1/* Safe automatic memory allocation. 2 Copyright (C) 2003, 2006-2007, 2009-2011 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, write to the Free Software Foundation, 17 Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */ 18 19#define _GL_USE_STDLIB_ALLOC 1 20#include <config.h> 21 22/* Specification. */ 23#include "malloca.h" 24 25#include "verify.h" 26 27/* The speed critical point in this file is freea() applied to an alloca() 28 result: it must be fast, to match the speed of alloca(). The speed of 29 mmalloca() and freea() in the other case are not critical, because they 30 are only invoked for big memory sizes. */ 31 32#if HAVE_ALLOCA 33 34/* Store the mmalloca() results in a hash table. This is needed to reliably 35 distinguish a mmalloca() result and an alloca() result. 36 37 Although it is possible that the same pointer is returned by alloca() and 38 by mmalloca() at different times in the same application, it does not lead 39 to a bug in freea(), because: 40 - Before a pointer returned by alloca() can point into malloc()ed memory, 41 the function must return, and once this has happened the programmer must 42 not call freea() on it anyway. 43 - Before a pointer returned by mmalloca() can point into the stack, it 44 must be freed. The only function that can free it is freea(), and 45 when freea() frees it, it also removes it from the hash table. */ 46 47#define MAGIC_NUMBER 0x1415fb4a 48#define MAGIC_SIZE sizeof (int) 49/* This is how the header info would look like without any alignment 50 considerations. */ 51struct preliminary_header { void *next; char room[MAGIC_SIZE]; }; 52/* But the header's size must be a multiple of sa_alignment_max. */ 53#define HEADER_SIZE \ 54 (((sizeof (struct preliminary_header) + sa_alignment_max - 1) / sa_alignment_max) * sa_alignment_max) 55struct header { void *next; char room[HEADER_SIZE - sizeof (struct preliminary_header) + MAGIC_SIZE]; }; 56verify (HEADER_SIZE == sizeof (struct header)); 57/* We make the hash table quite big, so that during lookups the probability 58 of empty hash buckets is quite high. There is no need to make the hash 59 table resizable, because when the hash table gets filled so much that the 60 lookup becomes slow, it means that the application has memory leaks. */ 61#define HASH_TABLE_SIZE 257 62static void * mmalloca_results[HASH_TABLE_SIZE]; 63 64#endif 65 66void * 67mmalloca (size_t n) 68{ 69#if HAVE_ALLOCA 70 /* Allocate one more word, that serves as an indicator for malloc()ed 71 memory, so that freea() of an alloca() result is fast. */ 72 size_t nplus = n + HEADER_SIZE; 73 74 if (nplus >= n) 75 { 76 char *p = (char *) malloc (nplus); 77 78 if (p != NULL) 79 { 80 size_t slot; 81 82 p += HEADER_SIZE; 83 84 /* Put a magic number into the indicator word. */ 85 ((int *) p)[-1] = MAGIC_NUMBER; 86 87 /* Enter p into the hash table. */ 88 slot = (unsigned long) p % HASH_TABLE_SIZE; 89 ((struct header *) (p - HEADER_SIZE))->next = mmalloca_results[slot]; 90 mmalloca_results[slot] = p; 91 92 return p; 93 } 94 } 95 /* Out of memory. */ 96 return NULL; 97#else 98# if !MALLOC_0_IS_NONNULL 99 if (n == 0) 100 n = 1; 101# endif 102 return malloc (n); 103#endif 104} 105 106#if HAVE_ALLOCA 107void 108freea (void *p) 109{ 110 /* mmalloca() may have returned NULL. */ 111 if (p != NULL) 112 { 113 /* Attempt to quickly distinguish the mmalloca() result - which has 114 a magic indicator word - and the alloca() result - which has an 115 uninitialized indicator word. It is for this test that sa_increment 116 additional bytes are allocated in the alloca() case. */ 117 if (((int *) p)[-1] == MAGIC_NUMBER) 118 { 119 /* Looks like a mmalloca() result. To see whether it really is one, 120 perform a lookup in the hash table. */ 121 size_t slot = (unsigned long) p % HASH_TABLE_SIZE; 122 void **chain = &mmalloca_results[slot]; 123 for (; *chain != NULL;) 124 { 125 if (*chain == p) 126 { 127 /* Found it. Remove it from the hash table and free it. */ 128 char *p_begin = (char *) p - HEADER_SIZE; 129 *chain = ((struct header *) p_begin)->next; 130 free (p_begin); 131 return; 132 } 133 chain = &((struct header *) ((char *) *chain - HEADER_SIZE))->next; 134 } 135 } 136 /* At this point, we know it was not a mmalloca() result. */ 137 } 138} 139#endif 140