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