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Initial revision

    1: /* Safe automatic memory allocation.
    2:    Copyright (C) 2003, 2006-2007 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: #include <config.h>
   20: 
   21: /* Specification.  */
   22: #include "malloca.h"
   23: 
   24: /* The speed critical point in this file is freea() applied to an alloca()
   25:    result: it must be fast, to match the speed of alloca().  The speed of
   26:    mmalloca() and freea() in the other case are not critical, because they
   27:    are only invoked for big memory sizes.  */
   28: 
   29: #if HAVE_ALLOCA
   30: 
   31: /* Store the mmalloca() results in a hash table.  This is needed to reliably
   32:    distinguish a mmalloca() result and an alloca() result.
   33: 
   34:    Although it is possible that the same pointer is returned by alloca() and
   35:    by mmalloca() at different times in the same application, it does not lead
   36:    to a bug in freea(), because:
   37:      - Before a pointer returned by alloca() can point into malloc()ed memory,
   38:        the function must return, and once this has happened the programmer must
   39:        not call freea() on it anyway.
   40:      - Before a pointer returned by mmalloca() can point into the stack, it
   41:        must be freed.  The only function that can free it is freea(), and
   42:        when freea() frees it, it also removes it from the hash table.  */
   43: 
   44: #define MAGIC_NUMBER 0x1415fb4a
   45: #define MAGIC_SIZE sizeof (int)
   46: /* This is how the header info would look like without any alignment
   47:    considerations.  */
   48: struct preliminary_header { void *next; char room[MAGIC_SIZE]; };
   49: /* But the header's size must be a multiple of sa_alignment_max.  */
   50: #define HEADER_SIZE \
   51:   (((sizeof (struct preliminary_header) + sa_alignment_max - 1) / sa_alignment_max) * sa_alignment_max)
   52: struct header { void *next; char room[HEADER_SIZE - sizeof (struct preliminary_header) + MAGIC_SIZE]; };
   53: /* Verify that HEADER_SIZE == sizeof (struct header).  */
   54: typedef int verify1[2 * (HEADER_SIZE == sizeof (struct header)) - 1];
   55: /* We make the hash table quite big, so that during lookups the probability
   56:    of empty hash buckets is quite high.  There is no need to make the hash
   57:    table resizable, because when the hash table gets filled so much that the
   58:    lookup becomes slow, it means that the application has memory leaks.  */
   59: #define HASH_TABLE_SIZE 257
   60: static void * mmalloca_results[HASH_TABLE_SIZE];
   61: 
   62: #endif
   63: 
   64: void *
   65: mmalloca (size_t n)
   66: {
   67: #if HAVE_ALLOCA
   68:   /* Allocate one more word, that serves as an indicator for malloc()ed
   69:      memory, so that freea() of an alloca() result is fast.  */
   70:   size_t nplus = n + HEADER_SIZE;
   71: 
   72:   if (nplus >= n)
   73:     {
   74:       char *p = (char *) malloc (nplus);
   75: 
   76:       if (p != NULL)
   77: 	{
   78: 	  size_t slot;
   79: 
   80: 	  p += HEADER_SIZE;
   81: 
   82: 	  /* Put a magic number into the indicator word.  */
   83: 	  ((int *) p)[-1] = MAGIC_NUMBER;
   84: 
   85: 	  /* Enter p into the hash table.  */
   86: 	  slot = (unsigned long) p % HASH_TABLE_SIZE;
   87: 	  ((struct header *) (p - HEADER_SIZE))->next = mmalloca_results[slot];
   88: 	  mmalloca_results[slot] = p;
   89: 
   90: 	  return p;
   91: 	}
   92:     }
   93:   /* Out of memory.  */
   94:   return NULL;
   95: #else
   96: # if !MALLOC_0_IS_NONNULL
   97:   if (n == 0)
   98:     n = 1;
   99: # endif
  100:   return malloc (n);
  101: #endif
  102: }
  103: 
  104: #if HAVE_ALLOCA
  105: void
  106: freea (void *p)
  107: {
  108:   /* mmalloca() may have returned NULL.  */
  109:   if (p != NULL)
  110:     {
  111:       /* Attempt to quickly distinguish the mmalloca() result - which has
  112: 	 a magic indicator word - and the alloca() result - which has an
  113: 	 uninitialized indicator word.  It is for this test that sa_increment
  114: 	 additional bytes are allocated in the alloca() case.  */
  115:       if (((int *) p)[-1] == MAGIC_NUMBER)
  116: 	{
  117: 	  /* Looks like a mmalloca() result.  To see whether it really is one,
  118: 	     perform a lookup in the hash table.  */
  119: 	  size_t slot = (unsigned long) p % HASH_TABLE_SIZE;
  120: 	  void **chain = &mmalloca_results[slot];
  121: 	  for (; *chain != NULL;)
  122: 	    {
  123: 	      if (*chain == p)
  124: 		{
  125: 		  /* Found it.  Remove it from the hash table and free it.  */
  126: 		  char *p_begin = (char *) p - HEADER_SIZE;
  127: 		  *chain = ((struct header *) p_begin)->next;
  128: 		  free (p_begin);
  129: 		  return;
  130: 		}
  131: 	      chain = &((struct header *) ((char *) *chain - HEADER_SIZE))->next;
  132: 	    }
  133: 	}
  134:       /* At this point, we know it was not a mmalloca() result.  */
  135:     }
  136: }
  137: #endif