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/* GObject - GLib Type, Object, Parameter and Signal Library
* Copyright (C) 2009 Benjamin Otte <>
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2 of the License, or (at your option) any later version.
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* Lesser General Public License for more details.
* You should have received a copy of the GNU Lesser General
* Public License along with this library; if not, see <>.
#include "config.h"
#include <string.h>
#include "gatomicarray.h"
/* A GAtomicArray is a growable, mutable array of data
* generally of the form of a header of a specific size and
* then a array of items of a fixed size.
* It is possible to do lock-less read transactions from the
* array without any protection against other reads or writes,
* but such read operation must be aware that the data in the
* atomic array can change at any time during the transaction,
* and only at the end can we verify if the transaction succeeded
* or not. Thus the reading transaction cannot for instance
* dereference a pointer in the array inside the transaction.
* The size of an array however cannot change during a read
* transaction.
* Writes to the array is done in a copy-update style, but there
* is no real protection against multiple writers overwriting each
* others updates, so writes must be protected by an external lock.
typedef struct _FreeListNode FreeListNode;
struct _FreeListNode {
FreeListNode *next;
/* This is really a list of array memory blocks, using the
* first item as the next pointer to chain them together.
* Protected by array lock */
static FreeListNode *freelist = NULL;
/* must hold array lock */
static gpointer
freelist_alloc (gsize size, gboolean reuse)
gpointer mem;
FreeListNode *free, **prev;
gsize real_size;
if (reuse)
for (free = freelist, prev = &freelist; free != NULL; prev = &free->next, free = free->next)
if (G_ATOMIC_ARRAY_DATA_SIZE (free) == size)
*prev = free->next;
return (gpointer)free;
real_size = sizeof (gsize) + MAX (size, sizeof (FreeListNode));
mem = g_slice_alloc (real_size);
mem = ((char *) mem) + sizeof (gsize);
return mem;
/* must hold array lock */
static void
freelist_free (gpointer mem)
FreeListNode *free;
free = mem;
free->next = freelist;
freelist = free;
_g_atomic_array_init (GAtomicArray *array)
array->data = NULL;
/* Get a copy of the data (if non-NULL) that
* can be changed and then re-applied with
* g_atomic_array_update().
* If additional_element_size is > 0 then
* then the new memory chunk is that much
* larger, or there were no data we return
* a chunk of header_size + additional_element_size.
* This means you can use this to grow the
* array part and it handles the first element
* being added automatically.
* We don't support shrinking arrays, as if
* we then re-grow we may reuse an old pointer
* value and confuse the transaction check.
_g_atomic_array_copy (GAtomicArray *array,
gsize header_size,
gsize additional_element_size)
guint8 *new, *old;
gsize old_size, new_size;
G_LOCK (array);
old = g_atomic_pointer_get (&array->data);
if (old)
old_size = G_ATOMIC_ARRAY_DATA_SIZE (old);
new_size = old_size + additional_element_size;
/* Don't reuse if copying to same size, as this may end
up reusing the same pointer for the same array thus
confusing the transaction check */
new = freelist_alloc (new_size, additional_element_size != 0);
memcpy (new, old, old_size);
else if (additional_element_size != 0)
new_size = header_size + additional_element_size;
new = freelist_alloc (new_size, TRUE);
new = NULL;
G_UNLOCK (array);
return new;
/* Replace the data in the array with the new data,
* freeing the old data (for reuse). The new data may
* not be smaller than the current data.
_g_atomic_array_update (GAtomicArray *array,
gpointer new_data)
guint8 *old;
G_LOCK (array);
old = g_atomic_pointer_get (&array->data);
g_assert (old == NULL || G_ATOMIC_ARRAY_DATA_SIZE (old) <= G_ATOMIC_ARRAY_DATA_SIZE (new_data));
g_atomic_pointer_set (&array->data, new_data);
if (old)
freelist_free (old);
G_UNLOCK (array);