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/* Safe automatic memory allocation.
Copyright (C) 2003, 2006-2007, 2009-2011 Free Software Foundation, Inc.
Written by Bruno Haible <>, 2003.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3, or (at your option)
any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software Foundation,
Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */
#include <config.h>
/* Specification. */
#include "malloca.h"
#include "verify.h"
/* The speed critical point in this file is freea() applied to an alloca()
result: it must be fast, to match the speed of alloca(). The speed of
mmalloca() and freea() in the other case are not critical, because they
are only invoked for big memory sizes. */
/* Store the mmalloca() results in a hash table. This is needed to reliably
distinguish a mmalloca() result and an alloca() result.
Although it is possible that the same pointer is returned by alloca() and
by mmalloca() at different times in the same application, it does not lead
to a bug in freea(), because:
- Before a pointer returned by alloca() can point into malloc()ed memory,
the function must return, and once this has happened the programmer must
not call freea() on it anyway.
- Before a pointer returned by mmalloca() can point into the stack, it
must be freed. The only function that can free it is freea(), and
when freea() frees it, it also removes it from the hash table. */
#define MAGIC_NUMBER 0x1415fb4a
#define MAGIC_SIZE sizeof (int)
/* This is how the header info would look like without any alignment
considerations. */
struct preliminary_header { void *next; char room[MAGIC_SIZE]; };
/* But the header's size must be a multiple of sa_alignment_max. */
#define HEADER_SIZE \
(((sizeof (struct preliminary_header) + sa_alignment_max - 1) / sa_alignment_max) * sa_alignment_max)
struct header { void *next; char room[HEADER_SIZE - sizeof (struct preliminary_header) + MAGIC_SIZE]; };
verify (HEADER_SIZE == sizeof (struct header));
/* We make the hash table quite big, so that during lookups the probability
of empty hash buckets is quite high. There is no need to make the hash
table resizable, because when the hash table gets filled so much that the
lookup becomes slow, it means that the application has memory leaks. */
#define HASH_TABLE_SIZE 257
static void * mmalloca_results[HASH_TABLE_SIZE];
void *
mmalloca (size_t n)
/* Allocate one more word, that serves as an indicator for malloc()ed
memory, so that freea() of an alloca() result is fast. */
size_t nplus = n + HEADER_SIZE;
if (nplus >= n)
char *p = (char *) malloc (nplus);
if (p != NULL)
size_t slot;
/* Put a magic number into the indicator word. */
((int *) p)[-1] = MAGIC_NUMBER;
/* Enter p into the hash table. */
slot = (unsigned long) p % HASH_TABLE_SIZE;
((struct header *) (p - HEADER_SIZE))->next = mmalloca_results[slot];
mmalloca_results[slot] = p;
return p;
/* Out of memory. */
return NULL;
if (n == 0)
n = 1;
# endif
return malloc (n);
freea (void *p)
/* mmalloca() may have returned NULL. */
if (p != NULL)
/* Attempt to quickly distinguish the mmalloca() result - which has
a magic indicator word - and the alloca() result - which has an
uninitialized indicator word. It is for this test that sa_increment
additional bytes are allocated in the alloca() case. */
if (((int *) p)[-1] == MAGIC_NUMBER)
/* Looks like a mmalloca() result. To see whether it really is one,
perform a lookup in the hash table. */
size_t slot = (unsigned long) p % HASH_TABLE_SIZE;
void **chain = &mmalloca_results[slot];
for (; *chain != NULL;)
if (*chain == p)
/* Found it. Remove it from the hash table and free it. */
char *p_begin = (char *) p - HEADER_SIZE;
*chain = ((struct header *) p_begin)->next;
free (p_begin);
chain = &((struct header *) ((char *) *chain - HEADER_SIZE))->next;
/* At this point, we know it was not a mmalloca() result. */