new gtk2-perl

This document began its life as a post to gtk-perl-list about a redesign of the fundamentals of the bindings; it rapidly grew too large, and a pointer to the document got posted instead. As a redesign description, it also included a fair amount of comparison and justification.

The proposal was accepted, and despite evolutionary changes, this remains the most complete documentation for the technical details of the bindings. Thus, i've tried to keep it up to date with important changes in implementation and API. Eventually all the "why not inline" justification will go away, but i'm leaving it for now because we haven't hit a stable release yet and people may still be wondering why we started over, or if they can use Inline to write their own extensions.

If you come across anything outdated or incorrect, please let me know.
-- muppet <scott at asofyet dot org>
(address mangled to foil robots; replace the words with the punctuation!)

In the following i refer to the original Gtk-Perl-0.7008 as gtk-perl, the existing unfinished Inline-based perl bindings for gtk2 as gtk2-perl, and my new unfinished XS-based gtk2 bindings as new-gtk2-perl.

Basic Philosophy

I designed my package with a few basic tenets in mind.

The Glib Module

In keeping with the tenet of not requiring the entire car for someone who only needs a single wheel, I broke the glib/gobject library family into its own module and namespace. This has proved to be a godsend, as it has made things very easy to debug; there's a clean separation between the base of the type system and the stuff on top of it.

Actually, the Glib module was originally going to be the only thing I implemented, but it turned out that I couldn't test the code because the types it defined were all abstract. So, I started implementing Gtk2 as well, and that's that.

The Glib module takes care of all the basic types handled by the GObject library --- GEnum, GFlags, GBoxed, GObject, GValue, GClosure --- as well has signal marshalling and such in GSignal. I'll discuss each of these separately.


[NOTE: this stuff is due for reimplementation after 0.24, in order to make it behave more correctly. the basic usage will not change, but several important details will change for the better.]

In order to use the GObject types from perl, we need to wrap those objects up in perl objects. The basic perl wrapper is a blessed reference to a scalar which holds the actual underlying object's pointer value. Perl scalars are reference counted, and thus we can rely on perl to keep track of when the wrapper is no longer needed.

The fundamental point about the lifetime of an object and its wrapper is two-fold: the C object must always outlive the perl wrapper, and the perl wrapper must never point to something invalid.

If an object is created by a function that returns directly to perl, then the wrapper returned by that function should "own" the object. If no other code assumes ownership of that object (by ref'ing a GObject or copying a GBoxed), then the object should be destroyed when the perl scalar is destroyed (actually, as part of its destruction).

If a function returns a preexisting object owned by someone else, then the bindings should NOT destroy the object with the perl wrapper. How we handle this for the various types is described below.

GType to Package Mappings

Both gtk-perl and gtk2-perl use substitution rules to map GType classes to corresponding perl packages. The fundamental flaw here is that the substitution rules are not easily extendable and are easily broken by extension packages which don't follow the naming conventions.

To circumvent this shortcoming, I built in the idea of explicit mappings. There is no chance of tricking a substitution mechanism, and at runtime you do a hash table lookup instead of a complete regex match and sub. This also allows us to track which classes are and are not registered and catch other fun things.

In addition, the type system tries as hard as it can to recover when things don't go well, using the GType system to its advantage. If you return a C object of a type that is not registered with Gperl, such as MyCustomTypeFoo, gperl_new_object (see below) will warn you that it has blessed the unknown MyCustomTypeFoo into the first known package in its ancestry, Gtk2::VBox.

GBoxed and GObject have distinct mapping registries to avoid cross-pollination and mistakes in the type system. See below.

To assist in handling inheritance that isn't specified directly by the GType system, the function gperl_set_isa allows you to add elements to the @ISA for a package. gperl_register_object does this for you, but you may need to add additional parents, e.g., for implementing GInterfaces. (see Gtk2/xs/GtkEntry.xs for an example)

GEnums and GFlags

These are largely unchanged from gtk2-perl. My only thought is towards changing the function names from gperl_convert_enum, gperl_convert_back_enum, etc to gperl_enum_from_sv, gperl_sv_from_enum, etc.


GBoxed provides a way to register functions that create, copy, and destroy opaque structures. For our purposes, we'll allow any perl package to inherit from Glib::Boxed and implement accessors for the struct members, but Glib::Boxed will handle the object and wrapper lifetime issues.

There are two functions for creating boxed wrappers:

SV * gperl_new_boxed (gpointer boxed, GType gtype, gboolean own);
SV * gperl_new_boxed_copy (gpointer boxed, GType gtype);

If own is TRUE, the wrapper returned by gperl_new_boxed will take boxed with it when it dies. In the case of a copy, own is implied, so there's a separate function which doesn't need the own option.

To get a boxed pointer out of a scalar wrapper, you just call gperl_get_boxed_check --- this will croak if the sv is undef or not blessed into the specified package. In general, you'll want to call this function from C preprocessor macros used in a typemap; see the Gtk2 autogeneration description below.

When you register a boxed type you get the option of supplying a table of function pointers describing how the boxed object should be wrapped, unwrapped, and destroyed. This allows you to decide in the wrapping function what subclass of the boxed type's class the wrapper should actually take (a trick used by Gtk2::Gdk::Event), or represent a boxed type as a native perl type (such as using array references for Gnome2::Canvas::Point objects). All of this happens automagically, behind the scenes, and most types assume the default wrapper class. See the commentary in gperl.h for more information.


The GObject knows its own type. Thus, we need only one parameter to create a GObject wrapper. In reality, we ask for two:

SV * gperl_new_object (GObject * object, gboolean own);

The wrapper SV will be blessed into the package corresponding to the gtype returned by G_OBJECT_TYPE (object), that is, the bottommost type in the inheritance chain. If that bottommost type is not known, the function walks back up the tree until it finds one that's known, blesses the reference into that package, and spits out a warning on stderr. To hush the warning, you need merely call

In general, this process will claim a reference on the GObject (with g_object_ref()), so that the C object stays alive so long as there is a perl wrapper for it. If own is set to TRUE, the perl wrapper will claim ownership of the C object by removing that reference; in theory, for a new GObject, fresh from a constructor, this leaves the object with a single reference owned by the perl object. The next question out of your mouth should be, "But what about GObject derivatives that require sinking or other strange methods to claim ownership?" For the answer, see the GtkObject section's description of sink functions.

void gperl_register_object (GType gtype, const char * package);

This magical function also sets up the @ISA for the package to point to the package corresponding to g_type_parent (gtype). [Since this requires the parent package to be registered, there is a simple deferral mechanism, which means your @ISA might not be set until the next call to gperl_register_object.]

There are two ways to get an object out of an SV (though I think only one is really needed):

GObject * gperl_get_object (SV * sv);
GObject * gperl_get_object_check (SV * sv, GType gtype);

The second one is like the first, but croaks if the object is not derived from gtype.

You can get and set object data and object parameters just like you'd expect.


All of this GObject stuff wouldn't be very useful if you couldn't connect signals and closures. I got most of my handling code from gtk2-perl and pygtk, and it's pretty straightforward. The data member is optional, and must be a scalar. Callbacks are not eval'd (I kept ignoring missing functions because they didn't kill my app).

To connect perl subroutines to GSignals I use GClosures, which require the handling of GValues. Again, largely borrowed from working code.


Use a GPerlClosure wherever you could use a GClosure and things should work out great. FIXME say more here


Function pointers are required in many places throughout gtk+, usually for a callback to be used as a "foreach" function or for some other purpose. Unfortunately, a majority of these spots aren't designed to work with GClosures (usually by lacking a way to destroy data associated with the callback when it is no longer needed). For this purpose, the GPerlCallback wraps up the gruntwork of using perl's call_sv to use a callback function directly. FIXME say more here

The Gtk2 Module

After I got the Glib module partially implemented I wanted to test it, but realized that its types were all abstract and largely not instantiable. So, I had to implement some client code. Since there were questions about how to handle GtkObjects with their floating references and GdkEvents and all this stuff, I decided to get enough going to run the scribble example from the gtk source. Since Gtk is a lot larger than GObject, I also chose to use autogeneration to idiot-proof a lot of the more tedious stuff. Also, I intended to include Gdk, Atk, and Pango under the Gtk2 namespace, in the style of the existing gtk2-perl project.


GtkObject adds the idea of a floating reference to GObject. A GObject is created with one reference which must be explicitly removed by its owner. GtkObject has a floating reference which is sunk by the code which wants to own it. This makes it less painful to create lots of objects in a row (you don't have to unref them).

To allow for this difference in procedure for taking ownership of an object, Glib allows you to register a "sink" function for a particular class. When asked to create a wrapper that owns the object, gperl_new_object will compare the list of registered sink functions with the type of the object; if the object is descended from a type, that sink function will be run on the object. The default one is g_object_unref(), of course. (this is inspired by pygtk.)

Thus, in Gtk2::Object's boot code, we register gtk_object_sink as the sink func for types derived from GtkObject. Now all wrappers for these types will be owned the proper way.

Of course, since gtk_object_sink() does nothing if the object isn't floating, it doesn't hurt anything if you always call gperl_new_object with "own" set to TRUE. So, to make life a little easier, Gtk2 defines another function

SV * gtk2perl_new_gtkobject (GtkObject * o);

Which does nothing more than

return gperl_new_object (G_OBJECT (o), TRUE);

(This changed as of 0.24; previously this section warned you that you had to use gtk2perl_new_object to create GtkObjects; the addition of the generic object constructor Glib::Object->new makes that impractical, hence this change.)

It's also important to know that this is largely done for you by the typemap.

Typemap scheme

In the same way that the Glib module uses explicit one-to-one GType to package registrations, I decided it was most foolproof to use an explicit, even exhaustive XS typemap. In this way I could avoid problems such as finding the proper set of regexes to map $var to the type macro and all sort of other problems of extensibility. This of course means it must be autogenerated, but that's easy.

The other main feature of the typemap is that it masks in a very sensible way the differences between GObject and GtkObject, and makes it very easy to specify whether a wrapper owns the object it wraps. This is handled through the idea of a "variant", which is a term I made up just now because it sounds about right.

Basically, a variant is the name of the class with some suffix. For example, for the a GBoxed subclass such as GdkEvent, a header would do this:

typedef GdkEvent GdkEvent_ornull;
typedef GdkEvent GdkEvent_own;

#define SvGdkEvent(s)           (gperl_get_boxed_check ((s), GDK_TYPE_EVENT))
#define SvGdkEvent_ornull(s)    ((s)==&PL_sv_undef ? NULL : SvGdkEvent(s))

#define newSVGdkEvent(e)        (gperl_new_boxed ((e), GDK_TYPE_EVENT, FALSE))
#define newSVGdkEvent_own(e)    (gperl_new_boxed ((e), GDK_TYPE_EVENT, TRUE))
#define newSVGdkEvent_ornull(e) (e == NULL ? &PL_sv_undef ? newSVGdkEvent (e))

Then the typemap entries for its various variants would look like this:


	$var = SvGdkEvent ($arg);
	$var = SvGdkEvent_ornull ($arg); 

	$arg = newSVGdkEvent ($var); 
	$arg = newSVGdkEvent_ornull ($var); 
	$arg = newSVGdkEvent_own ($var); 

And with that, your XS wrapper code can look as simple as this:

GdkEvent_own *
gdk_get_event (class)
        SV * class

gdk_event_get_time (event)
        GdkEvent * event

Isn't that nice and simple?

The variants for the various types go like this:

/* no ext */ object will not be destroyed with wrapper
_own object will be destroyed with wrapper
_copy object will be copied (and copy will be owned)
_ornull undef/NULL is legal
/* no ext */ object's refcount will be increased (=>not owned)
_noinc object's refcount will not be increased (=>owned)
_ornull undef/NULL is legal
/* no ext */ everything is peachy
_ornull undef/NULL is legal

Obviously, this scheme calls for autogeneration for any number of classes larger than just four or five.


Auto-generated code is known for having problems because it doesn't pay enough attention to special cases, but it's also great for situations in which a human writing or maintaining it would simply go insane with the tedium. Handling the magnitude of classes in Gtk is something like that' Here's a description of what gets autogenerated in Gtk2.

Oh yeah, the generation takes place in two places: the boot.xsh is created by code in Gtk2/Makefile.PL, and Gtk2/, which is called by Gtk2/Makefile.PL, generates the rest.


This is the starting point for autogeneration. This map file serves the purpose of a defs file in other binding packages; it is the input to the code generator. This map lists the TYPE macro for each of the GObject types in all of the gtk headers (including gdk, gdk-pixbuf, atk, and pango), along with the actual name of the class, name of the package into which it is to be blessed, and the base type (not exactly the fundamental type). Most of those should be obvious except for the base type. The base type is one of GEnum, GFlags, GBoxed, GObject, GInterface, or GtkObject. This is the important flag which determines what kind of code gets created for each record; the GtkObject wrapper must be created differently from the GObject wrapper, for instance.

In this file, you can change the explicit name of an object. If you don't like PangoFontDescription being Gtk2::Pango::FontDescription, you can change it to Gtk2::Pango::Font::Desc::ription if you were so inclined (but please don't).

I have a script called that actually scans the gtk header files and creates and runs a small program to generate the maps file. The advantage here is that the type information comes directly from the code and I don't have to worry about clerical errors making the software incorrect. In practice, this should need to be run only when new classes are added to the base libraries.


This file contains the typedefs and cast macros. This includes all the variant stuff described above.


The exhaustive typemap uses the macros defined in gtk2perl-autogen.h so that you are assured to get the same results from typemap generated code as from hand-written perl stack manipulation.


Included from the boot code of the toplevel Gtk2 module, this file lists all of the types in the maps file as a series of calls to the appropriate package registration functions (gperl_register_boxed or gperl_register_object). This is done before the boot code below so that hand-written code may override it. This code gets called when your program does a "use Gtk2".


The Gtk2 module is made up of dozens of XS files but only one PM file. calls bootstrap on Gtk2, but not on any of the others (because it doesn't know about them). It is a module's boot code which binds the xsubs into perl, so it's imperative that the modules get booted!

So, Makefile.PL scans the xs/ subdirectory for all the MODULE = ... lines in the XS files. It maps these to boot code symbols, and generates code to call these symbols in boot.xsh, which is then included by the boot code for the toplevel module, right after register.xsh. (The generation code takes steps to avoid spitting out the same symbol more than once, and will not emit code to boot the toplevel module (or else you get an infinite loop).

Just a point of style; you can change packages in an XS file by repeating the MODULE = ... line with a different PACKAGE (and possibly PREFIX) value. It's a good idea, however, to keep the MODULE the same, so that only one boot symbol gets generated per file.

Why Inline Is Not the Way To Go for Gtk2-Perl

I couldn't have done this in the Inline framework. Trust me, I tried. I looked at this stuff for weeks, trying to figure out how to change the underlying code without breaking the whole shebang, and there just wasn't a feasible way. On the other hand, I'm writing this monster document only 7 days after starting the pure-XS bindings project.

I originally just wanted to play with reference counts, but the entire Inline build system got very much in my way. There was just too much infrastructure there; this file loads that file, which decides what the symbol prefix should be based on some hard-coded list of substitutions, and then it bootstraps this module, which has perl code that calls an XS wrapper to a C wrapper function that calls all sorts of helper functions before finally calling the actual Gtk function. I couldn't see the forest for the trees, and I found myself having to dig into the undocumented internals of things far too often.

So, I switched to pure XS for my sandbox. Using a combination of typemaps and castmacros (borrowed from gtk2-perl) serves to make the XS implementation very clean.

Basically, this code from the Inline version of gtk2-perl

SV* gtkperl_message_dialog_new (char* class, SV* parent, SV* flags, SV* type, SV* buttons, char* message)
    return gtk2_perl_new_object(gtk_message_dialog_new(SvGtkWindow_nullok(parent),
                                SvGtkDialogFlags(flags), SvGtkMessageType(type),
                                SvGtkButtonsType(buttons), message));

Becomes this in XS:

GtkWidget *
gtk_message_dialog_new (class, parent, flags, type, buttons, message)
        SV * class
        GtkWindow_ornull * parent
        GtkDialogFlags flags
        GtkMessageType type
        GtkButtonsType buttons
        gchar * message
        parent, flags, type, buttons, message

Personally, I think the XS version is easier to figure out, quicker to use, and more maintainable in the long run. This works for the vast majority of functions, most of which don't even need the C_ARGS line. For others you have the complete XS back of tricks at your disposal: reducing code size by using the ALIAS keyword; using default parameters and vararg functions to make life easier on the perl side; and a wealth of documentation on the XS API both in pod and published paperback form.

I know there has been some reluctance on this list to look at XS because of its learning curve, but let me give you a laundry list of reasons that XS is the better choice for this project:

Where to Go From Here

As stated above, I think the inline architecture stinks and we would greatly benefit from moving to XS. I have a mostly functional XS code tree sitting here, in my hands, which I intend to use from here on out because it fits my needs to extensibility. I want to continue contributing to the public gtk2-perl project, and I hereby give you all the code.

While it may seem rash to throw away the Inline tree in favor of another, I don't see it that way: