17 |
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| 18 | #define _GL_USE_STDLIB_ALLOC 1
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| 19 | #include <config.h>
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| 20 |
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| 21 | /* Specification. */
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| 22 | #include "malloca.h"
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| 23 |
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| 24 | #include <stdint.h>
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| 25 |
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| 26 | #include "verify.h"
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| 27 |
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| 28 | /* The speed critical point in this file is freea() applied to an alloca()
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| 29 | result: it must be fast, to match the speed of alloca(). The speed of
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| 30 | mmalloca() and freea() in the other case are not critical, because they
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| 31 | are only invoked for big memory sizes. */
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| 32 |
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| 33 | #if HAVE_ALLOCA
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| 34 |
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| 35 | /* Store the mmalloca() results in a hash table. This is needed to reliably
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| 36 | distinguish a mmalloca() result and an alloca() result.
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| 37 |
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| 38 | Although it is possible that the same pointer is returned by alloca() and
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| 39 | by mmalloca() at different times in the same application, it does not lead
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| 40 | to a bug in freea(), because:
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| 41 | - Before a pointer returned by alloca() can point into malloc()ed memory,
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| 42 | the function must return, and once this has happened the programmer must
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| 43 | not call freea() on it anyway.
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| 44 | - Before a pointer returned by mmalloca() can point into the stack, it
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| 45 | must be freed. The only function that can free it is freea(), and
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| 46 | when freea() frees it, it also removes it from the hash table. */
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| 47 |
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| 48 | #define MAGIC_NUMBER 0x1415fb4a
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| 49 | #define MAGIC_SIZE sizeof (int)
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| 50 | /* This is how the header info would look like without any alignment
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| 51 | considerations. */
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| 52 | struct preliminary_header { void *next; char room[MAGIC_SIZE]; };
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| 53 | /* But the header's size must be a multiple of sa_alignment_max. */
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| 54 | #define HEADER_SIZE \
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| 55 | (((sizeof (struct preliminary_header) + sa_alignment_max - 1) / sa_alignment_max) * sa_alignment_max)
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| 56 | struct header { void *next; char room[HEADER_SIZE - sizeof (struct preliminary_header) + MAGIC_SIZE]; };
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| 57 | verify (HEADER_SIZE == sizeof (struct header));
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| 58 | /* We make the hash table quite big, so that during lookups the probability
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| 59 | of empty hash buckets is quite high. There is no need to make the hash
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| 60 | table resizable, because when the hash table gets filled so much that the
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| 61 | lookup becomes slow, it means that the application has memory leaks. */
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| 62 | #define HASH_TABLE_SIZE 257
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| 63 | static void * mmalloca_results[HASH_TABLE_SIZE];
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| 64 |
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| 65 | #endif
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| 66 |
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| 67 | void *
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| 68 | mmalloca (size_t n)
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| 69 | {
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| 70 | #if HAVE_ALLOCA
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| 71 | /* Allocate one more word, that serves as an indicator for malloc()ed
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| 72 | memory, so that freea() of an alloca() result is fast. */
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| 73 | size_t nplus = n + HEADER_SIZE;
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| 74 |
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| 75 | if (nplus >= n)
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| 76 | {
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| 77 | char *p = (char *) malloc (nplus);
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| 78 |
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| 79 | if (p != NULL)
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| 80 | {
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| 81 | size_t slot;
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| 82 |
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| 83 | p += HEADER_SIZE;
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| 84 |
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| 85 | /* Put a magic number into the indicator word. */
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| 86 | ((int *) p)[-1] = MAGIC_NUMBER;
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| 87 |
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| 88 | /* Enter p into the hash table. */
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| 89 | slot = (uintptr_t) p % HASH_TABLE_SIZE;
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| 90 | ((struct header *) (p - HEADER_SIZE))->next = mmalloca_results[slot];
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| 91 | mmalloca_results[slot] = p;
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| 92 |
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| 93 | return p;
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| 94 | }
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| 95 | }
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| 96 | /* Out of memory. */
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| 97 | return NULL;
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| 98 | #else
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| 99 | # if !MALLOC_0_IS_NONNULL
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| 100 | if (n == 0)
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| 101 | n = 1;
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| 102 | # endif
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| 103 | return malloc (n);
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| 104 | #endif
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| 105 | }
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| 106 |
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| 107 | #if HAVE_ALLOCA
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| 108 | void
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| 109 | freea (void *p)
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| 110 | {
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| 111 | /* mmalloca() may have returned NULL. */
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| 112 | if (p != NULL)
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| 113 | {
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| 114 | /* Attempt to quickly distinguish the mmalloca() result - which has
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| 115 | a magic indicator word - and the alloca() result - which has an
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| 116 | uninitialized indicator word. It is for this test that sa_increment
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| 117 | additional bytes are allocated in the alloca() case. */
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| 118 | if (((int *) p)[-1] == MAGIC_NUMBER)
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| 119 | {
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| 120 | /* Looks like a mmalloca() result. To see whether it really is one,
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| 121 | perform a lookup in the hash table. */
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| 122 | size_t slot = (uintptr_t) p % HASH_TABLE_SIZE;
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| 123 | void **chain = &mmalloca_results[slot];
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| 124 | for (; *chain != NULL;)
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| 125 | {
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| 126 | if (*chain == p)
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| 127 | {
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| 128 | /* Found it. Remove it from the hash table and free it. */
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| 129 | char *p_begin = (char *) p - HEADER_SIZE;
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| 130 | *chain = ((struct header *) p_begin)->next;
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| 131 | free (p_begin);
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| 132 | return;
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| 133 | }
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| 134 | chain = &((struct header *) ((char *) *chain - HEADER_SIZE))->next;
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| 135 | }
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| 136 | }
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| 137 | /* At this point, we know it was not a mmalloca() result. */
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| 138 | }
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| 139 | }
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| 140 | #endif
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