1devel(3) User Contributed Perl Documentation devel(3)
2
6 Glib::devel - Binding developer's overview of Glib's internals
7
9 Do you need to know how the gtk2-perl language bindings work, or need
10 to write your own language bindings for a Glib/Gtk2-based library?
11 Then you've come to the right place. If you are just a perl developer
12 wanting to write programs with Glib or Gtk2, then this is probably way
13 over your head.
14 This document began its life as a post to gtk-perl-list about a
16 redesign of the fundamentals of the bindings; today it is the reference
17 documentation for the developers of the bindings.
18 To reduce confusion, refer to GLib, the C library, with a capital L,
20 and Glib the perl module with a lower-case l. While the Gtk2 module is
21 the primary client of Glib, it is not necessarily the only one; in
22 fact, the perl bindings for the GStreamer library build directly atop
23 Glib. Therefore, this document describes just the GLib/Glib basics.
24 For details on how Gtk2 extends upon the concepts presented here, see
25 Gtk2::devel.
26 In various places, we use the name GPerl to refer to the actual binding
28 subsystem.
29 In order to avoid getting very quickly out of date, this document
31 doesn't go into great detail on APIs. gperl.h is rather heavily
32 commented, and should be considered the canonical source of correct API
33 information.
34
36 GLib is a portability library for C programs, providing a common set of
37 APIs and services on various platforms. Along with that you get
38 libgobject, which provides an inheritance-based type system and other
39 spiffy things.
40 Glib, as a perl module, must decide which portions of GLib's facilities
42 to map to perl and which to abstract and encapsulate.
43 In the grand scheme, the bindings have been designed with a few basic
45 tenets in mind:
46 - Stick close to the C API, to allow a perl developer to use
48 knowledge from the C API and API reference docs with the perl
49 bindings; this is overruled in some places by the remaining tenets.
50 - Be perlish. This is the most important. The user of the perl
52 bindings should not have to worry about memory management,
53 reference counting, freeing objects, and all that stuff, else he
54 might as well go write in C instead.
55 - Leave out deprecated functionality.
57 - Don't add new functionality. The exceptions to this rule are
59 consolidation of methods where default parameters may be used, or
60 where the direct analog from C is not practical.
61 - Be lightweight. As little indirection and bloat as possible. If
63 possible, implement each toplevel module (e.g., Glib, Gtk2, Gnome2,
64 GtkHTML, etc) as one .pm and one .so.
65 - Be extensible. Export header files and typemaps so that other
67 modules can easily chain off of our base. Do not require the
68 entirely of Gtk2 for someone who needs only to build atop GObject.
69
71 In keeping with the tenet of not requiring the entire car for someone
72 who only needs a single wheel, I broke the glib/gobject library family
73 into its own module and namespace. This has proved to be a godsend, as
74 it has made things very easy to debug; there's a clean separation
75 between the base of the type system and the stuff on top of it.
76 The Glib module takes care of all the basic types handled by the
78 GObject library --- GEnum, GFlags, GBoxed, GObject, GValue, GClosure
79 --- as well has signal marshalling and such in GSignal. I'll discuss
80 each of these separately.
81 In practice, you will rarely see direct calls to the functions that
83 convert objects in and out of perl. Most code should use the C
84 preprocessor to provide easier-to-remember names that follow the perl
85 API style, e.g., newSVGObject(obj) rather than
86 gperl_new_object(type,obj) and SvGObject(sv) instead of
87 gperl_get_gobject(type, sv). The convention used in all of gtk2-perl
88 is described in Gtk2::devel.
89 Wrappers
91 FIXME maybe this section should be rolled into the GBoxed and GObject
92 sections?
93 In order to use the C data structures from Perl, we need to wrap those
95 objects up in Perl objects. In general, a Perl object is simply a
96 blessed reference. A typical scheme for representing C objects in perl
97 is bless a reference to a scalar holding the C pointer value; perl will
98 destroy the reference-counted scalar when there are no more references
99 to it, and one would normally destroy the underlying data structure at
100 this point. However, GLib is a little more complex than your typical C
101 library, so this easy, typical setup won't work for us.
102 GBoxed types are opaque wrappers for C structures, providing copy and
104 free functions, to allow the types to be used generically. For the
105 most part we can get away with using the typical scheme described above
106 to provide an opaque object, but in some instances an opaque object is
107 very alien in perl. The Glib::Boxed section explains how we get around
108 this.
109 GObject, on the other hand, is a type-aware, reference-counted object
111 with lifetime semantics that differ somewhat from perl SVs. Thus we
112 need something a bit more sophisticated than a plain old opaque
113 wrapper; in fact, we use a blessed hash reference with the pointer to
114 the C object tucked away in attached magic, and a pointer to the SV
115 stored in the GObject's user data. The combined perl/C object does
116 some nifty reference-count borrowing to ensure that object lifetime is
117 managed correctly.
118 If an object is created by a function that returns directly to perl,
120 then the wrapper returned by that function should "own" the object. If
121 no other code assumes ownership of that object (by ref'ing a GObject or
122 copying a GBoxed), then the object should be destroyed when the perl
123 scalar is destroyed (actually, as part of its destruction).
124 If a function returns a preexisting object owned by someone else, then
126 the bindings should NOT destroy the object with the perl wrapper. How
127 we handle this for the various types is described below.
128 GType to Package Mappings
130 GType is the GObject library's unique type identifier; this is a
131 runtime variable, because GLib types may be loaded dynamically. The
132 direct analog in perl is the package name, which uniquely specifies an
133 object's class. Since these do about the same thing, we completely
134 replace the GType with the perl package.
135 For various reasons, mostly to do with robustness and performance,
137 there is a one-to-one mapping between GType classes and perl package
138 names. These must be registered, usually as part of the module
139 initialization process.
140 In addition, the type system tries as hard as it can to recover when
142 things don't go well, using the GType system to its advantage. If you
143 return a C object of a type that is not registered with Gperl, such as
144 MyCustomTypeFoo, gperl_new_object (see below) will warn you that it has
145 blessed the unknown MyCustomTypeFoo into the first known package in its
146 ancestry, Gtk2::VBox.
147 GBoxed and GObject have distinct mapping registries to avoid cross-
149 pollination and mistakes in the type system. See below.
150 To assist in handling inheritance that isn't specified directly by the
152 GType system, the function gperl_set_isa allows you to add elements to
153 the @ISA for a package. gperl_register_object does this for you, but
154 you may need to add additional parents, e.g., for implementing
155 GInterfaces. (see Gtk2/xs/GtkEntry.xs for an example)
156 You may be thinking that we could use substitution rules to map the
158 GObject classes to perl packages. In practice, this is a bad idea,
159 fraught with problems; the substitution rules are not easily extendable
160 and are easily broken by extension packages which don't follow the
161 naming conventions.
162 GEnums and GFlags
164 GLib provides a mechanism for creating runtime type information about
165 enumeration and flag types. Enumerations are lists of specific values,
166 one of which may be used at at time, whereas multiple flag values may
167 be supplied at a time. In C flags are meant to be used with bitfields.
168 A GType is associated with the various valid values for a given GEnum
169 or GFlags type as strings, in both full-name and nickname forms.
170 GPerl uses this mechanism to avoid the need to know integer values for
172 enum and flag types at the perl level. An enum value is just a string;
173 a bitfield of flag values is represented as a reference to an array of
174 strings. These strings are the GLib-provided nicknames. For the
175 convenience of a perl developer, the bindings treat '-' and '_' as
176 equivalent when looking up the corresponding integer values during
177 conversion.
178 A GEnum or GFlags type mapping should be registered with
180 void gperl_register_fundamental (GType gtype, const char * package);
182 so that their package names can be used where a GType is required (for
184 example, as GObject property types or GtkTreeModel column types).
185 The basic functions for converting between C and perl values are
187 /* croak if val is not part of type, otherwise return
189 * corresponding value. this is the general case. */
190 gint gperl_convert_enum (GType type, SV * val);
191 /* return a scalar which is the nickname of the enum value
193 * val, or croak if val is not a member of the enum. */
194 SV * gperl_convert_back_enum (GType type, gint val);
195 /* collapse a list of strings to an integer with all the
197 * correct bits set, croak if anything is invalid. */
198 gint gperl_convert_flags (GType type, SV * val);
199 /* convert a bitfield to a list of strings, or croak. */
201 SV * gperl_convert_back_flags (GType type, gint val);
202 Other utility functions allow for finer-grained control, such as the
204 ability to pass unknown values, which can be necessary in special
205 cases. In general, each of these functions raises an exception when
206 something goes wrong. To be helpful, they croak with a message listing
207 the valid values when they encounter invalid input.
208 GBoxed
210 GBoxed provides a way to register functions that create, copy, and
211 destroy opaque structures. For our purposes, we'll allow any perl
212 package to inherit from Glib::Boxed and implement accessors for the
213 struct members, but Glib::Boxed will handle the object and wrapper
214 lifetime issues.
215 There are two functions for creating boxed wrappers:
217 SV * gperl_new_boxed (gpointer boxed, GType gtype, gboolean own);
219 SV * gperl_new_boxed_copy (gpointer boxed, GType gtype);
220 If own is TRUE, the wrapper returned by gperl_new_boxed will take boxed
222 with it when it dies. In the case of a copy, own is implied, so
223 there's a separate function which doesn't need the own option.
224 To get a boxed pointer out of a scalar wrapper, you just call
226 gperl_get_boxed_check --- this will croak if the sv is undef or not
227 blessed into the specified package.
228 When you register a boxed type you get the option of supplying a table
230 of function pointers describing how the boxed object should be wrapped,
231 unwrapped, and destroyed. This allows you to decide in the wrapping
232 function what subclass of the boxed type's class the wrapper should
233 actually take (a trick used by Gtk2::Gdk::Event), or represent a boxed
234 type as a native perl type (such as using array references for
235 Gnome2::Canvas::Point objects). All of this happens automagically,
236 behind the scenes, and most types assume the default wrapper class.
237 See the commentary in gperl.h for more information.
239 GObject
241 The GObject knows its own type. Thus, we need only one parameter to
242 create a GObject wrapper. In reality, we ask for two:
243 SV * gperl_new_object (GObject * object, gboolean own);
245 The wrapper SV will be blessed into the package corresponding to the
247 gtype returned by G_OBJECT_TYPE (object), that is, the bottommost type
248 in the inheritance chain. If that bottommost type is not known, the
249 function walks back up the tree until it finds one that's known,
250 blesses the reference into that package, and spits out a warning on
251 stderr. To hush the warning, you need merely call
252 In general, this process will claim a reference on the GObject (with
254 g_object_ref()), so that the C object stays alive so long as there is a
255 perl wrapper for it. If <i>own</i> is set to TRUE, the perl wrapper
256 will claim ownership of the C object by removing that reference; in
257 theory, for a new GObject, fresh from a constructor, this leaves the
258 object with a single reference owned by the perl object. The next
259 question out of your mouth should be, "But what about GObject
260 derivatives that require sinking or other strange methods to claim
261 ownership?" For the answer, see the GtkObject section's description of
262 sink functions.
263 void gperl_register_object (GType gtype, const char * package);
265 This magical function also sets up the @ISA for the package to point to
267 the package corresponding to g_type_parent (gtype). [Since this
268 requires the parent package to be registered, there is a simple
269 deferral mechanism, which means your @ISA might not be set until the
270 next call to gperl_register_object.]
271 There are two ways to get an object out of an SV (though I think only
273 one is really needed):
274 GObject * gperl_get_object (SV * sv);
276 GObject * gperl_get_object_check (SV * sv, GType gtype);
277 The second one is like the first, but croaks if the object is not
279 derived from gtype.
280 You can get and set object data and object parameters just like you'd
282 expect.
283 GSignal
285 All of this GObject stuff wouldn't be very useful if you couldn't
286 connect signals and closures. I got most of my handling code from
287 gtk2-perl and pygtk, and it's pretty straightforward. The data member
288 is optional, and must be a scalar.
289 To connect perl subroutines to GSignals I use GClosures, which require
291 the handling of GValues.
292 GPerlClosure
294 Use a GPerlClosure wherever you could use a GClosure and things should
295 work out great. FIXME say more here
296 GPerlCallback
298 Function pointers are required in many places throughout gtk+, usually
299 for a callback to be used as a "foreach" function or for some other
300 purpose. Unfortunately, a majority of these spots aren't designed to
301 work with GClosures (usually by lacking a way to destroy data
302 associated with the callback when it is no longer needed). For this
303 purpose, the GPerlCallback wraps up the gruntwork of using perl's
304 call_sv to use a callback function directly.
305
307 perl(1), perlxs(1), perlguts(1), perlapi(1), perlxstut(1),
308 ExtUtils::Depends(3pm), ExtUtils::PkgConfig(3pm) Glib(3pm),
309 Glib::Object::Subclass(3pm), Glib::xsapi(3pm)
310
312 muppet <scott at asofyet.org>
313
315 Copyright (C) 2003 by the gtk2-perl team (see the file AUTHORS for the
316 full list)
317 This library is free software; you can redistribute it and/or modify it
319 under the terms of the GNU Library General Public License as published
320 by the Free Software Foundation; either version 2.1 of the License, or
321 (at your option) any later version.
322 This library is distributed in the hope that it will be useful, but
324 WITHOUT ANY WARRANTY; without even the implied warranty of
325 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
326 Library General Public License for more details.
327 You should have received a copy of the GNU Library General Public
329 License along with this library; if not, write to the Free Software
330 Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
331 02110-1301 USA.
332perl v5.28.1 2012-01-06 devel(3)