glib/docs/reference/gobject/tut_intro.xml

<chapter>
<title>Background</title>

<para>
GObject, and its lower-level type system, GType, are used by GTK+ and most Gnome libraries to
provide:
<itemizedlist>
<listitem><para>object-oriented C-based APIs and</para></listitem>
<listitem><para>automatic transparent API bindings to other compiled 
or interpreted languages.</para></listitem>
</itemizedlist>
</para>

<para>A lot of programmers are used to work with compiled-only or dynamically interpreted-only
languages and do not understand the challenges associated with cross-language interoperability.
This introduction tries to provide an insight into these challenges. describes briefly 
the solution choosen by GLib.
</para>

<para>The following chapters go into greater detail into how GType and GObject work and
how you can use them as a C programmer. It is useful to keep in mind that
allowing access to C objects from other interpreted languages was one of the major design
goals: this can often explain the sometimes rather convoluted APIs and features present
in this library.
</para>

<sect1>
<title>Data types and programming</title>

<para>
One could say (I have seen such definitions used in some textbooks on programming language theory)
that a programming language is merely a way to create data types and manipulate them. Most languages
provide a number of language-native types and a few primitives to create more complex types based
on these primitive types.
</para>

<para>
In C, the language provides types such as <emphasis>char</emphasis>, <emphasis>long</emphasis>, 
<emphasis>pointer</emphasis>. During compilation of C code, the compiler maps these
language types to the compiler's target architecture machine types. If you are using a C interpreter
(I have never seen one myself but it is possible :), the interpreter (the program which interprets 
the source code and executes it) maps the language types to the machine types of the target machine at 
runtime, during the program execution (or just before execution if it uses a Just In Time compiler engine).
</para>

<para>Perl and Python which are interpreted languages do not really provide type definitions similar
to those used by C. Perl and Python programmers manipulate variables and the type of the variables
is decided only upon the first assignment or upon the first use which forces a type on the variable.
The interpreter also often provides a lot of automatic conversions from one type to the other. For example,
in Perl, a variable which holds an integer can be automatically converted to a string given the
required context:
<programlisting>
my $tmp = 10;
print "this is an integer converted to a string:" . $tmp . "\n";
</programlisting>
Of course, it is also often possible to explicitely specify conversions when the default conversions provided
by the language are not intuitive.
</para>

</sect1>

<sect1>
<title>Exporting a C API</title>

<para>C APIs are defined by a set of functions and global variables which are usually exported from a 
binary. C functions have an arbitrary number of arguments and one return value. Each function is thus
uniquely identified by the function name and the set of C types which describe the function arguments
and return value. The global variables exported by the API are similarly identified by their name and 
their type.
</para>

<para>
A C API is thus merely defined by a set of names to which a set of types are associated. If you know the
function calling convention and the mapping of the C types to the machine types used by the platform you 
are on, you can resolve the name of each function to find where the code associated to this function 
is located in memory, and then construct a valid argument list for the function. Finally, all you have to 
do is triger a call to the target C function with the argument list.
</para>

<para>
For the sake of discussion, here is a sample C function and the associated 32 bit x86 
assembly code generated by gcc on my linux box:
<programlisting>
static void function_foo (int foo)
{}

int main (int argc, char *argv[])
{

        function_foo (10);

        return 0;
}

push   $0xa
call   0x80482f4 &lt;function_foo>
</programlisting>
The assembly code shown above is pretty straightforward: the first instruction pushes
the hexadecimal value 0xa (decimal value 10) as a 32 bit integer on the stack and calls 
<function>function_foo</function>. As you can see, C function calls are implemented by
gcc by native function calls (this is probably the fastest implementation possible).
</para>

<para>
Now, let's say we want to call the C function <function>function_foo</function> from 
a python program. To do this, the python interpreter needs to:
<itemizedlist>
<listitem><para>Find where the function is located. This means probably find the binary generated by the C compiler
which exports this functions.</para></listitem>
<listitem><para>Load the code of the function in executable memory.</para></listitem>
<listitem><para>Convert the python parameters to C-compatible parameters before calling 
the function.</para></listitem>
<listitem><para>Call the function with the right calling convention</para></listitem>
<listitem><para>Convert the return values of the C function to python-compatible
variables to return them to the python code.</para></listitem>
</itemizedlist>
</para>

<para>The process described above is pretty complex and there are a lot of ways to make it entirely automatic
and transparent to the C and the Python programmers:
<itemizedlist>
<listitem><para>The first solution is to write by hand a lot of glue code, once for each function exported or imported,
which does the python to C parameter conversion and the C to python return value conversion. This glue code is then 
linked with the interpreter which allows python programs to call a python functions which delegates the work to the 
C function.</para></listitem>
<listitem><para>Another nicer solution is to automatically generate the glue code, once for each function exported or
imported, with a special compiler which
reads the original function signature.</para></listitem>
<listitem><para>The solution used by GLib is to use the GType library which holds at runtime a description of
all the objects manipulated by the programmer. This so-called <emphasis>dynamic type</emphasis><footnote>
<para>
	There are numerous different implementations of dynamic type systems: all C++ 
	compilers have one, Java and .NET have one too. A dynamic type system allows you
	to get information about every instantiated object at runtime. It can be implemented
	by a process-specific database: every new object created registers the characteristics 
	of its associated type in the type system. It can also be implemented by introspection
	interfaces. The common point between all these different type systems and implementations
	is that they all allow you to query for object metadata at runtime.
</para>
</footnote>

 library is then
used by special generic glue code to automatically convert function parameters and function calling conventions
between different runtime domains.</para></listitem>
</itemizedlist>
The greatest advantage of the solution implemented by GType is that the glue code sitting at the runtime domain 
boundaries is written once: the figure below states this more clearly.
<figure>
  <mediaobject>
    <imageobject> <!-- this is for HTML output -->
      <imagedata fileref="glue.png" format="png" align="center"/>
    </imageobject>
    <imageobject> <!-- this is for PDF output -->
      <imagedata fileref="glue.jpg" format="jpg" align="center"/>
    </imageobject>
  </mediaobject>
</figure>

Currently, there exist at least Python and Perl generic glue code which makes it possible to use
C objects written with GType directly in Python or Perl, with a minimum amount of work: there
is no need to generate huge amounts of glue code either automatically or by hand.
</para>

<para>Although that goal was arguably laudable, its pursuit has had a major influence on
the whole GType/GObject library. C programmers are likely to be puzzled at the complexity 
of the features exposed in the following chapters if they forget that the GType/GObject library
was not only designed to offer OO-like features to C programmers but also transparent 
cross-langage interoperability.
</para>

</sect1>

</chapter>