Reference counted data

Reference counted data — Allocated memory with reference counting semantics

Includes

#include <glib.h>
#include <glib/gi18n.h>

Description

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:

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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():

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// 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:

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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():

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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, see the atomic reference counted data API.

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():

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():

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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_clear)

Functions

g_rc_box_alloc ()

gpointer
g_rc_box_alloc (gsize block_size);

Allocates block_size bytes of memory, and adds reference counting semantics to it.

The data will be freed when its reference count drops to zero.

Parameters

block_size

the size of the allocation, must be greater than 0

 

Returns

a pointer to the allocated memory.

[transfer full][not nullable]

Since: 2.58


g_rc_box_alloc0 ()

gpointer
g_rc_box_alloc0 (gsize block_size);

Allocates block_size bytes of memory, and adds reference counting semantics to it.

The contents of the returned data is set to zero.

The data will be freed when its reference count drops to zero.

Parameters

block_size

the size of the allocation, must be greater than 0

 

Returns

a pointer to the allocated memory.

[transfer full][not nullable]

Since: 2.58


g_rc_box_new()

#define             g_rc_box_new(type)

A convenience macro to allocate reference counted data with the size of the given type .

This macro calls g_rc_box_alloc() with sizeof (@type) and casts the returned pointer to a pointer of the given type , avoiding a type cast in the source code.

Parameters

type

the type to allocate, typically a structure name

 

Returns

a pointer to the allocated memory, cast to a pointer for the given type .

[transfer full][not nullable]

Since: 2.58


g_rc_box_new0()

#define             g_rc_box_new0(type)

A convenience macro to allocate reference counted data with the size of the given type , and set its contents to zero.

This macro calls g_rc_box_alloc0() with sizeof (@type) and casts the returned pointer to a pointer of the given type , avoiding a type cast in the source code.

Parameters

type

the type to allocate, typically a structure name

 

Returns

a pointer to the allocated memory, cast to a pointer for the given type .

[transfer full][not nullable]

Since: 2.58


g_rc_box_dup ()

gpointer
g_rc_box_dup (gsize block_size,
              gconstpointer mem_block);

Allocates a new block of data with reference counting semantics, and copies block_size bytes of mem_block into it.

Parameters

block_size

the number of bytes to copy, must be greater than 0

 

mem_block

the memory to copy.

[not nullable]

Returns

a pointer to the allocated memory.

[transfer full][not nullable]

Since: 2.58


g_rc_box_acquire ()

gpointer
g_rc_box_acquire (gpointer mem_block);

Acquires a reference on the data pointed by mem_block .

Parameters

mem_block

a pointer to reference counted data.

[not nullable]

Returns

a pointer to the data, with its reference count increased.

[transfer full][not nullable]

Since: 2.58


g_rc_box_release ()

void
g_rc_box_release (gpointer mem_block);

Releases a reference on the data pointed by mem_block .

If the reference was the last one, it will free the resources allocated for mem_block .

Parameters

mem_block

a pointer to reference counted data.

[transfer full][not nullable]

Since: 2.58


g_rc_box_release_full ()

void
g_rc_box_release_full (gpointer mem_block,
                       GDestroyNotify clear_func);

Releases a reference on the data pointed by mem_block .

If the reference was the last one, it will call clear_func to clear the contents of mem_block , and then will free the resources allocated for mem_block .

Parameters

mem_block

a pointer to reference counted data.

[transfer full][not nullable]

clear_func

a function to call when clearing the data.

[not nullable]

Since: 2.58


g_rc_box_get_size ()

gsize
g_rc_box_get_size (gpointer mem_block);

Retrieves the size of the reference counted data pointed by mem_block .

Parameters

mem_block

a pointer to reference counted data.

[not nullable]

Returns

the size of the data, in bytes

Since: 2.58

Types and Values