Reference counting types

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.

Reference counted data

A “reference counted box”, or “RcBox”, is an opaque wrapper data type that is guaranteed to be as big as the size of a given data type, and which augments the given data type with reference counting semantics for its memory management.

RcBox is useful if you have a plain old data type, like a structure typically placed on the stack, and you wish to provide additional API to use it on the heap; or if you want to implement a new type to be passed around by reference without necessarily implementing copy/free semantics or your own reference counting.

The typical use is:

typedef struct {
  char *name;
  char *address;
  char *city;
  char *state;
  int age;
} Person;

Person *
person_new (void)
{
  return g_rc_box_new0 (Person);
}

Every time you wish to acquire a reference on the memory, you should call g_rc_box_acquire(); similarly, when you wish to release a reference you should call g_rc_box_release():

// Add a Person to the Database; the Database acquires ownership
// of the Person instance
void
add_person_to_database (Database *db, Person *p)
{
  db->persons = g_list_prepend (db->persons, g_rc_box_acquire (p));
}

// Removes a Person from the Database; the reference acquired by
// add_person_to_database() is released here
void
remove_person_from_database (Database *db, Person *p)
{
  db->persons = g_list_remove (db->persons, p);
  g_rc_box_release (p);
}

If you have additional memory allocated inside the structure, you can use g_rc_box_release_full(), which takes a function pointer, which will be called if the reference released was the last:

void
person_clear (Person *p)
{
  g_free (p->name);
  g_free (p->address);
  g_free (p->city);
  g_free (p->state);
}

void
remove_person_from_database (Database *db, Person *p)
{
  db->persons = g_list_remove (db->persons, p);
  g_rc_box_release_full (p, (GDestroyNotify) person_clear);
}

If you wish to transfer the ownership of a reference counted data type without increasing the reference count, you can use g_steal_pointer():

  Person *p = g_rc_box_new (Person);

  // fill_person_details() is defined elsewhere
  fill_person_details (p);

  // add_person_to_database_no_ref() is defined elsewhere; it adds
  // a Person to the Database without taking a reference
  add_person_to_database_no_ref (db, g_steal_pointer (&p));

Thread safety

The reference counting operations on data allocated using g_rc_box_alloc(), g_rc_box_new(), and g_rc_box_dup() are not thread safe; it is your code’s responsibility to ensure that references are acquired are released on the same thread.

If you need thread safe reference counting, you should use the g_atomic_rc_* API:

Operation Atomic equivalent
g_rc_box_alloc() g_atomic_rc_box_alloc()
g_rc_box_new() g_atomic_rc_box_new()
g_rc_box_dup() g_atomic_rc_box_dup()
g_rc_box_acquire() g_atomic_rc_box_acquire()
g_rc_box_release() g_atomic_rc_box_release()
g_rc_box_release_full() g_atomic_rc_box_release_full()

The reference counting operations on data allocated using g_atomic_rc_box_alloc(), g_atomic_rc_box_new(), and g_atomic_rc_box_dup() are guaranteed to be atomic, and thus can be safely be performed by different threads. It is important to note that only the reference acquisition and release are atomic; changes to the content of the data are your responsibility.

It is a programmer error to mix the atomic and non-atomic reference counting operations.

Automatic pointer clean up

If you want to add g_autoptr() support to your plain old data type through reference counting, you can use the G_DEFINE_AUTOPTR_CLEANUP_FUNC() and g_rc_box_release():

G_DEFINE_AUTOPTR_CLEANUP_FUNC (MyDataStruct, g_rc_box_release)

If you need to clear the contents of the data, you will need to use an ancillary function that calls g_rc_box_release_full():

static void
my_data_struct_release (MyDataStruct *data)
{
  // my_data_struct_clear() is defined elsewhere
  g_rc_box_release_full (data, (GDestroyNotify) my_data_struct_clear);
}

G_DEFINE_AUTOPTR_CLEANUP_FUNC (MyDataStruct, my_data_struct_release)