/* grefcount.c: Reference counting

 *

 * Copyright 2018  Emmanuele Bassi

 *

 * SPDX-License-Identifier: LGPL-2.1-or-later

 *

 * 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 2.1 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

 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU

 * 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 <http://www.gnu.org/licenses/>.

 */

/**

 * SECTION:refcount

 * @Title: Reference counting

 * @Short_description: Reference counting types and functions

 *

 * Reference counting is a garbage collection mechanism that is based on

 * assigning a counter to a data type, or any memory area; the counter is

 * increased whenever a new reference to that data type is acquired, and

 * decreased whenever the reference is released. Once the last reference

 * is released, the resources associated to that data type are freed.

 *

 * GLib uses reference counting in many of its data types, and provides

 * the #grefcount and #gatomicrefcount types to implement safe and atomic

 * reference counting semantics in new data types.

 *

 * It is important to note that #grefcount and #gatomicrefcount should be

 * considered completely opaque types; you should always use the provided

 * API to increase and decrease the counters, and you should never check

 * their content directly, or compare their content with other values.

 *

 * Since: 2.58

 */

#include "config.h"

#include "grefcount.h"

#include "gatomic.h"

#include "gmessages.h"

/**

 * grefcount:

 *

 * A type for implementing non-atomic reference count semantics.

 *

 * Use g_ref_count_init() to initialize it; g_ref_count_inc() to

 * increase the counter, and g_ref_count_dec() to decrease it.

 *

 * It is safe to use #grefcount only if you're expecting to operate

 * on the reference counter from a single thread. It is entirely up

 * to you to ensure that all reference count changes happen in the

 * same thread.

 *

 * See also: #gatomicrefcount

 *

 * Since: 2.58

 */

/**

 * gatomicrefcount:

 *

 * A type for implementing atomic reference count semantics.

 *

 * Use g_atomic_ref_count_init() to initialize it; g_atomic_ref_count_inc()

 * to increase the counter, and g_atomic_ref_count_dec() to decrease it.

 *

 * It is safe to use #gatomicrefcount if you're expecting to operate on the

 * reference counter from multiple threads.

 *

 * See also: #grefcount

 *

 * Since: 2.58

 */

/**

 * g_ref_count_init:

 * @rc: the address of a reference count variable

 *

 * Initializes a reference count variable to 1.

 *

 * Since: 2.58

 */

void

(g_ref_count_init) (grefcount *rc)

{

  g_return_if_fail (rc != NULL);

  /* Non-atomic refcounting is implemented using the negative range

   * of signed integers:

   *

   * G_MININT                 Z¯< 0 > Z⁺                G_MAXINT

   * |----------------------------|----------------------------|

   *

   * Acquiring a reference moves us towards MININT, and releasing a

   * reference moves us towards 0.

   */

  *rc = -1;

}

/**

 * g_ref_count_inc:

 * @rc: the address of a reference count variable

 *

 * Increases the reference count.

 *

 * Since: 2.58

 */

void

(g_ref_count_inc) (grefcount *rc)

{

  grefcount rrc;

  g_return_if_fail (rc != NULL);

  rrc = *rc;

  g_return_if_fail (rrc < 0);

  /* Check for saturation */

  if (rrc == G_MININT)

    {

      g_critical ("Reference count %p has reached saturation", rc);

      return;

    }

  rrc -= 1;

  *rc = rrc;

}

/**

 * g_ref_count_dec:

 * @rc: the address of a reference count variable

 *

 * Decreases the reference count.

 *

 * If %TRUE is returned, the reference count reached 0. After this point, @rc

 * is an undefined state and must be reinitialized with

 * g_ref_count_init() to be used again.

 *

 * Returns: %TRUE if the reference count reached 0, and %FALSE otherwise

 *

 * Since: 2.58

 */

gboolean

(g_ref_count_dec) (grefcount *rc)

{

  grefcount rrc;

  g_return_val_if_fail (rc != NULL, FALSE);

  rrc = *rc;

  g_return_val_if_fail (rrc < 0, FALSE);

  rrc += 1;

  if (rrc == 0)

    return TRUE;

  *rc = rrc;

  return FALSE;

}

/**

 * g_ref_count_compare:

 * @rc: the address of a reference count variable

 * @val: the value to compare

 *

 * Compares the current value of @rc with @val.

 *

 * Returns: %TRUE if the reference count is the same

 *   as the given value

 *

 * Since: 2.58

 */

gboolean

(g_ref_count_compare) (grefcount *rc,

                       gint       val)

{

  grefcount rrc;

  g_return_val_if_fail (rc != NULL, FALSE);

  g_return_val_if_fail (val >= 0, FALSE);

  rrc = *rc;

  if (val == G_MAXINT)

    return rrc == G_MININT;

  return rrc == -val;

}

/**

 * g_atomic_ref_count_init:

 * @arc: the address of an atomic reference count variable

 *

 * Initializes a reference count variable to 1.

 *

 * Since: 2.58

 */

void

(g_atomic_ref_count_init) (gatomicrefcount *arc)

{

  g_return_if_fail (arc != NULL);

  /* Atomic refcounting is implemented using the positive range

   * of signed integers:

   *

   * G_MININT                 Z¯< 0 > Z⁺                G_MAXINT

   * |----------------------------|----------------------------|

   *

   * Acquiring a reference moves us towards MAXINT, and releasing a

   * reference moves us towards 0.

   */

  *arc = 1;

}

/**

 * g_atomic_ref_count_inc:

 * @arc: the address of an atomic reference count variable

 *

 * Atomically increases the reference count.

 *

 * Since: 2.58

 */

void

(g_atomic_ref_count_inc) (gatomicrefcount *arc)

{

  gint old_value;

  g_return_if_fail (arc != NULL);

  old_value = g_atomic_int_add (arc, 1);

  g_return_if_fail (old_value > 0);

  if (old_value == G_MAXINT)

    g_critical ("Reference count has reached saturation");

}

/**

 * g_atomic_ref_count_dec:

 * @arc: the address of an atomic reference count variable

 *

 * Atomically decreases the reference count.

 *

 * If %TRUE is returned, the reference count reached 0. After this point, @arc

 * is an undefined state and must be reinitialized with

 * g_atomic_ref_count_init() to be used again.

 *

 * Returns: %TRUE if the reference count reached 0, and %FALSE otherwise

 *

 * Since: 2.58

 */

gboolean

(g_atomic_ref_count_dec) (gatomicrefcount *arc)

{

  gint old_value;

  g_return_val_if_fail (arc != NULL, FALSE);

  old_value = g_atomic_int_add (arc, -1);

  g_return_val_if_fail (old_value > 0, FALSE);

  return old_value == 1;

}

/**

 * g_atomic_ref_count_compare:

 * @arc: the address of an atomic reference count variable

 * @val: the value to compare

 *

 * Atomically compares the current value of @arc with @val.

 *

 * Returns: %TRUE if the reference count is the same

 *   as the given value

 *

 * Since: 2.58

 */

gboolean

(g_atomic_ref_count_compare) (gatomicrefcount *arc,

                              gint             val)

{

  g_return_val_if_fail (arc != NULL, FALSE);

  g_return_val_if_fail (val >= 0, FALSE);

  return g_atomic_int_get (arc) == val;

}