path: root/docs/faq/gtkfaq.sgml

<!doctype linuxdoc system>

<article>

<!-- Title information -->

<title>GTK+ FAQ

<!-- NOTE: Use only one author tag, otherwise sgml2txt barfs - TRG --> 
<author>Tony Gale, Shawn T. Amundson, Emmanuel Deloget, Nathan Froyd

<date>October 30th 1999

<abstract> This document is intended to answer questions that are likely to be 
frequently asked by programmers using GTK+ or people who are just looking at 
using GTK+. 
</abstract>

<!-- Table of contents -->
<toc>

<!-- Begin the document -->

<!-- ***************************************************************** -->
<sect>General Information

<!-- ----------------------------------------------------------------- -->
<sect1>Before anything else: the greetings
<p>
The FAQ authors want to thank:
<itemize>
<item>Havoc Pennington
<item>Erik Mouw
<item>Owen Taylor
<item>Tim Janik
<item>Thomas Mailund Jensen
<item>Joe Pfeiffer
<item>Andy Kahn
<item>Federico Mena Quintero
<item>Damon Chaplin
<item>and all the members of the GTK+ lists
</itemize>
If we forgot you, please email us !
Thanks again (I know, it's really short :)

<!-- ----------------------------------------------------------------- -->
<sect1>Authors
<p>
The original authors of GTK+ were:

<itemize>
<item>Peter Mattis
<item>Spencer Kimball
<item>Josh MacDonald
</itemize>

Since then, much has been added by others. Please see the AUTHORS
file in the distribution for the GTK+ Team.

<!-- ----------------------------------------------------------------- -->
<sect1>What is GTK+?
<p>
GTK+ is a small and efficient widget set designed with the general
look and feel of Motif.  In reality, it looks much better than Motif.
It contains common widgets and some more complex widgets such as a
file selection, and color selection widgets.

GTK+ provides some unique features. (At least, I know of no other
widget library which provides them). For example, a button does not
contain a label, it contains a child widget, which in most instances
will be a label.  However, the child widget can also be a pixmap,
image or any combination possible the programmer desires.  This
flexibility is adhered to throughout the library.

<!-- ----------------------------------------------------------------- -->
<sect1>What is the + in GTK+?
<P>
Peter Mattis informed the gtk mailing list that:
<quote>
"I originally wrote gtk which included the three libraries, libglib,
libgdk and libgtk. It featured a flat widget hierarchy. That is, you
couldn't derive a new widget from an existing one. And it contained
a more standard callback mechanism instead of the signal mechanism now
present in gtk+. The + was added to distinguish between the original
version of gtk and the new version. You can think of it as being an
enhancement to the original gtk that adds object oriented features."
</quote>

<!-- ----------------------------------------------------------------- -->
<sect1>Does the G in GTK+, GDK and GLib stand for?
<p>
GTK+ == Gimp Toolkit

GDK == Gtk+ Drawing Kit

GLib == G Libray

<!-- ----------------------------------------------------------------- -->
<sect1>Where is the documentation for GTK+?
<p>
In the GTK+ distribution's doc/ directory you will find the
reference material for both GTK and GDK, this FAQ and the
GTK Tutorial.

In addition, you can find links to HTML versions of these documents 
by going to <htmlurl url="http://www.gtk.org/" 
name="http://www.gtk.org/">. A packaged version of the GTK Tutorial,
with SGML, HTML, Postscript, DVI and text versions can be found in
<htmlurl url="ftp://ftp.gtk.org/pub/gtk/tutorial"
name="ftp://ftp.gtk.org/pub/gtk/tutorial">

There are now a couple of books available that deal with programming
GTK+, GDK and GNOME:
<itemize>
<item> Eric Harlows book entitled "Developing Linux Applications with
GTK+ and GDK". The ISBN is 0-7357-0021-4
<P>
The example code from Eric's book is available on-line at
<htmlurl url="http://www.bcpl.net/~eharlow/book"
name="http://www.bcpl.net/~eharlow/book">

<item> Havoc Pennington has released a book called "GTK+/GNOME
Application Development". The ISBN is 0-7357-0078-8
<P>
The free version of the book lives here:
<htmlurl url="http://developer.gnome.org/doc/GGAD/"
name="http://developer.gnome.org/doc/GGAD/">
<P>
And Havoc maintains information about it and errata here:
<htmlurl url="http://pobox.com/~hp/gnome-app-devel.html"
name="http://pobox.com/~hp/gnome-app-devel.html">
</itemize>

<!-- ----------------------------------------------------------------- -->
<sect1>Is there a mailing list (or mailing list archive) for GTK+?
<p>
Information on mailing lists relating to GTK+ can be found at:

<htmlurl url="http://www.gtk.org/mailinglists.html"
name="http://www.gtk.org/mailinglists.html">

<!-- ----------------------------------------------------------------- -->
<sect1>How to get help with GTK+
<p>
First, make sure your question isn't answered in the documentation,
this FAQ or the tutorial. Done that? You're sure you've done that,
right? In that case, the best place to post questions is to the GTK+
mailing list.

<!-- ----------------------------------------------------------------- -->
<sect1>How to report bugs in GTK+
<p>
Bugs should be reported to the GNOME bug tracking
system (<htmlurl url="http://bugs.gnome.org"
name="http://bugs.gnome.org">). To report a problem about GTK+, send
mail to submit@bugs.gnome.org.

The subject of the mail should describe your problem. In the body of
the mail, you should first include a "pseudo-header" that gives the
package and version number. This should be separated by a blank line
from the actual headers.

<verb>
 Package: gtk+
 Version: 1.2.0
</verb>

Substitute 1.2.0 with the version of GTK+ that you have installed.

Then describe the bug. Include:

<itemize>
<item> Information about your system. For instance:
   <itemize>
   <item> What operating system and version
   <item> What version of X
   <item> For Linux, what version of the C library
   </itemize>
  And anything else you think is relevant.

<item> How to reproduce the bug. 

  If you can reproduce it with the testgtk program that is built in
  the gtk/ subdirectory, that will be most convenient. Otherwise,
  please include a short test program that exhibits the behavior. As
  a last resort, you can also provide a pointer to a larger piece of
  software that can be downloaded.

  (Bugs that can be reproduced within the GIMP are almost as good as
  bugs that can be reproduced in testgtk. If you are reporting a bug
  found with the GIMP, please include the version number of the GIMP
  you are using)

<item> If the bug was a crash, the exact text that was printed out
  when the crash occured.

<item> Further information such as stack traces may be useful, but are
  not necessary. If you do send a stack trace, and the error is an X
  error, it will be more useful if the stacktrace is produced running
  the test program with the <tt/--sync/ command line option.
</itemize>

<!-- ----------------------------------------------------------------- -->
<sect1>Is there a Windows version of GTK+?
<p>
There is an on going port of GTK+ to the Windows platform which is
making impressive progress.

See <htmlurl url="http://www.iki.fi/tml/gimp/win32"
name="http://www.iki.fi/tml/gimp/win32"> for more information.

<!-- ----------------------------------------------------------------- -->
<sect1>What applications have been written with GTK+?
<p>
A list of some GTK+ based application can be found on the GTK+ web
server at <htmlurl url="http://www.gtk.org/apps/"
name="http://www.gtk.org/apps/"> and contains more than 350
applications.

Failing that, look for a project to work on for the GNOME project,
<htmlurl url="http://www.gnome.org/" name="http://www.gnome.org/">
Write a game. Write something that is useful.

Some of these are:
<itemize>
<item>GIMP (<htmlurl url="http://www.gimp.org/" 
                  name="http://www.gimp.org/">), 
        an image manipulation program
<item>AbiWord (<htmlurl url="http://www.abisource.com/" 
        name="http://www.abisource.com/">),
        a professional word processor
<item>GUBI (<htmlurl url="http://www.SoftHome.net/pub/users/timj/gubi/index.htm"
            name="http://www.SoftHome.net/pub/users/timj/gubi/index.htm">),
        a user interface builder
<item>Gzilla (<htmlurl url="http://www.levien.com/gzilla/" 
        name="http://www.levien.com/gzilla/">),
        a web browser
<item>SANE (<htmlurl url="http://www.azstarnet.com/~axplinux/sane/" 
        name="http://www.azstarnet.com/~axplinux/sane/"> ),
        a universal scanner interface
<item>XQF (<htmlurl url="http://www.botik.ru/~roma/quake/" 
        name="http://www.botik.ru/~roma/quake/">),
        a QuakeWorld/Quake2 server browser and launcher
<item>ElectricEyes (<htmlurl url="http://www.labs.redhat.com/ee.shtml" 
        name="http://www.labs.redhat.com/ee.shtml">),
        an image viewer that aims to be a free replacement for xv
<item>GPK - the General Proxy Kit (<htmlurl url="http://www.humanfactor.com/gpk/" 
        name="http://www.humanfactor.com/gpk/">),
        an add-on library to permit thread-safe access to GTK+
<item>GCK - the General Convenience Kit (<htmlurl url="http://www.ii.uib.no/~tomb/gck.html" 
        name="http://www.ii.uib.no/~tomb/gck.html">),
        miscellaneous functions intended to ease color handling, UI construction,
        vector operations, and math functions
<item>GDK Imlib (<htmlurl url="http://www.labs.redhat.com/imlib/" 
        name="http://www.labs.redhat.com/imlib/">),
        a fast image loading and manipulation library for GDK
<item>Glade (<htmlurl url="http://glade.pn.org/"
        name="http://glade.pn.org/">),
        a GTK+ based RAD tool which produces GTK+ applications
</itemize>
<p>
In addition to the above, the GNOME project (<htmlurl
url="http://www.gnome.org" name="http://www.gnome.org">) is using GTK+
to build a free desktop for Linux. Many more programs can be found
there.

<!-- ----------------------------------------------------------------- -->
<sect1>I'm looking for an application to write in GTK+. How about an IRC client? 
<p> 

Ask on gtk-list for suggestions. There are at least three IRC clients
already under development (probably more in fact. The server at
<htmlurl url="http://www.forcix.cx/irc-clients.html"
name="http://www.forcix.cx/irc-clients.html"> list a bunch of them).

<itemize>
<item>X-Chat.
<item>girc. (Included with GNOME)
<item>gsirc. (In the gnome CVS tree)
</itemize>

<!-- ***************************************************************** -->
<sect>How to find, configure, install, and troubleshoot GTK+

<!-- ***************************************************************** -->

<!-- ----------------------------------------------------------------- -->
<sect1>What do I need to run GTK+?
<p>
To compile GTK+, all you need is a C compiler (gcc) and the X Window System
and associated libraries on your system.

<!-- ----------------------------------------------------------------- -->
<sect1>Where can I get GTK+?
<p>
The canonical site is:
<verb>
ftp://ftp.gtk.org/pub/gtk
</verb>
This site tends to get busy around the time of a new GTK+ release
so try and use one of the mirror sites that are listed in
<htmlurl url="ftp://ftp.gtk.org/etc/mirrors"
name="ftp://ftp.gtk.org/etc/mirrors">

Here's a few mirror sites to get you started:
<itemize>
<item>Africa - ftp://ftp.is.co.za/applications/gimp/
<item>Australia - ftp://ftp.au.gimp.org/pub/gimp/
<item>Finland - ftp://ftp.funet.fi/pub/sci/graphics/packages/gimp
<item>Germany - ftp://infosoc.uni-koeln.de/pub/ftp.gimp.org/
<item>Japan - ftp://SunSITE.sut.ac.jp/pub/archives/packages/gimp/
<item>UK - ftp://ftp.flirble.org/pub/X/gimp/
<item>US - ftp://ftp.insync.net/pub/mirrors/ftp.gimp.org/
</itemize>

<sect1>How do I configure/compile GTK+?
<p>
Generally, all you will need to do is issue the commands:
<verb>
./configure
make
</verb>
in the gtk+-version/ directory.

<!-- ----------------------------------------------------------------- -->
<sect1>When compiling GTK+ I get an error like: 
<tt/make: file `Makefile' line 456: Syntax error/
<p>
Make sure that you are using GNU make (use <tt/make -v/ to check). There are
many weird and wonderful versions of make out there, and not all of them
handle the automatically generated Makefiles.

<!-- ----------------------------------------------------------------- -->
 
<sect1>I've compiled and installed GTK+, but I can't get any programs to link with it!
<p>
This problem is most often encountered when the GTK+ libraries can't be 
found or are the wrong version. Generally, the compiler will complain about an
'unresolved symbol'.  There are two things you need to check:
<itemize>
<item>Make sure that the libraries can be found. You want to edit 
/etc/ld.so.conf to include the directories which contain the GTK libraries,
 so it looks something like:
<verb>
/usr/X11R6/lib
/usr/local/lib
</verb>
Then you need to run /sbin/ldconfig as root. You can find what directory
GTK is in using
<verb>
gtk-config --libs
</verb>

If your system doesn't use ld.so to find libraries (such as Solaris), then
you will have to use the LD_LIBRARY_PATH environment variable (or compile
the path into your program, which I'm not going to cover here). So, with a
Bourne type shell you can do (if your GTK libraries are in /usr/local/lib):
<verb>
export LD_LIBRARY_PATH=/usr/local/lib
</verb>
and in a csh, you can do:
<verb>
setenv LD_LIBRARY_PATH /usr/local/lib
</verb>

<item>Make sure the linker is finding the correct set of libraries. If you
have a Linux distribution that installs GTK+ (e.g. RedHat 5.0) then this 
older version may be used. Now (assuming you have a RedHat
system), issue the command
<verb>
rpm -e gtk gtk-devel
</verb>
You may also want to remove the packages that depend on gtk (rpm will tell you
which ones they are).  If you don't have a RedHat Linux system, check to make sure
that neither <verb>/usr/lib</verb> or <verb>/usr/local/lib</verb> contain any of
the libraries libgtk, libgdk, libglib, or libgck.  If they do exist, remove them
(and any gtk include files, such as /usr/include/gtk and /usr/include/gdk) 
and reinstall gtk+.
</itemize>

<!-- ----------------------------------------------------------------- -->
<sect1>When compiling programs with GTK+, I get compiler error messages about not being able to find <tt/"glibconfig.h"/.
<p>
The header file "glibconfig.h" was moved to the directory
$exec_prefix/lib/glib/include/. $exec_prefix is the
directory that was specified by giving the --exec-prefix
flags to ./configure when compiling GTK+. It defaults to 
$prefix, (specified with --prefix), which in turn defaults
to /usr/local/.

This was done because "glibconfig.h" includes architecture
dependent information, and the rest of the include files
are put in $prefix/include, which can be shared between different
architectures. 

GTK+ includes a shell script, <tt/gtk-config/, that
makes it easy to find out the correct include paths.
The GTK+ tutorial includes an example of using <tt/gtk-config/
for simple compilation from the command line. For information
about more complicated configuration, see the file
docs/gtk-config.txt in the GTK+ distribution.

If you are trying to compile an old program, you may
be able to work around the problem by configuring it
with a command line like: 

<tscreen><verb>
CPPFLAGS="-I/usr/local/include/glib/include" ./configure
</verb></tscreen>

for Bourne-compatible shells like bash, or for csh variants:

<tscreen><verb>
setenv CPPFLAGS "-I/usr/local/include/glib/include" 
./configure
</verb></tscreen>

(Substitute the appropriate value of $exec_prefix for /usr/local.)

<!-- ----------------------------------------------------------------- -->
<sect1>When installing a GTK+ application, configure reports that it can't find GTK.
<p>
There are several common reasons for this:
<itemize>
<item>You have an old version of GTK installed somewhere. RedHat 5.0, for 
example, installs an older copy of GTK that may not work with the latest 
applications. You should remove this old copy, but note that in the case
of RedHat 5.0 this will break the <tt/control-panel/ applications.
<P>
<item><tt/gtk-config/ (or another component of GTK) isn't in your path, or
there is an old version on your system. Type:
<verb>
gtk-config --version
</verb>
to check for both of these. If it returns a value different from what
you expect, then you have an old version of GTK on your system.
<P>
<item>The ./configure script can't find the GTK libraries. As ./configure
compiles various test programs, it needs to be able to find the GTK
libraries. See the question above for help on this.
</itemize>
<p>
If none of the above help, then have a look in config.log, which is
generated by ./configure as it runs. At the bottom will be the last
action it took before failing. If it is a section of source code, copy
the source code to a file and compile it with the line just above it in
config.log. If the compilation is successful, try executing it.

<!-- ***************************************************************** -->
<sect>Development of GTK+
<!-- ***************************************************************** -->

<!-- ----------------------------------------------------------------- -->
<sect1>Whats this CVS thing that everyone keeps talking about, and how do I access it?
<p>
CVS is the Concurent Version System and is a very popular means of 
version control for software projects. It is designed to allow multiple 
authors to be able to simultanously operate on the same source tree. 
This source tree is centrally maintained, but each developer has a
local mirror of this repository that they make there changes to.

The GTK+ developers use a CVS repository to store the master copy of
the current development version of GTK+. As such, people wishing to
contribute patches to GTK+ should generate them against the CVS version.
Normal people should use the packaged releases.

The CVS toolset is available as RPM packages from the usual RedHat sites.
The latest version is available at 
<htmlurl url="http://download.cyclic.com/pub/" 
name="&lt;http://download.cyclic.com/pub/&gt;">

Anyone can download the latest CVS version of GTK+ by using anonymous access
using the following steps:
<itemize>
<item> In a bourne shell descendant (e.g. bash) type:
<verb>
CVSROOT=':pserver:anonymous@anoncvs.gnome.org:/cvs/gnome'
export CVSROOT
</verb>
<item>Next, the first time the source tree is checked out, a cvs login 
is needed. 
<verb>
cvs login
</verb>
This will ask you for a password. There is no password for cvs.gimp.org, 
so just enter a carriage return. 
<item>To get the tree and place it in a subdir of your current working
directory, issue the command: 
<verb>
cvs -z3 get gtk+
</verb>

Note that with the GTK+ 1.1 tree, glib has been moved to a separate CVS
module, so if you don't have glib installed you will need to get that
as well:
<verb>
cvs -z3 get glib
</verb>

</itemize>
<!-- ----------------------------------------------------------------- -->
<sect1>How can I contribute to GTK+?
<p>
It's simple. If something doesn't work like you think it should in a program,
check the documentation to make sure you're not missing something. If it is a
true bug or missing feature, track it down in the GTK+ source, change it, 
and then generate a patch in the form of a 'context diff'. This can be done
using a command such as <tt/diff -ru &lt;oldfile&gt; &lt;newfile&gt;/. 
Then upload the patchfile to:
<verb>
ftp://ftp.gtk.org/incoming
</verb>
along with a README file.  Make sure you follow the naming conventions or your
patch will just be deleted! The filenames should be of this form:
<verb>
gtk-<username>-<date yymmdd-n>.patch.gz
gtk-<username>-<date yymmdd-n>.patch.README
</verb>
The "n" in the date indicates a unique number (starting from 0)
of patches you uploaded that day.  It should be 0, unless you
upload more than one patch in the same day.

Example:
<verb>
gtk-gale-982701-0.patch.gz
gtk-gale-982701-0.patch.README
</verb>
Once you upload <em>anything</em>, send the README to ftp-admin@gtk.org

<!-- ----------------------------------------------------------------- -->
<sect1>How do I know if my patch got applied, and if not, why not?
<p>
Uploaded patches will be moved to <tt>ftp://ftp.gtk.org/pub/gtk/patches</tt>
where one of the GTK+ development team will pick them up. If applied, they
will be moved to <tt>/pub/gtk/patches/old</tt>.

Patches that aren't applied, for whatever reason, are moved to 
<tt>/pub/gtk/patches/unapplied</tt> or <tt>/pub/gtk/patches/outdated</tt>.
At this point you can ask on the <tt/gtk-list/ mailing list why your patch
wasn't applied. There are many possible reasons why patches may not be
applied, ranging from it doesn't apply cleanly, to it isn't right. Don't
be put off if your patch didn't make it first time round.

<!-- ----------------------------------------------------------------- -->
<sect1>What is the policy on incorporating new widgets into the library?
<p>
This is up to the authors, so you will have to ask them once you
are done with your widget. As a general guideline, widgets that are 
generally useful, work, and are not a disgrace to the widget set will 
gladly be included.

<!-- ----------------------------------------------------------------- -->
<sect1>Is anyone working on bindings for languages other than C?
<p>
The GTK+ home page (<htmlurl url="http://www.gtk.org/" 
name="http://www.gtk.org/">) presents a list of GTK+ bindings.

<itemize>
<item>There are several C++ wrappers for GTK+.
  <itemize>
  
  <item>the gtk-- package, which is a very small wrapper for GTK+.
  You can find the home page at:
  <htmlurl url="http://www.cs.tut.fi/~p150650/gtk/gtk--.html"
  name="http://www.cs.tut.fi/~p150650/gtk/gtk--.html">. 
  The FTP site is 
  <htmlurl url="ftp://ftp.gtk.org/pub/gtk/gtk--"
  name="ftp://ftp.gtk.org/pub/gtk/gtk--">.
  
  <item>the VDK package, which was built as the base package of a GTK+
  application Borland-like builder. The home page can be found at
  <htmlurl url="www.guest.net/homepages/mmotta/VDKHome" 
  name="www.guest.net/homepages/mmotta/VDKHome">.

  <item>The wxWindows/Gtk package, a free C++ library for cross-platform 
  GUI development. The home page of this package is
  <htmlurl url="http://www.freiburg.linux.de/~wxxt/"
  name="http://www.freiburg.linux.de/~wxxt/">.

  </itemize> 
<p>

<item>There are three known Objective-c bindings currently in development:

  <itemize>

  <item>The <htmlurl url="http://www.gnome.org/" name="GNOME project's"> package
  of choice is objgtk. Objgtk is based on the Object class and is maintained by
  <htmlurl url="mailto:sopwith@cuc.edu" name="Elliot Lee">. Apparently, objgtk
  is being accepted as the `standard' Objective-C binding for GTK+.

  <item>If you are more inclined towards the 
  <htmlurl url="http://www.gnustep.org/" name="GNUstep project">,
  you may want to check out GTKKit by 
  <htmlurl url="mailto:helge@mdlink.de" name="Helge He&szlig;">.
  The intention is to setup a GTK+ binding using the FoundationKit. 
  GTKKit includes nicities like writing a XML-type template file to 
  construct a GTK+ interface.

  <item>The GToolKit package, which can be found at
  <htmlurl url="ftp://ftp.gtk.org/pub/gtk/objc-gtoolkit/" 
  name="ftp://ftp.gtk.org/pub/gtk/objc-gtoolkit/">.

 </itemize> 
<p>               
<item>Perl bindings
  <htmlurl url="ftp://ftp.gtk.org/pub/gtk/perl"
  name="ftp://ftp.gtk.org/pub/gtk/perl">
<P>
<item>Guile bindings. The home page is at
  <htmlurl url="http://www.ping.de/sites/zagadka/guile-gtk"
  name="http://www.ping.de/sites/zagadka/guile-gtk">.
  By the way, Guile is the GNU Project's implemention of R4RS Scheme (the
  standard). If you like Scheme, you may want to take a look at this.
<p>

<item>David Monniaux reports:
  <quote>I've started a gtk-O'Caml binding system.
  The basics of the system, including callbacks, work fine.

  The current development is in
  <htmlurl url="http://www.ens-lyon.fr/~dmonniau/arcs"
  name="http://www.ens-lyon.fr/~dmonniau/arcs">
  </quote>

<item> Several python bindings have been done:
<p>
  <itemize>
  <item>pygtk is at 
  <htmlurl url="http://www.daa.com.au/~james/pygtk" 
  name="http://www.daa.com.au/~james/pygtk"> and 
  <htmlurl url="ftp://ftp.gtk.org/pub/gtk/python"
  name="ftp://ftp.gtk.org/pub/gtk/python">

  <item>python-gtk is at
  <htmlurl url="http://www.ucalgary.ca/~nascheme/python-gtk" 
  name="http://www.ucalgary.ca/~nascheme/python-gtk">
  </itemize>
<p>
<item>There's are a couple of OpenGL/Mesa widgets available for
GTK+. I suggest you start at
  <htmlurl url="http://www.student.oulu.fi/~jlof/gtkglarea/index.html" 
  name="http://www.student.oulu.fi/~jlof/gtkglarea/index.html">
<p>
<item>Last, there are a lot of other language bindings for languages such as 
  Eiffel, TOM, Pascal, Pike, etc.

</itemize>

<!-- ***************************************************************** -->
<sect>Development with GTK+: the begining
<!-- ***************************************************************** -->
<!-- ----------------------------------------------------------------- -->
<sect1>How do I get started?
<p>
So, after you have installed GTK+ there are a couple of things that can
ease you into developing applications with it. There is the
GTK+ Tutorial <htmlurl url="http://www.gtk.org/tutorial/" 
name="&lt;http://www.gtk.org/tutorial/&gt;">, which is undergoing 
development. This will introduce you to writing applications using C.

The Tutorial doesn't (yet) contain information on all of the widgets
that are in GTK+. For example code on how to use the basics of all the
GTK+ widgets you should look at the file gtk/testgtk.c (and associated
source files) within the GTK+ distribution. Looking at these examples will
give you a good grounding on what the widgets can do.

<!-- ----------------------------------------------------------------- -->
<sect1>I tried to compile a small <tt/Hello World/ of mine, but it failed. Any clue?
<p>
Since you are good at coding, we will not deal with compile time error here :).

The classic command line to compile a GTK+ based program is

<verb>
gcc -o myprg [c files list] `gtk-config --cflags --libs`
</verb>

You should notice the backquote character which is used in this command line.
A common mistake when you start a GTK+ based development is to use quote 
instead of backquotes. If you do so, the compiler will complain about an 
unknown file called 'gtk-config --cflags --libs'. The text in
backquotes is an instruction to your shell to substitute the output of
executing this text into the command line.

The command line above ensure that:
<itemize>

  <item>the correct C compiler flags will be used to compile the program
        (including the complete C header directory list)

  <item>your program will be linked with the needed libraries.

</itemize>

<sect1>What about using the <tt/make/ utility?
<p>
This is a sample makefile which compile a GTK+ based program:

<tscreen><verb>
# basic GTK+ app makefile
SOURCES = myprg.c foo.c bar.c
OBJS    = ${SOURCES:.c=.o}
CFLAGS  = `gtk-config --cflags`
LDADD   = `gtk-config --libs`
CC      = gcc
PACKAGE = myprg

all : ${OBJS}
        ${CC} -o ${PACKAGE} ${OBJS} ${LDADD}

.c.o:
        ${CC} ${CFLAGS} -c $<

# end of file
</verb></tscreen>

For more information about the <tt/make/ utility, you should read either the
related man page or the relevant info file.

<sect1>I use the backquote stuff in my makefiles, but my make process failed.
<p>
The backquote construction seems to not be accepted by some old <tt/make/
utilities. If you use one of these, the make process will probably fail.
In order to have the backquote syntax working again, you should use the
GNU make utility (get it on the GNU ftp server at
<htmlurl url="ftp://ftp.gnu.org/" name="ftp://ftp.gnu.org/">).

<!-- ----------------------------------------------------------------- -->
<sect1>I want to add some configure stuff, how could I do this?
<p>
To use autoconf/automake, you must first install the relevant packages. These
are:

<itemize>
  <item>the m4 preprocessor v1.4 or better
  <item>autoconf v2.13 or better
  <item>automake v1.4 or better
</itemize>

You'll find these packages on the GNU main ftp server (<htmlurl 
url="ftp://ftp.gnu.org/" name="ftp://ftp.gnu.org/">) or on any GNU mirror.

In order to use the powerfull autoconf/automake scheme, you must create
a configure.in which may look like:

<tscreen><verb>
dnl Process this file with autoconf to produce a configure script.
dnl configure.in for a GTK+ based program

AC_INIT(myprg.c)dnl
AM_INIT_AUTOMAKE(mypkbname,0.0.1)dnl
AM_CONFIG_HEADER(config.h)dnl

dnl Checks for programs.
AC_PROG_CC dnl check for the c compiler
dnl you should add CFLAGS="" here, 'cos it is set to -g by PROG_CC

dnl Checks for libraries.
AM_PATH_GTK(1.2.0,,AC_MSG_ERROR(mypkgname 0.1 needs GTK))dnl

AC_OUTPUT(
	Makefile
)dnl
</verb></tscreen>

You must add a Makefile.am file:

<tscreen><verb>
bin_PROGRAMS    = myprg
myprg_SOURCES   = myprg.c foo.c bar.c
INCLUDES        = @GTK_CFLAGS@
LDADD           = @GTK_LIBS@
CLEANFILES      = *~
DISTCLEANFILES  = .deps/*.P
</verb></tscreen>

then, to use these, simply type the following commands:

<verb>
aclocal
autoheader
autoconf
automake --add-missing --include-deps --foreign 
</verb>

For further informations, you should look at the autoconf and the automake
documentation (the shipped info files are really easy to understand, and there
are plenty of web resources that deal with autoconf and/or automake).

<!-- ----------------------------------------------------------------- -->
<sect1>I try to debug my GTK+ application with gdb, but it hangs my X server when I hit some breakpoint. Any Idea ?
<p>
From Federico Mena Quintero:
<quote>
X is not locked up.  It is likely that you are hitting a breakpoint
inside a callback that is called from a place in Gtk that has a mouse
grab.
<P>
Run your program with the "--sync" option; it will make it easier to
debug. Also, you may want to use the console for running the
debugger, and just let the program run in another console with the X
server.
</quote>

Eric Mouw had another solution:
<quote>
An old terminal connected to an otherwise unused serial port is also great
for debugging X programs. Old vt100/vt220 terminals are dirt cheap but a
bit hard to get (here in The Netherlands, YMMV).
</quote>

<!-- ***************************************************************** -->
<sect>Development with GTK+: general questions
<!-- ***************************************************************** -->
<!-- ----------------------------------------------------------------- -->
<sect1>What widgets are in GTK?
<p>
The GTK+ Tutorial lists the following widgets:

<verb>
  GtkObject
   +GtkData
   | +GtkAdjustment
   | `GtkTooltips
   `GtkWidget
     +GtkContainer
     | +GtkBin
     | | +GtkAlignment
     | | +GtkEventBox
     | | +GtkFrame
     | | | `GtkAspectFrame
     | | +GtkHandleBox
     | | +GtkItem
     | | | +GtkListItem
     | | | +GtkMenuItem
     | | | | `GtkCheckMenuItem
     | | | |   `GtkRadioMenuItem
     | | | `GtkTreeItem
     | | +GtkViewport
     | | `GtkWindow
     | |   +GtkColorSelectionDialog
     | |   +GtkDialog
     | |   | `GtkInputDialog
     | |   `GtkFileSelection
     | +GtkBox
     | | +GtkButtonBox
     | | | +GtkHButtonBox
     | | | `GtkVButtonBox
     | | +GtkHBox
     | | | +GtkCombo
     | | | `GtkStatusbar
     | | `GtkVBox
     | |   +GtkColorSelection
     | |   `GtkGammaCurve
     | +GtkButton
     | | +GtkOptionMenu
     | | `GtkToggleButton
     | |   `GtkCheckButton
     | |     `GtkRadioButton
     | +GtkCList
     |   `GtkCTree
     | +GtkFixed
     | +GtkList
     | +GtkMenuShell
     | | +GtkMenuBar
     | | `GtkMenu
     | +GtkNotebook
     | +GtkPaned
     | | +GtkHPaned
     | | `GtkVPaned
     | +GtkScrolledWindow
     | +GtkTable
     | +GtkToolbar
     | `GtkTree
     +GtkDrawingArea
     | `GtkCurve
     +GtkEditable
     | +GtkEntry
     | | `GtkSpinButton
     | `GtkText
     +GtkMisc
     | +GtkArrow
     | +GtkImage
     | +GtkLabel
     | | `GtkTipsQuery
     | `GtkPixmap
     +GtkPreview
     +GtkProgressBar
     +GtkRange
     | +GtkScale
     | | +GtkHScale
     | | `GtkVScale
     | `GtkScrollbar
     |   +GtkHScrollbar
     |   `GtkVScrollbar
     +GtkRuler
     | +GtkHRuler
     | `GtkVRuler
     `GtkSeparator
       +GtkHSeparator
       `GtkVSeparator
</verb>

<!-- ----------------------------------------------------------------- -->
<sect1>Is GTK+ thread safe? How do I write multi-threaded GTK+ applications?
<p>
The GLib library can be used in a thread-safe mode by calling
g_thread_init() before making any other GLib calls. In this mode GLib
automatically locks all internal data structures as needed.  This
does not mean that two threads can simultaneously access, for
example, a single hash table, but they can access two different hash
tables simultaneously. If two different threads need to access the
same hash table, the application is responsible for locking
itself.

When GLib is intialized to be thread-safe, GTK+ is
<em>thread aware</em>. There is a single global lock
that you must acquire with gdk_threads_enter() before
making any GDK calls, and release with gdk_threads_leave()
afterwards.

A minimal main program for a threaded GTK+ application
looks like:

<verb>
int
main (int argc, char *argv[])
{
  GtkWidget *window;

  g_thread_init(NULL);
  gtk_init(&amp;argc, &amp;argv);

  window = create_window();
  gtk_widget_show(window);

  gdk_threads_enter();
  gtk_main();
  gdk_threads_leave();

  return(0);
}
</verb>

Callbacks require a bit of attention. Callbacks from GTK+
(signals) are made within the GTK+ lock. However callbacks
from GLib (timeouts, IO callbacks, and idle functions)
are made outside of the GTK+ lock. So, within a signal
handler you do not need to call gdk_threads_enter(), but
within the other types of callbacks, you do.

<!-- This is the old answer - TRG


Although GTK+, like many X toolkits, isn't thread safe, this does
not prohibit the development of multi-threaded applications with
GTK+. 

Rob Browning (rlb@cs.utexas.edu) describes threading techniques for
use with GTK+ (slightly edited): 

There are basically two main approaches, the first is simple, and the
second complicated. In the first, you just make sure that all GTK+ (or
X) interactions are handled by one, and
only one, thread. Any other thread that wants to draw something has
to somehow notify the "GTK+" thread, and let it handle the
actual work. 

The second approach allows you to call GTK+ (or X) functions from any
thread, but it requires some careful synchronization. The
basic idea is that you create an X protection mutex, and no one may
make any X calls without first acquiring this mutex. 

Note that this is a little effort, but it allows you to be
potentially more efficient than a completely thread safe GTK+. You
get to decide the granularity of the thread locking. You also have to
make sure that the thread that calls <tt/gtk_main()/ is holding the lock when
it calls <tt/gtk_main()/.

The next thing to worry about is that since you were holding the
global mutex when you entered <tt/gtk_main()/, all callbacks will also be
holding it. This means that the callback must release it if it's
going to call any other code that might reacquire it. Otherwise
you'll get deadlock. Also, you must be holding the mutex when you
finally return from the callback. 

In order to allow threads other than the one calling <tt/gtk_main/ to
get access to the mutex, we also need to register a work function
with GTK that allows us to release the mutex periodically.

Why can't GTK+ be thread safe by default?                           

Complexity, overhead, and manpower.  The proportion of threaded
programs is still reasonably small, and getting thread safety right is
both quite difficult and takes valuable time away from the main work
of getting a good graphics library finished.  It would be nice to have
GTK+ thread safe "out of the box", but that's not practical right now,
and it also might make GTK+ substantially less efficient if not handled
carefully.

Regardless, it's especially not a priority since relatively good
workarounds exist. -->
<!-- ----------------------------------------------------------------- -->
<sect1>Why do this strange 'x io error' occur when I <tt/fork()/ in my GTK+ app?
<p>
This is not really a GTK+ problem, and the problem is not related to <tt/fork()/
either. If the 'x io error' occurs then you probably use the <tt/exit()/ function
in order to exit from the child process.

When GDK opens an X display, it creates a socket file descriptor. When you use
the <tt/exit()/ function, you implicitly close all the open file descriptors,
and the underlying X library really doesn't like this.

The right function to use here is <tt/_exit()/. 

Erik Mouw gave the following code example to illustrate handling
fork() and exit().

<tscreen><verb>
/*-------------------------------------------------------------------------
 * Filename:      gtk-fork.c
 * Version:       0.99.1
 * Copyright:     Copyright (C) 1999, Erik Mouw
 * Author:        Erik Mouw <J.A.K.Mouw@its.tudelft.nl>
 * Description:   GTK+ fork example
 * Created at:    Thu Sep 23 21:37:55 1999
 * Modified by:   Erik Mouw <J.A.K.Mouw@its.tudelft.nl>
 * Modified at:   Thu Sep 23 22:39:39 1999
 *-----------------------------------------------------------------------*/
/*
 * Compile with:
 *
 * cc -o gtk-fork gtk-fork.c `gtk-config --cflags --libs`
 *
 */

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <gtk/gtk.h>

void sigchld_handler(int num)
{
  sigset_t set, oldset;
  pid_t pid;
  int status, exitstatus;

  /* block other incoming SIGCHLD signals */
  sigemptyset(&amp;set);
  sigaddset(&amp;set, SIGCHLD);
  sigprocmask(SIG_BLOCK, &amp;set, &amp;oldset);

  /* wait for child */
  while((pid = waitpid((pid_t)-1, &amp;status, WNOHANG)) > 0)
    {
      if(WIFEXITED(status))
	{
	  exitstatus = WEXITSTATUS(status);

	  fprintf(stderr, 
		  "Parent: child exited, pid = %d, exit status = %d\n", 
		  (int)pid, exitstatus);
	}
      else if(WIFSIGNALED(status))
	{
	  exitstatus = WTERMSIG(status);

	  fprintf(stderr,
		  "Parent: child terminated by signal %d, pid = %d\n",
		  exitstatus, (int)pid);
	}
      else if(WIFSTOPPED(status))
	{
	  exitstatus = WSTOPSIG(status);

	  fprintf(stderr,
		  "Parent: child stopped by signal %d, pid = %d\n",
		  exitstatus, (int)pid);
	}
      else
	{
	  fprintf(stderr,
		  "Parent: child exited magically, pid = %d\n",
		  (int)pid);
	}
    }

  /* re-install the signal handler (some systems need this) */
  signal(SIGCHLD, sigchld_handler);
  
  /* and unblock it */
  sigemptyset(&amp;set);
  sigaddset(&amp;set, SIGCHLD);
  sigprocmask(SIG_UNBLOCK, &amp;set, &amp;oldset);
}

gint delete_event(GtkWidget *widget, GdkEvent *event, gpointer data)
{
  return(FALSE);
}

void destroy(GtkWidget *widget, gpointer data)
{
  gtk_main_quit();
}

void fork_me(GtkWidget *widget, gpointer data)
{
  pid_t pid;

  pid = fork();

  if(pid == -1)
    {
      /* ouch, fork() failed */
      perror("fork");
      exit(-1);
    }
  else if(pid == 0)
    {
      /* child */
      fprintf(stderr, "Child: pid = %d\n", (int)getpid());

      execlp("ls", "ls", "-CF", "/", NULL);
      
      /* if exec() returns, there is something wrong */
      perror("execlp");

      /* exit child. note the use of _exit() instead of exit() */
      _exit(-1);
    }
  else
    {
      /* parent */
      fprintf(stderr, "Parent: forked a child with pid = %d\n", (int)pid);
    }
}

int main(int argc, char *argv[])
{
  GtkWidget *window;
  GtkWidget *button;

  gtk_init(&amp;argc, &amp;argv);

  /* the basic stuff: make a window and set callbacks for destroy and
   * delete events 
   */
  window = gtk_window_new(GTK_WINDOW_TOPLEVEL);

  gtk_signal_connect(GTK_OBJECT (window), "delete_event",
		     GTK_SIGNAL_FUNC(delete_event), NULL);
          
  gtk_signal_connect(GTK_OBJECT (window), "destroy",
		     GTK_SIGNAL_FUNC(destroy), NULL);

#if (GTK_MAJOR_VERSION == 1) && (GTK_MINOR_VERSION == 0)
  gtk_container_border_width(GTK_CONTAINER (window), 10);
#else  
  gtk_container_set_border_width(GTK_CONTAINER (window), 10);
#endif

  /* add a button to do something usefull */
  button = gtk_button_new_with_label("Fork me!");
          
  gtk_signal_connect(GTK_OBJECT (button), "clicked",
		     GTK_SIGNAL_FUNC(fork_me), NULL);

  gtk_container_add(GTK_CONTAINER(window), button);
          
  /* show everything */
  gtk_widget_show (button);
  gtk_widget_show (window);


  /* install a signal handler for SIGCHLD signals */
  signal(SIGCHLD, sigchld_handler);

  
  /* main loop */
  gtk_main ();

  exit(0);         
}
</verb></tscreen>

<!-- ----------------------------------------------------------------- -->
<sect1>Why don't the contents of a button move when the button is pressed? Here's a patch to make it work that way...
<p>
From: Peter Mattis

<quote>
The reason buttons don't move their child down and to the right when
they are depressed is because I don't think that's what is happening
visually. My view of buttons is that you are looking at them straight
on. That is, the user interface lies in a plane and you're above it
looking straight at it. When a button gets pressed it moves directly
away from you. To be absolutely correct I guess the child should
actually shrink a tiny amount. But I don't see why the child should
shift down and to the left. Remember, the child is supposed to be
attached to the buttons surface. Its not good for it to appear like
the child is slipping on the surface of the button.
<P>
On a more practical note, I did implement this at one point and
determined it didn't look good and removed it.
</quote>     

<!-- ----------------------------------------------------------------- -->
<sect1>How to I identifiy a widgets top level window or other ancestor?
<p>
There are a couple of ways to find the top level parent of a
widget. The easier way is to call the <tt/gtk_widget_top_level()/
function that returns a pointer to a GtkWidget that is the top level
window.

A more complicated way to do this (but less limited, as it allows
the user to get the closest ancestor of a known type) is to use
<tt/gtk_widget_get_ancestor()/ as in:

<tscreen><verb>
      GtkWidget       *widget;
      
      widget = gtk_widget_get_ancestor(w, GTK_TYPE_WINDOW);
</verb></tscreen>

Since virtually all the GTK_TYPEs can be used as the second parameter of
this function, you can get any parent widget of a particular
widget. Suppose you have an hbox which contains a vbox, which in turn contains
some other atomic widget (entry, label, etc. To find the master hbox
using the <tt/entry/ widget simply use:

<tscreen><verb>
      GtkWidget       *hbox;
      hbox = gtk_widget_get_ancestor(w, GTK_TYPE_HBOX);
</verb></tscreen>

<!-- ----------------------------------------------------------------- -->
<sect1>How do I get the Window ID of a GtkWindow?
<p>
The actual Gdk/X window will be created when the widget gets
realized. You can get the Window ID with:

<verb>
#include <gdk/gdkx.h>

Window xwin = GDK_WINDOW_XWINDOW (GTK_WIDGET (my_window)->window);
</verb>

<!-- ----------------------------------------------------------------- -->
<sect1>How do I catch a double click event (in a list widget, for example)?
<p>
Tim Janik wrote to gtk-list (slightly modified):

Define a signal handler:

<tscreen><verb>
gint
signal_handler_event(GtkWiget *widget, GdkEvenButton *event, gpointer func_data)
{
  if (GTK_IS_LIST_ITEM(widget) &&
       (event->type==GDK_2BUTTON_PRESS ||
        event->type==GDK_3BUTTON_PRESS) ) {
    printf("I feel %s clicked on button %d\",
           event->type==GDK_2BUTTON_PRESS ? "double" : "triple",
           event->button);
  }

  return FALSE;
}
</verb></tscreen>

And connect the handler to your object:

<tscreen><verb>
{
  /* list, list item init stuff */     

  gtk_signal_connect(GTK_OBJECT(list_item),
                     "button_press_event",
                     GTK_SIGNAL_FUNC(signal_handler_event),
                     NULL);

  /* and/or */

  gtk_signal_connect(GTK_OBJECT(list_item),
                     "button_release_event",
                     GTK_SIGNAL_FUNC(signal_handler_event),
                     NULL);

  /* something else */
}
</verb></tscreen>

and, Owen Taylor wrote:

Note that a single button press will be received beforehand, and
if you are doing this for a button, you will therefore also get a
"clicked" signal for the button. (This is going to be true for
any toolkit, since computers aren't good at reading one's
mind.)
                                       
<!-- ----------------------------------------------------------------- -->
<sect1>By the way, what are the differences between signals and events?
<p>
First of all, Havoc Pennington gives a rather complete description of the
differences between events and signals in his free book (two chapters can
be found at <htmlurl url="http://www106.pair.com/rhp/sample_chapters.html"
name="http://www106.pair.com/rhp/sample_chapters.html">).

Moreover, Havoc posted this to the <tt/gtk-list/
<quote>
  Events are a stream of messages received from the X server. They drive the
  Gtk main loop; which more or less amounts to "wait for events, process
  them" (not exactly, it is really more general than that and can wait on
  many different input streams at once). Events are a Gdk/Xlib concept.
<P>
  Signals are a feature of GtkObject and its subclasses. They have nothing
  to do with any input stream; really a signal is just a way to keep a list
  of callbacks around and invoke them ("emit" the signal). There are lots of
  details and extra features of course. Signals are emitted by object
  instances, and are entirely unrelated to the Gtk main loop.
  Conventionally, signals are emitted "when something changes" about the
  object emitting the signal.
<P>
  Signals and events only come together because GtkWidget happens to emit
  signals when it gets events. This is purely a convenience, so you can
  connect callbacks to be invoked when a particular widget receives a
  particular event. There is nothing about this that makes signals and
  events inherently related concepts, any more than emitting a signal when
  you click a button makes button clicking and signals related concepts.
</quote>

<!-- ----------------------------------------------------------------- -->
<sect1>I have my signal connected to the the (whatever) event, but it seems I don't catch it. What's wrong?
<p>
There is some special initialisation to do in order to catch some 
particular events. In fact, you must set the correct event mask bit of your
widget before getting some particular events.

For example, 

<tscreen><verb>
  gtk_widget_add_events(window, GDK_KEY_RELEASE_MASK);
</verb></tscreen>

lets you catch the key release events. If you want to catch every events,
simply us the GDK_ALL_EVENTS_MASK event mask.

All the event masks are defined in the <tt/gdktypes.h/ file.

<!-- ----------------------------------------------------------------- -->
<sect1>Is it possible to get some text displayed which is truncated to fit inside its allocation? 
<p>
GTK's behavior (no clipping) is a consequence of its attempts to
conserve X resources. Label widgets (among others) don't get their own
X window - they just draw their contents on their parent's window.
While it might be possible to have clipping occur by setting the clip
mask before drawing the text, this would probably cause a substantial
performance penalty.

Its possible that, in the long term, the best solution to such
problems might be just to change gtk to give labels X windows.
A short term workaround is to put the label widget inside another
widget that does get it's own window - one possible candidate would
be the viewport widget.

<tscreen><verb>
viewport = gtk_viewport (NULL, NULL);
gtk_widget_set_usize (viewport, 50, 25);
gtk_viewport_set_shadow_type (GTK_VIEWPORT(viewport), GTK_SHADOW_NONE);
gtk_widget_show(viewport);

label = gtk_label ("a really long label that won't fit");
gtk_container_add (GTK_CONTAINER(viewport), label);
gtk_widget_show (label);
</verb></tscreen>

If you were doing this for a bunch of widgets, you might want to
copy gtkviewport.c and strip out the adjustment and shadow
functionality (perhaps you could call it GtkClipper).

<!-- ----------------------------------------------------------------- -->

<sect1>How do I make my window modal? / How do I make a single window active?
<p>
After you create your window, do <tt/gtk_grab_add(my_window)/. And after 
closing the window do <tt/gtk_grab_remove(my_window)/.

<!-- ----------------------------------------------------------------- -->
<sect1>Why doesn't my widget (e.g. progressbar) update?
<p>

You are probably doing all the changes within a function 
without returning control to <tt/gtk_main()/. This may be the case if you do some 
lengthy calculation in your code. Most drawing updates are only 
placed on a queue, which is processed within <tt/gtk_main()/. You can
force the drawing queue to be processed using something like:

<tscreen><verb>
while (gtk_main_iteration(FALSE));
</verb></tscreen>

inside you're function that changes the widget.

What the above snippet does is run all pending events and high priority 
idle functions, then return immediately (the drawing is done in a 
high priority idle function).

<!-- ----------------------------------------------------------------- -->
<sect1>How do I attach data to some GTK+ object/widget?
<p>
First of all, the attached data is stored in the object_data field of a 
GtkObject. The type of this field is GData, which is defined in glib.h. 
So you should read the gdataset.c file in your glib source directory very 
carefully.

There are two (easy) ways to attach some data to a gtk object. 
Using <tt/gtk_object_set_data()/ and <tt/gtk_object_get_data()/ seems to be the 
most common way to do this, as it provides a powerfull interface 
to connect objects and data.

<tscreen><verb>
void gtk_object_set_data(GtkObject *object, const gchar *key, gpointer data);
gpointer gtk_object_get_data(GtkObject *object, const gchar *key);
</verb></tscreen>

Since a short example is better than any lengthy speech:

<tscreen><verb>
struct my_struct	p1,p2,*result;
GtkWidget		*w;

gtk_object_set_data(GTK_OBJECT(w),"p1 data",(gpointer)&amp;p1);
gtk_object_set_data(GTK_OBJECT(w),"p2 data",(gpointer)&amp;p2);

result = gtk_object_get_data(GTK_OBJECT(w),"p1 data");
</verb></tscreen>

The <tt/gtk_object_set_user_data()/ and <tt/gtk_object_get_user_data()/
functions does exactly the same thing
as the functions above, but does not let you specify the "key" parameter.
Instead, it uses a standard "user_data" key. Note that the use of these
functions is deprecated in 1.2. They only provide a compatibility mode 
with some old gtk packages.

<!-- ----------------------------------------------------------------- -->
<sect1>How do I remove the data I have attached to an object?
<p>
When attaching the data to the object, you can use the 
<tt/gtk_object_set_data_full()/
function. The three first arguments of the function are the same as in
<tt/gtk_object_set_data()/. The fourth one is a pointer to a callback function
which is called when the data is destroyed. The data is destroyed when
you:

<itemize>
<item> destroy the object
<item> replace the data with a new one (with the same key)
<item> replace the data with NULL (with the same key)
</itemize>

<!-- ----------------------------------------------------------------- -->
<sect1>How could I get any widgets position?
<p>
As Tim Janik pointed out, there are different cases, and each case requires
a different solution.

<itemize>
<item>  If you want the position of a widget relative to its parent, you should
        use <tt/widget->allocation.x/ and <tt/widget->allocation.y/.
<item>  If you want the position of a window relative to the X root window,
        you should use <tt/gdk_window_get_geometry()/ or 
        <tt/gdk_window_get_origin()/.
<item>  Last but not least, if you want to get a Window Manager frame position,
        you should use <tt/gdk_window_get_deskrelative_origin()/.
</itemize>

<!-- ----------------------------------------------------------------- -->
<sect1>How do I set the size of a widget/window? How do I prevent the user resizing my window?
<p>
The <tt/gtk_widget_set_uposition()/ function is used to set the
position of any widget.

The <tt/gtk_widget_set_usize()/ function is used to set the
size of a widget. In order to use all the features that are provided by
this function when it acts on a window, you may want to use the
<tt/gtk_window_set_policy/ function. The definition of these functions
is:

<tscreen><verb>
void        gtk_widget_set_usize (GtkWidget *widget,
                                  gint width,
                                  gint height);

void        gtk_window_set_policy (GtkWindow *window,
                                   gint allow_shrink,
                                   gint allow_grow,
                                   gint auto_shrink);
</verb></tscreen>

<tt/Auto_shrink/ will automatically shrink the window when the
requested size of the child widgets goes below the current size of the
window. <tt/Allow_shrink/ will give the user the authorisation to
make the window smaller that it should normally be. <tt/Allow_grow/
will give the user will have the ability to make the window
bigger. The default values for these parameters are:

<tscreen><verb>
allow_shrink = FALSE
allow_grow   = TRUE
auto_shrink  = FALSE
</verb></tscreen>

<!-- ----------------------------------------------------------------- -->
<sect1>How do I add a popup menu to my GTK+ application?
<p>
The <tt/menu/ example in the examples/menu directory of the GTK+ distribution
implements a popup menu with this technique :

<tscreen><verb>
static gint button_press (GtkWidget *widget, GdkEvent *event)
{

    if (event->type == GDK_BUTTON_PRESS) {
        GdkEventButton *bevent = (GdkEventButton *) event; 
        gtk_menu_popup (GTK_MENU(widget), NULL, NULL, NULL, NULL,
                        bevent->button, bevent->time);
        /* Tell calling code that we have handled this event; the buck
         * stops here. */
        return TRUE;
    }

    /* Tell calling code that we have not handled this event; pass it on. */
    return FALSE;
}
</verb></tscreen>

<!-- ----------------------------------------------------------------- -->
<sect1>How do I disable or enable a widget, such as a button?
<p>
To disable (or to enable) a widget, use the <tt/gtk_widget_set_sensitive()/
function. The first parameter is you widget pointer. The second parameter
is a boolean value: when this value is TRUE, the widget is enabled.

<!-- ----------------------------------------------------------------- -->
<sect1>Shouldn't the text argument in the gtk_clist_* functions be declared const?
<p>
For example:
<verb>
gint gtk_clist_prepend (GtkCList    *clist,
                        gchar       *text[]);
</verb>

Answer: No, while a type "gchar*" (pointer to char) can automatically
be cast into "const gchar*" (pointer to const char), this does not
apply for "gchar *[]" (array of an unspecified number of pointers to
char) into "const gchar *[]" (array of an unspecified number of
pointers to const char).

The type qualifier "const" may be subject to automatic casting, but
in the array case, it is not the array itself that needs the (const)
qualified cast, but its members, thus changing the whole type.

<!-- ----------------------------------------------------------------- -->
<sect1>How do I render pixels (image data) to the screen?
<p>
There are several ways to approach this. The simplest way is to use
GdkRGB, see gdk/gdkrgb.h. You create an RGB buffer, render to your RGB
buffer, then use GdkRGB routines to copy your RGB buffer to a drawing
area or custom widget. The book "GTK+/Gnome Application Development"
gives some details; GdkRGB is also documented in the GTK+ reference
documentation.

If you're writing a game or other graphics-intensive application, you
might consider a more elaborate solution. OpenGL is the graphics
standard that will let you access hardware accelaration in future
versions of XFree86; so for maximum speed, you probably want to use
OpenGL. A GtkGLArea widget is available for using OpenGL with GTK+
(but GtkGLArea does not come with GTK+ itself). There are also several
open source game libraries, such as ClanLib and Loki's Simple
DirectMedia Layer library (SDL).

You do NOT want to use <tt/gdk_draw_point()/, that will be extremely
slow.

<!-- ***************************************************************** -->
<sect>Development with GTK+: widget specific questions 
<!-- ***************************************************************** -->

<!-- ----------------------------------------------------------------- -->
<sect1>How do I find out about the selection of a GtkList?
<p>
Get the selection something like this:
<tscreen><verb>
GList *sel;
sel = GTK_LIST(list)->selection;
</verb></tscreen>

This is how GList is defined (quoting glist.h):
<tscreen><verb>
typedef struct _GList GList;

struct _GList
{
  gpointer data;
  GList *next;
  GList *prev;
};
</verb></tscreen>

A GList structure is just a simple structure for doubly linked lists.
there exist several g_list_*() functions to modify a linked list in
glib.h.  However the GTK_LIST(MyGtkList)->selection is maintained
by the gtk_list_*() functions and should not be modified.

The selection_mode of the GtkList determines the selection
facilities of a GtkList and therefore the contents
of GTK_LIST(AnyGtkList)->selection:

<verb>
selection_mode          GTK_LIST()->selection contents
------------------------------------------------------

GTK_SELECTION_SINGLE)   selection is either NULL
                        or contains a GList* pointer
                        for a single selected item.

GTK_SELECTION_BROWSE)   selection is NULL if the list
                        contains no widgets, otherwise
                        it contains a GList* pointer
                        for one GList structure.
GTK_SELECTION_MULTIPLE) selection is NULL if no listitems
                        are selected or a a GList* pointer
                        for the first selected item. that
                        in turn points to a GList structure
                        for the second selected item and so
                        on

GTK_SELECTION_EXTENDED) selection is NULL.
</verb>

The data field of the GList structure GTK_LIST(MyGtkList)->selection points
to the first GtkListItem that is selected.  So if you would like to determine 
which listitems are selected you should go like this:

Upon Initialization:
<tscreen><verb>
{
        gchar           *list_items[]={
                                "Item0",
                                "Item1",
                                "foo",
                                "last Item",
                        };
        guint           nlist_items=sizeof(list_items)/sizeof(list_items[0]);
        GtkWidget       *list_item;
        guint           i;

        list=gtk_list_new();
        gtk_list_set_selection_mode(GTK_LIST(list), GTK_SELECTION_MULTIPLE);
        gtk_container_add(GTK_CONTAINER(AnyGtkContainer), list);
        gtk_widget_show (list);

        for (i = 0; i < nlist_items; i++)
        {
                list_item=gtk_list_item_new_with_label(list_items[i]);
                gtk_object_set_user_data(GTK_OBJECT(list_item), (gpointer)i);
                gtk_container_add(GTK_CONTAINER(list), list_item);
                gtk_widget_show(list_item);
        }
}
</verb></tscreen>

To get known about the selection:
<tscreen><verb>
{
        GList   *items;

        items=GTK_LIST(list)->selection;

        printf("Selected Items: ");
        while (items) {
                if (GTK_IS_LIST_ITEM(items->data))
                        printf("%d ", (guint) 
                gtk_object_get_user_data(items->data));
                items=items->next;
        }
        printf("\n");
}
</verb></tscreen>

<!-- ----------------------------------------------------------------- --><p>
<sect1>I don't want the user of my applications to enter text into a GtkCombo. Any idea?
<p>
A GtkCombo has an associated entry which can be accessed using the
following expression:

<tscreen><verb>
      GTK_COMBO(combo_widget)->entry
</verb></tscreen>

If you don't want the user to be able to modify the content of this
entry, you can use the gtk_entry_set_editable() function:

<tscreen><verb>
      void gtk_entry_set_editable(GtkEntry *entry, 
                                  gboolean editable);
</verb></tscreen>

Set the editable parameter to FALSE to disable typing into the entry.

<!-- ----------------------------------------------------------------- -->
<sect1>How do I catch a combo box change?
<p>
The entry which is associated to your GtkCombo send a "changed" signal when:
<itemize>
      <item>some text is typed in
      <item>the selection of the combo box is changed
</itemize>

To catch any combo box change, simply connect your signal handler with

<tscreen><verb>
      gtk_signal_connect(GTK_COMBO(cb)->entry,
                         "changed",
                         GTK_SIGNAL_FUNC(my_cb_change_handler),
                         NULL);
</verb></tscreen>

<!-- ----------------------------------------------------------------- -->
<sect1>How can I define a separation line in a menu? 
<p>
See the <htmlurl url="http://www.gtk.org/tutorial/"
name="Tutorial"> for information on how to create menus.
However, to create a separation line in a menu, just insert an
empty menu item:

<tscreen><verb>
menuitem = gtk_menu_item_new();
gtk_menu_append(GTK_MENU(menu), menuitem);
gtk_widget_show(menuitem);
</verb></tscreen>

<!-- ----------------------------------------------------------------- -->
<sect1>How can I right justify a menu, such as Help? 
<p>
Depending on if you use the MenuFactory or not, there are two ways to proceed.
With the MenuFactory, use something like the following:

<tscreen><verb>
menu_path = gtk_menu_factory_find (factory,  "<MyApp>/Help");
gtk_menu_item_right_justify(menu_path->widget);
</verb></tscreen>

If you do not use the MenuFactory, you should simply use:

<tscreen><verb>
gtk_menu_item_right_justify(my_menu_item);
</verb></tscreen>

<!-- ----------------------------------------------------------------- -->
<sect1>How do I add some underlined accelerators to menu items?
<p>
Damon Chaplin, the technical force behind the Glade project, provided the
following code sample (this code is an output from Glade). It creates a
small <tt/File/ menu item with only one child (<tt/New/). The F in <tt/File/ 
and the N in <tt/New/ are underlined, and the relevant accelerators are 
created.

<tscreen><verb>
  menubar1 = gtk_menu_bar_new ();
  gtk_object_set_data (GTK_OBJECT (window1), "menubar1", menubar1);
  gtk_widget_show (menubar1);
  gtk_box_pack_start (GTK_BOX (vbox1), menubar1, FALSE, FALSE, 0);

  file1 = gtk_menu_item_new_with_label ("");
  tmp_key = gtk_label_parse_uline (GTK_LABEL (GTK_BIN (file1)->child),
                                   _("_File"));
  gtk_widget_add_accelerator (file1, "activate_item", accel_group,
                              tmp_key, GDK_MOD1_MASK, 0);
  gtk_object_set_data (GTK_OBJECT (window1), "file1", file1);
  gtk_widget_show (file1);
  gtk_container_add (GTK_CONTAINER (menubar1), file1);

  file1_menu = gtk_menu_new ();
  file1_menu_accels = gtk_menu_ensure_uline_accel_group (GTK_MENU (file1_menu));
  gtk_object_set_data (GTK_OBJECT (window1), "file1_menu", file1_menu);
  gtk_menu_item_set_submenu (GTK_MENU_ITEM (file1), file1_menu);

  new1 = gtk_menu_item_new_with_label ("");
  tmp_key = gtk_label_parse_uline (GTK_LABEL (GTK_BIN (new1)->child),
                                   _("_New"));
  gtk_widget_add_accelerator (new1, "activate_item", file1_menu_accels,
                              tmp_key, 0, 0);
  gtk_object_set_data (GTK_OBJECT (window1), "new1", new1);
  gtk_widget_show (new1);
  gtk_container_add (GTK_CONTAINER (file1_menu), new1);
</verb></tscreen>

<!-- ----------------------------------------------------------------- -->
<sect1>How do I right (or otherwise) justify a GtkLabel?
<p>
Are you sure you want to <em>justify</em> the labels?  The label class contains 
the <tt/gtk_label_set_justify()/ function that is used to control the 
justification of a multi-line label.

What you probably want is to set the <em>alignment</em> of the label, ie right 
align it, center it or left align it. If you want to do this, you 
should use:

<tscreen><verb>
void gtk_misc_set_alignment     (GtkMisc *misc,
                                gfloat xalign,
                                gfloat yalign);
</verb></tscreen>

where the <tt/xalign/ and <tt/yalign/ values are floats in [0.00;1.00].

<tscreen><verb>
GtkWidget       *label;

/* horizontal : left align, vertical : top */
gtk_misc_set_alignment(GTK_MISK(label), 0.0f, 0.0f);

/* horizontal : centered, vertical : centered */
gtk_misc_set_alignment(GTK_MISK(label), 0.5f, 0.5f);

/* horizontal : right align, vertical : bottom */
gtk_misc_set_alignment(GTK_MISK(label), 1.0f, 1.0f);
</verb></tscreen>

<!-- ----------------------------------------------------------------- -->
<sect1>How do I set the color and font of a GtkLabel using a Resource File?
<p>
The widget name path constructed for a Label consists of the widget
names of its object hierarchy as well, e.g.

<verb>
window (name: humphrey)
  hbox
    label (name: mylabel)
</verb>

The widget path your pattern needs to match would be:
<tt/humphrey.GtkHBox.mylabel/

The resource file may look something like:

<verb>
style "title"
{
      fg[NORMAL] = {1.0, 0.0, 0.0}
      font = "-adobe-helvetica-bold-r-normal--*-140-*-*-*-*-*-*"
}
widget "*mylabel" style "title"
</verb>

In your program, you would also need to give a name to the Label
widget, which can be done using:
<verb>
  label = gtk_label_new("Some Label Text");
  gtk_widget_set_name(label, "mylabel");
  gtk_widget_show(label);
</verb>

<!-- ----------------------------------------------------------------- -->
<sect1>How do I configure Tooltips in a Resource File?
<p>
The tooltip's window is named "gtk-tooltips", GtkTooltips in itself is not
a GtkWidget (though a GtkObject) and as such is not attempted to match any
widget styles.

So, you resource file should look something like:
<verb> 
style "postie"
{
      bg[NORMAL] = {1.0, 1.0, 0.0}
}
widget "gtk-tooltips*" style "postie"
</verb>

<!-- ----------------------------------------------------------------- -->
<sect1>How do I use horizontal scrollbars with a GtkText widget?
<p>
The short answer is that you can't. The current version of the GtkText
widget does not support horizontal scrolling. There is an intention to
completely rewrite the GtkText widget, at which time this limitation
will be removed.

<!-- ----------------------------------------------------------------- -->
<sect1>I can't add more than (something like) 2000 chars in a GtkEntry. What's wrong?
<p>
There is now a known problem in the GtkEntry widget. In the
<tt/gtk_entry_insert_text()/ function, the following lines limit
the number of chars in the entry to 2047.

<tscreen><verb>
  /* The algorithms here will work as long as, the text size (a
   * multiple of 2), fits into a guint16 but we specify a shorter
   * maximum length so that if the user pastes a very long text, there
   * is not a long hang from the slow X_LOCALE functions.  */

  if (entry->text_max_length == 0)
    max_length = 2047;
  else
    max_length = MIN (2047, entry->text_max_length);
</verb></tscreen>

<!-- ----------------------------------------------------------------- -->
<sect1>How do I change the font of a GtkText widget?
<p>
There are a couple of ways of doing this. As GTK+ allows the
appearance of applications to be changed at run time using resources
you can use something like the following in the appropriate
file:

<tscreen><verb>
style "text"
{
  font = "-adobe-helvetica-medium-r-normal--*-100-*-*-*-*-*-*"
}
</verb></tscreen>

Another way to do this is to load a font within your program, and then
use this in the functions for adding text to the text widget. You can
load a font using, for example:

<tscreen><verb>
  GdkFont *font;
  font = gdk_font_load("-adobe-helvetica-medium-r-normal--*-140-*-*-*-*-*-*");
</verb></tscreen>

<!-- ----------------------------------------------------------------- -->
<sect1>How do I set the cursor position in a GtkText object?
<p>
Notice that the response is valid for any object that inherits from the 
GtkEditable class.

Are you sure that you want to move the cursor position? Most of the time,
while the cursor position is good, the insertion point does not match the
cursor position. If this apply to what you really want, then you should use
the <tt/gtk_text_set_point()/ function. If you want to set the insertion point
at the current cursor position, use the following:

<tscreen><verb>
  gtk_text_set_point(GTK_TEXT(text),
      gtk_editable_get_position(GTK_EDITABLE(text)));
</verb></tscreen>

If you want the insertion point to follow the cursor at all time, you
should probably catch the button press event, and then move the
insertion point. Be careful : you'll have to catch it after the widget 
has changed the cursor position though. Thomas Mailund Jensen proposed the
following code:

<tscreen><verb>
static void
insert_bar (GtkWidget *text)
{
  /* jump to cursor mark */
  gtk_text_set_point (GTK_TEXT (text),
      gtk_editable_get_position (GTK_EDITABLE  (text)));

  gtk_text_insert (GTK_TEXT (text), NULL, NULL, NULL,
     "bar", strlen ("bar"));
}

int
main (int argc, char *argv[])
{
  GtkWidget *window, *text;

  gtk_init (&amp;argc, &amp;argv);

  window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
  text = gtk_text_new (NULL, NULL);
  gtk_text_set_editable (GTK_TEXT (text), TRUE);
  gtk_container_add (GTK_CONTAINER (window), text);

  /* connect after everything else */
  gtk_signal_connect_after (GTK_OBJECT(text), "button_press_event",
    GTK_SIGNAL_FUNC (insert_bar), NULL);

  gtk_widget_show_all(window);
  gtk_main();

  return 0;
}
</verb></tscreen>

Now, if you really want to change the cursor position, you should use the
<tt/gtk_editable_set_position()/ function.

<!-- ***************************************************************** -->
<sect>About gdk
<!-- ***************************************************************** -->

<!-- ----------------------------------------------------------------- -->
<sect1>What is GDK?
<p>
GDK is basically a wrapper around the standard Xlib function calls. If you are
at all familiar with Xlib, a lot of the functions in GDK will require little 
or no getting used to. All functions are written to provide an way 
to access Xlib functions in an easier and slightly more intuitive manner. 
In addition, since GDK uses GLib (see below), it will be more portable 
and safer to use on multiple platforms.

<!-- Examples, anybody? I've been mulling some over. NF -->
   
<!-- ----------------------------------------------------------------- -->
<sect1>How do I use color allocation?
<p>
One of the nice things about GDK is that it's based on top of Xlib; this is 
also a problem, especially in the area of color management. If you want 
to use color in your program (drawing a rectangle or such, your code 
should look something like this:
<tscreen>
<verb>
{
  GdkColor *color;
  int width, height;
  GtkWidget *widget;
  GdkGC *gc;

  ...
  
  /* first, create a GC to draw on */
  gc = gdk_gc_new(widget->window);

  /* find proper dimensions for rectangle */
  gdk_window_get_size(widget->window, &amp;width, &amp;height);

  /* the color we want to use */
  color = (GdkColor *)malloc(sizeof(GdkColor));
  
  /* red, green, and blue are passed values, indicating the RGB triple
   * of the color we want to draw. Note that the values of the RGB components
   * within the GdkColor are taken from 0 to 65535, not 0 to 255.
   */
  color->red = red * (65535/255);
  color->green = green * (65535/255);
  color->blue = blue * (65535/255);
  
  /* the pixel value indicates the index in the colormap of the color.
   * it is simply a combination of the RGB values we set earlier
   */
  color->pixel = (gulong)(red*65536 + green*256 + blue);

  /* However, the pixel valule is only truly valid on 24-bit (TrueColor)
   * displays. Therefore, this call is required so that GDK and X can
   * give us the closest color available in the colormap
   */
  gdk_color_alloc(gtk_widget_get_colormap(widget), color);

  /* set the foreground to our color */
  gdk_gc_set_foreground(gc, color);
  
  /* draw the rectangle */
  gdk_draw_rectangle(widget->window, gc, 1, 0, 0, width, height);

  ...
}
</verb>
</tscreen>

<!-- ***************************************************************** -->
<sect>About GLib
<!-- ***************************************************************** -->

<!-- ----------------------------------------------------------------- -->
<sect1>What is GLib?
<p>
GLib is a library of useful functions and definitions available for use 
when creating GDK and GTK applications. It provides replacements for some
standard libc functions, such as malloc, which are buggy on some systems.
<p>
It also provides routines for handling:
<itemize>
<item>Doubly Linked Lists
<item>Singly Linked Lists
<item>Timers
<item>String Handling
<item>A Lexical Scanner
<item>Error Functions
</itemize>

<!-- ----------------------------------------------------------------- -->
<sect1>How can I use the doubly linked lists?
<p>
The GList object is defined as:

<tscreen><verb>
typedef struct _GList GList;

struct _GList
{
  gpointer data;
  GList *next;
  GList *prev;
};
</verb></tscreen>

To use the GList objects, simply :

<tscreen><verb>
GList   *list = NULL;
GList   *listrunner;
gint    array[] = { 1, 2, 3, 4, 5, 6 };
gint    pos;
gint    *value;

/* add data to the list */
for (pos=0;pos < sizeof array; pos++) {
  list = g_list_append(list, (gpointer)&amp;array[pos]);
}

/* run through the list */
listrunner = g_list_first(list);
while (listrunner) {
  value = (gint *)listrunner->data;
  printf("%d\n", *value);
  listrunner = g_list_next(listrunner);
}

/* removing datas from the list */
listrunner = g_list_first(list);
list = g_list_remove_link(list, listrunner);
list = g_list_remove(list, &amp;array[4]);
</verb></tscreen>

The same code is usable with singly linked lists (GSList objects) by replacing
g_list_* functions with the relevant g_slist_* ones (g_slist_append, 
g_slist_remove, ...). Just remember that since you can't go backward in a singly
linked list, there is no g_slist_first function - you'll need to keep a 
reference on the first node of the list.

<!-- Some Examples might be useful here! NF -->
<!-- I believe it should be better :) ED -->
<!-- Linked lists are pretty standard data structures - don't want to
     over do it - TRG -->
<!-- ----------------------------------------------------------------- -->
<sect1>Why use g_print, g_malloc, g_strdup and fellow glib functions?  
<p>
Thanks to Tim Janik who wrote to gtk-list: (slightly modified)
<quote>
Regarding g_malloc(), g_free() and siblings, these functions are much safer
than their libc equivalents. For example, g_free() just returns if called 
with NULL. Also, if USE_DMALLOC is defined, the definition for these 
functions changes (in glib.h) to use MALLOC(), FREE() etc...  If MEM_PROFILE
or MEM_CHECK are defined, there are even small statistics made counting
the used block sizes (shown by g_mem_profile() / g_mem_check()).
<P>
Considering the fact that glib provides an interface for memory chunks
to save space if you have lots of blocks that are always the same size
and to mark them ALLOC_ONLY if needed, it is just straight forward to
create a small saver (debug able) wrapper around the normal malloc/free
stuff as well - just like gdk covers Xlib. ;)
<P>
Using g_error() and g_warning() inside of applications like the GIMP
that fully rely on gtk even gives the opportunity to pop up a window
showing the messages inside of a gtk window with your own handler
(by using g_set_error_handler()) along the lines of <tt/gtk_print()/
(inside of gtkmain.c).
</quote>

<!-- ----------------------------------------------------------------- -->
<sect1>What's a GScanner and how do I use one?  
<p>
A GScanner will tokenize your text, that is, it'll return an integer
for every word or number that appears in its input stream, following
certain (customizable) rules to perform this translation.
You still need to write the parsing functions on your own though.

Here's a little test program supplied by Tim Janik that will parse

<verb>
<SYMBOL> = <OPTIONAL-MINUS> <NUMBER> ;
</verb>

constructs, while skipping "#\n" and "/**/" style comments.

<verb>
#include <glib.h>

/* some test text to be fed into the scanner */
static const gchar *test_text =
( "ping = 5;\n"
  "/* slide in some \n"
  " * comments, just for the\n"
  " * fun of it \n"
  " */\n"
  "pong = -6; \n"
  "\n"
  "# the next value is a float\n"
  "zonk = 0.7;\n"
  "# redefine ping\n"
  "ping = - 0.5;\n" );

/* define enumeration values to be returned for specific symbols */
enum {
  SYMBOL_PING = G_TOKEN_LAST + 1,
  SYMBOL_PONG = G_TOKEN_LAST + 2,
  SYMBOL_ZONK = G_TOKEN_LAST + 3
};

/* symbol array */
static const struct {
  gchar *symbol_name;
  guint  symbol_token;
} symbols[] = {
  { "ping", SYMBOL_PING, },
  { "pong", SYMBOL_PONG, },
  { "zonk", SYMBOL_ZONK, },
  { NULL, 0, },
}, *symbol_p = symbols;

static gfloat ping = 0;
static gfloat pong = 0;
static gfloat zonk = 0;

static guint
parse_symbol (GScanner *scanner)
{
  guint symbol;
  gboolean negate = FALSE;

  /* expect a valid symbol */
  g_scanner_get_next_token (scanner);
  symbol = scanner->token;
  if (symbol < SYMBOL_PING ||
      symbol > SYMBOL_ZONK)
    return G_TOKEN_SYMBOL;

  /* expect '=' */
  g_scanner_get_next_token (scanner);
  if (scanner->token != '=')
    return '=';

  /* feature optional '-' */
  g_scanner_peek_next_token (scanner);
  if (scanner->next_token == '-')
    {
      g_scanner_get_next_token (scanner);
      negate = !negate;
    }

  /* expect a float (ints are converted to floats on the fly) */
  g_scanner_get_next_token (scanner);
  if (scanner->token != G_TOKEN_FLOAT)
    return G_TOKEN_FLOAT;

  /* make sure the next token is a ';' */
  if (g_scanner_peek_next_token (scanner) != ';')
    {
      /* not so, eat up the non-semicolon and error out */
      g_scanner_get_next_token (scanner);
      return ';';
    }

  /* assign value, eat the semicolon and exit successfully */
  switch (symbol)
    {
    case SYMBOL_PING:
      ping = negate ? - scanner->value.v_float : scanner->value.v_float;
      break;
    case SYMBOL_PONG:
      pong = negate ? - scanner->value.v_float : scanner->value.v_float;
      break;
    case SYMBOL_ZONK:
      zonk = negate ? - scanner->value.v_float : scanner->value.v_float;
      break;
    }
  g_scanner_get_next_token (scanner);

  return G_TOKEN_NONE;
}

int
main (int argc, char *argv[])
{
  GScanner *scanner;
  guint expected_token;

  scanner = g_scanner_new (NULL);

  /* adjust lexing behaviour to suit our needs
   */
  /* convert non-floats (octal values, hex values...) to G_TOKEN_INT */
  scanner->config->numbers_2_int = TRUE;
  /* convert G_TOKEN_INT to G_TOKEN_FLOAT */
  scanner->config->int_2_float = TRUE;
  /* don't return G_TOKEN_SYMBOL, but the symbol's value */
  scanner->config->symbol_2_token = TRUE;

  /* load symbols into the scanner */
  while (symbol_p->symbol_name)
    {
      g_scanner_add_symbol (scanner,
                            symbol_p->symbol_name,
                            GINT_TO_POINTER (symbol_p->symbol_token));
      symbol_p++;
    }

  /* feed in the text */
  g_scanner_input_text (scanner, test_text, strlen (test_text));

  /* give the error handler an idea on how the input is named */
  scanner->input_name = "test text";

  /* scanning loop, we parse the input untill it's end is reached,
   * the scanner encountered a lexing error, or our sub routine came
   * across invalid syntax
   */
  do
    {
      expected_token = parse_symbol (scanner);
      
      g_scanner_peek_next_token (scanner);
    }
  while (expected_token == G_TOKEN_NONE &&
         scanner->next_token != G_TOKEN_EOF &&
         scanner->next_token != G_TOKEN_ERROR);

  /* give an error message upon syntax errors */
  if (expected_token != G_TOKEN_NONE)
    g_scanner_unexp_token (scanner, expected_token, NULL, "symbol", NULL, NULL, TRUE);

  /* finsish parsing */
  g_scanner_destroy (scanner);

  /* print results */
  g_print ("ping: %f\n", ping);
  g_print ("pong: %f\n", pong);
  g_print ("zonk: %f\n", zonk);
  
  return 0;
}
</verb>

You need to understand that the scanner will parse it's input and
tokenize it, it is up to you to interpret these tokens, not define
their types before they get parsed, e.g. watch gscanner parse a string:
 
<verb>
"hi i am 17"
 |  | |  |
 |  | |  v
 |  | v  TOKEN_INT, value: 17
 |  v TOKEN_IDENTIFIER, value: "am"
 v  TOKEN_CHAR, value: 'i'
TOKEN_IDENTIFIER, value: "hi"
</verb>

If you configure the scanner with:
<verb>
scanner->config->int_2_float = TRUE;
scanner->config->char_2_token = TRUE;
scanner->config->scan_symbols = TRUE;
</verb>

and add "am" as a symbol with
<verb>
g_scanner_add_symbol (scanner, "am", "symbol value");
</verb>

GScanner will parse it as

<verb>
"hi i am 17"
 |  | |  |
 |  | |  v
 |  | v  TOKEN_FLOAT, value: 17.0  (automatic int->float conversion)
 |  | TOKEN_SYMBOL, value: "symbol value"  (a successfull hash table lookup
 |  |                                       turned a TOKEN_IDENTIFIER into a
 |  |                                       TOKEN_SYMBOL and took over the
 |  v                                       symbol's value)
 v  'i'  ('i' can be a valid token as well, as all chars >0 and <256)
TOKEN_IDENTIFIER, value: "hi"
</verb>

You need to match the token sequence with your code, and if you encounter
something that you don't want, you error out:

<verb>
/* expect an identifier ("hi") */
g_scanner_get_next_token (scanner);
if (scanner->token != G_TOKEN_IDENTIFIER)
  return G_TOKEN_IDENTIFIER;
/* expect a token 'i' */
g_scanner_get_next_token (scanner);
if (scanner->token != 'i')
  return 'i';
/* expect a symbol ("am") */
g_scanner_get_next_token (scanner);
if (scanner->token != G_TOKEN_SYMBOL)
  return G_TOKEN_SYMBOL;
/* expect a float (17.0) */
g_scanner_get_next_token (scanner);
if (scanner->token != G_TOKEN_FLOAT)
  return G_TOKEN_FLOAT;
</verb>

If you got past here, you have parsed "hi i am 17" and would have
accepted "dooh i am 42" and  "bah i am 0.75" as well, but you would
have not accepted "hi 7 am 17" or "hi i hi 17".

<!-- ***************************************************************** -->
<sect>GTK+ FAQ Contributions, Maintainers and Copyright
<p>
If you would like to make a contribution to the FAQ, send either one of us
an e-mail message with the exact text you think should be included (question and
answer).  With your help, this document can grow and become more useful!

This document is maintained by Nathan Froyd 
<htmlurl url="mailto:maestrox@geocities.com" 
name="&lt;maestrox@geocities.com&gt;">,
Tony Gale <htmlurl url="mailto:gale@gtk.org" 
name="&lt;gale@gtk.org&gt;"> and 
Emmanuel Deloget <htmlurl url="mailto:logout@free.fr"
name="&lt;logout@free.fr&gt;">.
This FAQ was created by Shawn T. Amundson 
<htmlurl url="mailto:amundson@gimp.org" 
name="&lt;amundson@gimp.org&gt;"> who continues to provide support.

The GTK+ FAQ is Copyright (C) 1997,1998, 1999 by Shawn T. Amundson, 
Nathan Froyd and Tony Gale, Emmanuel Deloget.

Permission is granted to make and distribute verbatim copies of 
this manual provided the copyright notice and this permission notice 
are preserved on all copies.

Permission is granted to copy and distribute modified versions of 
this document under the conditions for verbatim copying, provided 
that this copyright notice is included exactly as in the original, 
and that the entire resulting derived work is distributed under 
the terms of a permission notice identical to this one.

Permission is granted to copy and distribute translations of 
this document into another language,  under the above conditions 
for modified versions.

If you are intending to incorporate this document into a published work, 
please contact one of the maintainers, and we will make an effort to ensure 
that you have the most up to date information available.

There is no guarentee that this document lives up to its intended
purpose.  This is simply provided as a free resource.  As such,
the authors and maintainers of the information provided within can 
not make any guarentee that the information is even accurate.

</article>