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Annotation of embedaddon/libiconv/srclib/malloca.c, revision 1.1.1.1

1.1       misho       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