gl/allocsa.c

   1 /* Safe automatic memory allocation.
   2    Copyright (C) 2003, 2006 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 2, 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 #include <config.h>
  19
  20 /* Specification.  */
  21 #include "allocsa.h"
  22
  23 /* The speed critical point in this file is freesa() applied to an alloca()
  24    result: it must be fast, to match the speed of alloca().  The speed of
  25    mallocsa() and freesa() in the other case are not critical, because they
  26    are only invoked for big memory sizes.  */
  27
  28 #if HAVE_ALLOCA
  29
  30 /* Store the mallocsa() results in a hash table.  This is needed to reliably
  31    distinguish a mallocsa() result and an alloca() result.
  32
  33    Although it is possible that the same pointer is returned by alloca() and
  34    by mallocsa() at different times in the same application, it does not lead
  35    to a bug in freesa(), because:
  36      - Before a pointer returned by alloca() can point into malloc()ed memory,
  37        the function must return, and once this has happened the programmer must
  38        not call freesa() on it anyway.
  39      - Before a pointer returned by mallocsa() can point into the stack, it
  40        must be freed.  The only function that can free it is freesa(), and
  41        when freesa() frees it, it also removes it from the hash table.  */
  42
  43 #define MAGIC_NUMBER 0x1415fb4a
  44 #define MAGIC_SIZE sizeof (int)
  45 /* This is how the header info would look like without any alignment
  46    considerations.  */
  47 struct preliminary_header { void *next; char room[MAGIC_SIZE]; };
  48 /* But the header's size must be a multiple of sa_alignment_max.  */
  49 #define HEADER_SIZE \
  50   (((sizeof (struct preliminary_header) + sa_alignment_max - 1) / sa_alignment_max) * sa_alignment_max)
  51 struct header { void *next; char room[HEADER_SIZE - sizeof (struct preliminary_header) + MAGIC_SIZE]; };
  52 /* Verify that HEADER_SIZE == sizeof (struct header).  */
  53 typedef int verify1[2 * (HEADER_SIZE == sizeof (struct header)) - 1];
  54 /* We make the hash table quite big, so that during lookups the probability
  55    of empty hash buckets is quite high.  There is no need to make the hash
  56    table resizable, because when the hash table gets filled so much that the
  57    lookup becomes slow, it means that the application has memory leaks.  */
  58 #define HASH_TABLE_SIZE 257
  59 static void * mallocsa_results[HASH_TABLE_SIZE];
  60
  61 #endif
  62
  63 void *
  64 mallocsa (size_t n)
  65 {
  66 #if HAVE_ALLOCA
  67   /* Allocate one more word, that serves as an indicator for malloc()ed
  68      memory, so that freesa() of an alloca() result is fast.  */
  69   size_t nplus = n + HEADER_SIZE;
  70
  71   if (nplus >= n)
  72     {
  73       char *p = (char *) malloc (nplus);
  74
  75       if (p != NULL)
  76         {
  77           size_t slot;
  78
  79           p += HEADER_SIZE;
  80
  81           /* Put a magic number into the indicator word.  */
  82           ((int *) p)[-1] = MAGIC_NUMBER;
  83
  84           /* Enter p into the hash table.  */
  85           slot = (unsigned long) p % HASH_TABLE_SIZE;
  86           ((struct header *) (p - HEADER_SIZE))->next = mallocsa_results[slot];
  87           mallocsa_results[slot] = p;
  88
  89           return p;
  90         }
  91     }
  92   /* Out of memory.  */
  93   return NULL;
  94 #else
  95 # if !MALLOC_0_IS_NONNULL
  96   if (n == 0)
  97     n = 1;
  98 # endif
  99   return malloc (n);
 100 #endif
 101 }
 102
 103 #if HAVE_ALLOCA
 104 void
 105 freesa (void *p)
 106 {
 107   /* mallocsa() may have returned NULL.  */
 108   if (p != NULL)
 109     {
 110       /* Attempt to quickly distinguish the mallocsa() result - which has
 111          a magic indicator word - and the alloca() result - which has an
 112          uninitialized indicator word.  It is for this test that sa_increment
 113          additional bytes are allocated in the alloca() case.  */
 114       if (((int *) p)[-1] == MAGIC_NUMBER)
 115         {
 116           /* Looks like a mallocsa() result.  To see whether it really is one,
 117              perform a lookup in the hash table.  */
 118           size_t slot = (unsigned long) p % HASH_TABLE_SIZE;
 119           void **chain = &mallocsa_results[slot];
 120           for (; *chain != NULL;)
 121             {
 122               if (*chain == p)
 123                 {
 124                   /* Found it.  Remove it from the hash table and free it.  */
 125                   char *p_begin = (char *) p - HEADER_SIZE;
 126                   *chain = ((struct header *) p_begin)->next;
 127                   free (p_begin);
 128                   return;
 129                 }
 130               chain = &((struct header *) ((char *) *chain - HEADER_SIZE))->next;
 131             }
 132         }
 133       /* At this point, we know it was not a mallocsa() result.  */
 134     }
 135 }
 136 #endif