lib/malloca.c

   1 /* Safe automatic memory allocation.
   2    Copyright (C) 2003, 2006-2007, 2009-2014 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, see <http://www.gnu.org/licenses/>.  */
  17
  18 #define _GL_USE_STDLIB_ALLOC 1
  19 #include <config.h>
  20
  21 /* Specification.  */
  22 #include "malloca.h"
  23
  24 #include <stdint.h>
  25
  26 #include "verify.h"
  27
  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.  */
  32
  33 #if HAVE_ALLOCA
  34
  35 /* Store the mmalloca() results in a hash table.  This is needed to reliably
  36    distinguish a mmalloca() result and an alloca() result.
  37
  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.  */
  47
  48 #define MAGIC_NUMBER 0x1415fb4a
  49 #define MAGIC_SIZE sizeof (int)
  50 /* This is how the header info would look like without any alignment
  51    considerations.  */
  52 struct preliminary_header { void *next; int magic; };
  53 /* But the header's size must be a multiple of sa_alignment_max.  */
  54 #define HEADER_SIZE \
  55   (((sizeof (struct preliminary_header) + sa_alignment_max - 1) / sa_alignment_max) * sa_alignment_max)
  56 union header {
  57   void *next;
  58   struct {
  59     char room[HEADER_SIZE - MAGIC_SIZE];
  60     int word;
  61   } magic;
  62 };
  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];
  70
  71 #endif
  72
  73 void *
  74 mmalloca (size_t n)
  75 {
  76 #if HAVE_ALLOCA
  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;
  80
  81   if (nplus >= n)
  82     {
  83       void *p = malloc (nplus);
  84
  85       if (p != NULL)
  86         {
  87           size_t slot;
  88           union header *h = p;
  89
  90           p = h + 1;
  91
  92           /* Put a magic number into the indicator word.  */
  93           h->magic.word = MAGIC_NUMBER;
  94
  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;
  99
 100           return p;
 101         }
 102     }
 103   /* Out of memory.  */
 104   return NULL;
 105 #else
 106 # if !MALLOC_0_IS_NONNULL
 107   if (n == 0)
 108     n = 1;
 109 # endif
 110   return malloc (n);
 111 #endif
 112 }
 113
 114 #if HAVE_ALLOCA
 115 void
 116 freea (void *p)
 117 {
 118   /* mmalloca() may have returned NULL.  */
 119   if (p != NULL)
 120     {
 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)
 126         {
 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;)
 132             {
 133               union header *h = p;
 134               if (*chain == p)
 135                 {
 136                   /* Found it.  Remove it from the hash table and free it.  */
 137                   union header *p_begin = h - 1;
 138                   *chain = p_begin->next;
 139                   free (p_begin);
 140                   return;
 141                 }
 142               h = *chain;
 143               chain = &h[-1].next;
 144             }
 145         }
 146       /* At this point, we know it was not a mmalloca() result.  */
 147     }
 148 }
 149 #endif