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