1 /* Safe automatic memory allocation. 2 Copyright (C) 2003, 2006-2007, 2009-2015 Free Software Foundation, Inc. 3 Written by Bruno Haible <bruno@clisp.org>, 2003. 4 5 This program is free software; you can redistribute it and/or modify 6 it under the terms of the GNU General Public License as published by 7 the Free Software Foundation; either version 3, or (at your option) 8 any later version. 9 10 This program is distributed in the hope that it will be useful, 11 but WITHOUT ANY WARRANTY; without even the implied warranty of 12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 13 GNU General Public License for more details. 14 15 You should have received a copy of the GNU General Public License 16 along with this program; if not, see <http://www.gnu.org/licenses/>. */ 17 18 #define _GL_USE_STDLIB_ALLOC 1 19 #include <config.h> 20 21 /* Specification. */ 22 #include "malloca.h" 23 24 #include <stdint.h> 25 26 #include "verify.h" 27 28 /* The speed critical point in this file is freea() applied to an alloca() 29 result: it must be fast, to match the speed of alloca(). The speed of 30 mmalloca() and freea() in the other case are not critical, because they 31 are only invoked for big memory sizes. */ 32 33 #if HAVE_ALLOCA 34 35 /* Store the mmalloca() results in a hash table. This is needed to reliably 36 distinguish a mmalloca() result and an alloca() result. 37 38 Although it is possible that the same pointer is returned by alloca() and 39 by mmalloca() at different times in the same application, it does not lead 40 to a bug in freea(), because: 41 - Before a pointer returned by alloca() can point into malloc()ed memory, 42 the function must return, and once this has happened the programmer must 43 not call freea() on it anyway. 44 - Before a pointer returned by mmalloca() can point into the stack, it 45 must be freed. The only function that can free it is freea(), and 46 when freea() frees it, it also removes it from the hash table. */ 47 48 #define MAGIC_NUMBER 0x1415fb4a 49 #define MAGIC_SIZE sizeof (int) 50 /* This is how the header info would look like without any alignment 51 considerations. */ 52 struct preliminary_header { void *next; int magic; }; 53 /* But the header's size must be a multiple of sa_alignment_max. */ 54 #define HEADER_SIZE \ 55 (((sizeof (struct preliminary_header) + sa_alignment_max - 1) / sa_alignment_max) * sa_alignment_max) 56 union header { 57 void *next; 58 struct { 59 char room[HEADER_SIZE - MAGIC_SIZE]; 60 int word; 61 } magic; 62 }; 63 verify (HEADER_SIZE == sizeof (union header)); 64 /* We make the hash table quite big, so that during lookups the probability 65 of empty hash buckets is quite high. There is no need to make the hash 66 table resizable, because when the hash table gets filled so much that the 67 lookup becomes slow, it means that the application has memory leaks. */ 68 #define HASH_TABLE_SIZE 257 69 static void * mmalloca_results[HASH_TABLE_SIZE]; 70 71 #endif 72 73 void * 74 mmalloca (size_t n) 75 { 76 #if HAVE_ALLOCA 77 /* Allocate one more word, that serves as an indicator for malloc()ed 78 memory, so that freea() of an alloca() result is fast. */ 79 size_t nplus = n + HEADER_SIZE; 80 81 if (nplus >= n) 82 { 83 void *p = malloc (nplus); 84 85 if (p != NULL) 86 { 87 size_t slot; 88 union header *h = p; 89 90 p = h + 1; 91 92 /* Put a magic number into the indicator word. */ 93 h->magic.word = MAGIC_NUMBER; 94 95 /* Enter p into the hash table. */ 96 slot = (uintptr_t) p % HASH_TABLE_SIZE; 97 h->next = mmalloca_results[slot]; 98 mmalloca_results[slot] = p; 99 100 return p; 101 } 102 } 103 /* Out of memory. */ 104 return NULL; 105 #else 106 # if !MALLOC_0_IS_NONNULL 107 if (n == 0) 108 n = 1; 109 # endif 110 return malloc (n); 111 #endif 112 } 113 114 #if HAVE_ALLOCA 115 void 116 freea (void *p) 117 { 118 /* mmalloca() may have returned NULL. */ 119 if (p != NULL) 120 { 121 /* Attempt to quickly distinguish the mmalloca() result - which has 122 a magic indicator word - and the alloca() result - which has an 123 uninitialized indicator word. It is for this test that sa_increment 124 additional bytes are allocated in the alloca() case. */ 125 if (((int *) p)[-1] == MAGIC_NUMBER) 126 { 127 /* Looks like a mmalloca() result. To see whether it really is one, 128 perform a lookup in the hash table. */ 129 size_t slot = (uintptr_t) p % HASH_TABLE_SIZE; 130 void **chain = &mmalloca_results[slot]; 131 for (; *chain != NULL;) 132 { 133 union header *h = p; 134 if (*chain == p) 135 { 136 /* Found it. Remove it from the hash table and free it. */ 137 union header *p_begin = h - 1; 138 *chain = p_begin->next; 139 free (p_begin); 140 return; 141 } 142 h = *chain; 143 chain = &h[-1].next; 144 } 145 } 146 /* At this point, we know it was not a mmalloca() result. */ 147 } 148 } 149 #endif 150