1 /* Safe automatic memory allocation. 2 Copyright (C) 2003, 2006-2007, 2009-2013 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; char room[MAGIC_SIZE]; }; 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 struct header { void *next; char room[HEADER_SIZE - sizeof (struct preliminary_header) + MAGIC_SIZE]; }; 57 verify (HEADER_SIZE == sizeof (struct header)); 58 /* We make the hash table quite big, so that during lookups the probability 59 of empty hash buckets is quite high. There is no need to make the hash 60 table resizable, because when the hash table gets filled so much that the 61 lookup becomes slow, it means that the application has memory leaks. */ 62 #define HASH_TABLE_SIZE 257 63 static void * mmalloca_results[HASH_TABLE_SIZE]; 64 65 #endif 66 67 void * 68 mmalloca (size_t n) 69 { 70 #if HAVE_ALLOCA 71 /* Allocate one more word, that serves as an indicator for malloc()ed 72 memory, so that freea() of an alloca() result is fast. */ 73 size_t nplus = n + HEADER_SIZE; 74 75 if (nplus >= n) 76 { 77 char *p = (char *) malloc (nplus); 78 79 if (p != NULL) 80 { 81 size_t slot; 82 83 p += HEADER_SIZE; 84 85 /* Put a magic number into the indicator word. */ 86 ((int *) p)[-1] = MAGIC_NUMBER; 87 88 /* Enter p into the hash table. */ 89 slot = (uintptr_t) p % HASH_TABLE_SIZE; 90 ((struct header *) (p - HEADER_SIZE))->next = mmalloca_results[slot]; 91 mmalloca_results[slot] = p; 92 93 return p; 94 } 95 } 96 /* Out of memory. */ 97 return NULL; 98 #else 99 # if !MALLOC_0_IS_NONNULL 100 if (n == 0) 101 n = 1; 102 # endif 103 return malloc (n); 104 #endif 105 } 106 107 #if HAVE_ALLOCA 108 void 109 freea (void *p) 110 { 111 /* mmalloca() may have returned NULL. */ 112 if (p != NULL) 113 { 114 /* Attempt to quickly distinguish the mmalloca() result - which has 115 a magic indicator word - and the alloca() result - which has an 116 uninitialized indicator word. It is for this test that sa_increment 117 additional bytes are allocated in the alloca() case. */ 118 if (((int *) p)[-1] == MAGIC_NUMBER) 119 { 120 /* Looks like a mmalloca() result. To see whether it really is one, 121 perform a lookup in the hash table. */ 122 size_t slot = (uintptr_t) p % HASH_TABLE_SIZE; 123 void **chain = &mmalloca_results[slot]; 124 for (; *chain != NULL;) 125 { 126 if (*chain == p) 127 { 128 /* Found it. Remove it from the hash table and free it. */ 129 char *p_begin = (char *) p - HEADER_SIZE; 130 *chain = ((struct header *) p_begin)->next; 131 free (p_begin); 132 return; 133 } 134 chain = &((struct header *) ((char *) *chain - HEADER_SIZE))->next; 135 } 136 } 137 /* At this point, we know it was not a mmalloca() result. */ 138 } 139 } 140 #endif 141