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.
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)
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.
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/>. */
18 #define _GL_USE_STDLIB_ALLOC 1
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. */
35 /* Store the mmalloca() results in a hash table. This is needed to reliably
36 distinguish a mmalloca() result and an alloca() result.
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. */
48 #define MAGIC_NUMBER 0x1415fb4a
49 #define MAGIC_SIZE sizeof (int)
50 /* This is how the header info would look like without any alignment
52 struct preliminary_header
{ void *next
; int magic
; };
53 /* But the header's size must be a multiple of sa_alignment_max. */
55 (((sizeof (struct preliminary_header) + sa_alignment_max - 1) / sa_alignment_max) * sa_alignment_max)
59 char room
[HEADER_SIZE
- MAGIC_SIZE
];
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
];
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
;
83 void *p
= malloc (nplus
);
92 /* Put a magic number into the indicator word. */
93 h
->magic
.word
= MAGIC_NUMBER
;
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
;
106 # if !MALLOC_0_IS_NONNULL
118 /* mmalloca() may have returned NULL. */
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
)
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
;)
136 /* Found it. Remove it from the hash table and free it. */
137 union header
*p_begin
= h
- 1;
138 *chain
= p_begin
->next
;
146 /* At this point, we know it was not a mmalloca() result. */