lib/malloca.c

   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 Lesser 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 Lesser General Public License for more details.
  14
  15    You should have received a copy of the GNU Lesser 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.  */
  51 struct 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)
  55 struct header { void *next; char room[HEADER_SIZE - sizeof (struct preliminary_header) + MAGIC_SIZE]; };
  56 verify (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
  62 static void * mmalloca_results[HASH_TABLE_SIZE];
  63
  64 #endif
  65
  66 void *
  67 mmalloca (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
 107 void
 108 freea (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