gtk/REFCOUNTING

How ref counting works within Gdk and Gtk
=========================================

Each data structure that provides ref counting offers a bunch of
functions that follow these conventions:

  *_new:      Create a new structure with a reference count of 1.
  *_ref:      Increase ref count by one.
  *_unref:    Decrease ref count by one.  If the count drops to zero,
              run aprropriate finalization code and free the memory.
              No user visible actions should take place, like
              destryoing windows, etc.

Some structures also provide a *_destroy function, but it is generally
unrelated to freeing the memory.  `Destroying' merely renders an
object `unusable'.  But as long as there are references to it, it will
stick around.

GdkWindow
---------

A GdkWindow has to be explicitely destroyed with gdk_window_destroy.
This will send out a request to destroy this window and all its
children, and will decrement the ref_count of the GdkWindow by one.
Thus, it releases the inital reference created by gdk_window_new.

All GdkWindows are kept in a hash table to translate from their XId to
the actual structure and the pointer in the hash table is reflected in
the reference count.  When a DestroyNotify event is received for a
particular GdkWindow, it is removed from the hash table and the
ref_count is updated accordingly.

You can call gdk_window_destroy more than once on a particular
GdkWindow, it will only be destroyed when it hasn't been yet.  The
ref_count is *always* decremented, tho. Be careful.
 
GdkPixmap
---------

There is no gdk_pixmap_destroy function.  The Pixmap is destroyed when
the last reference to it vanishes.

GdkPixmaps are kept in the same hash table as GdkWindows but the
pointer in the hash table is *not* reflected in the ref_count.

This works only when Pixmaps never get XEvents.  I'm not sure if this
is the case.

GdkBitmap
---------

A GdkBitmap is only another name for a special use of GdkPixmap.

GdkVisual
---------

There are no *_new or *_destroy functions and the *_ref and *_unref
functions are noops.  GdkVisuals are static structures and thus do not
need reference counting.  The ref counting functions are only there
for extra defensive programming.

GdkColormap
-----------

Nothing special.  There is no gdk_colormap_destroy function.

GdkFont / GdkFontSet
--------------------

GdkFont and GdkFontSet are equivalent as far as ref counting is
concerned.  Use gdk_font_ref and gdk_font_unref for both.

There is no gdk_font_free or gdk_fontset_free function.

GtkAcceleratorTable
-------------------

There is no gtk_accelerator_table_destroy function.

GtkTooltips
-----------

There is no gtk_tooltips_destroy function.

GtkStyle
--------

There is no gtk_style_destroy function.

GtkObject
---------

GtkObjects follow the usual ref_counting strategy, but with a twist.

They are created with a ref_count of 1.  GtkObjects are able able to
run finalization code when the ref_count drops to zero but you cannot
register arbitrary signal handlers to run at finalization time.

There is also the old gtk_object_destroy function and the "destroy"
signal but they are somewhat independent from finalization.  Just as
stated at the top of this text, gtk_object_destroy merely renders an
object unusable.  When the object is a container widget for example,
it unrealizes that widget, removes all children and disconnects all
signal handlers.  The finalization code is different, it would for
example free associated memory for text strings and release the
attached style.

[This is the biggest change.  Every widget must be revised to have a
 proper "destroy" function, etc.  Such a destroy function must be able
 to be called any number of times and generally leave the widget in a
 minimal but consistent state.  The "finalization" function is new and
 should perform last-minute cleanup actions.  It can assume that the
 "destroy" function has been called as the last function on this
 widget.

 Essentially, the old "destroy" function has been split into a
 "finalize" plus a "destroy" function.]

It is not possible to create GtkObjects with a ref_count of 0 (as it
is done now) because the first ref/unref pair will destroy it
unintentionally.

To be mostly backward compatible with existing practice, a GtkObject
leads a more complicated life than the other reference counted structures.

When a GtkObject is created, it starts out in a special state called
"floating" (this is the twist).  This means that it is alive and has a
reference to it, but the `owner' of this reference is not known.
There are certain `potential owners' that will adopt a floating
GtkObject.  For GtkWidgets the most common adopters are the parent
widget.

When you want to adopt a possibly floating GtkObject, you call
gtk_object_sink on it.  This clears the floating state of the
GtkObject and decrements the ref_count by one, if it has been floating
previously.  Once the floating state has been cleared, it will never
be set again.

All widgets that are part of the display are linked into a
parent/child tree.  The link from the parent to a child is reflected
in the ref_count of the child, but the link from the child to the
parent is not reflected in the ref_count of the parent.

Like a GtkObject, a GtkWidget is created with a ref_count of 1 and
initially flagged as `floating'.  As soon as it is added as a child to
a parent, the `floating' flag is cleared and never will be set again.
Not even when it is later unparented.  The act of clearing the
`floating' flag also decrements the ref_count of the widget by one.

When the widget is unparented, its underlying GdkWindow is destroyed
(when it has one), it loses its reference from the parent and
naturally the ref_count is decremented.

It is considered a bug if a widget still has a GdkWindow when it is
being freed.

Toplevel widgets, which don't have a `natural' parent, are adopted by
a special widget, maybe a GtkDisplay or GtkScreen.  This special
parent of all toplevel widgets is never freed.  The toplevel widgets
are added to this parent as soon as they are created. [Maybe this
special widget will only exist conceptually because toplevel widgets
are identified by parent == NULL through-out the code.]

So, the typical career of a GtkWindow and the GtkButton that sits in
it looks like this:

  window = gtk_window_new (GTK_WINDOW_TOPLEVEL);

  // window is created with ref_count == 1.  It is not flagged as
  // `floating' because it has already been added to the special
  // parent of all toplevel widgets.

  button = gtk_button_new_with_label ("Yo!");

  // button->ref_count == 1 and it is flagged as `floating'.

  gtk_container_add (window, button);

  // button->ref_count still == 1, but it is no longer `floating'.

  gtk_widget_show (button);
  gtk_widget_show (window);

  // The widgets get their GdkWindows, nothing significant happens to
  // the ref_counts.

Then, when the user wants to get rid of the window:

  gtk_widget_destroy (window);

  // The GdkWindow of `window' and all its child GdkWindows are
  // destroyed.

  // window is removed from its (conceptual) parent and its ref_count
  // drops to zero.  The destroy code of `window' destroyes `button'.

  // The destriction of the button removes it from its parent, the
  // button->ref_count drops to zero and the button is freed, too.


- Marius Vollmer <mvo@zagadka.ping.de>