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1 office 1 <?xml version='1.0' encoding="UTF-8"?>
2 <!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
3 "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
4 ]>
5 <chapter id="chapter-intro">
6 <title>Background</title>
7  
8 <para>
9 GObject, and its lower-level type system, GType, are used by GTK+ and most GNOME libraries to
10 provide:
11 <itemizedlist>
12 <listitem><para>object-oriented C-based APIs and</para></listitem>
13 <listitem><para>automatic transparent API bindings to other compiled
14 or interpreted languages.</para></listitem>
15 </itemizedlist>
16 </para>
17  
18 <para>
19 A lot of programmers are used to working with compiled-only or dynamically interpreted-only
20 languages and do not understand the challenges associated with cross-language interoperability.
21 This introduction tries to provide an insight into these challenges and briefly describes
22 the solution chosen by GLib.
23 </para>
24  
25 <para>
26 The following chapters go into greater detail into how GType and GObject work and
27 how you can use them as a C programmer. It is useful to keep in mind that
28 allowing access to C objects from other interpreted languages was one of the major design
29 goals: this can often explain the sometimes rather convoluted APIs and features present
30 in this library.
31 </para>
32  
33 <sect1>
34 <title>Data types and programming</title>
35  
36 <para>
37 One could say
38 that a programming language is merely a way to create data types and manipulate them. Most languages
39 provide a number of language-native types and a few primitives to create more complex types based
40 on these primitive types.
41 </para>
42  
43 <para>
44 In C, the language provides types such as <emphasis>char</emphasis>, <emphasis>long</emphasis>,
45 <emphasis>pointer</emphasis>. During compilation of C code, the compiler maps these
46 language types to the compiler's target architecture machine types. If you are using a C interpreter
47 (assuming one exists), the interpreter (the program which interprets
48 the source code and executes it) maps the language types to the machine types of the target machine at
49 runtime, during the program execution (or just before execution if it uses a Just In Time compiler engine).
50 </para>
51  
52 <para>
53 Perl and Python are interpreted languages which do not really provide type definitions similar
54 to those used by C. Perl and Python programmers manipulate variables and the type of the variables
55 is decided only upon the first assignment or upon the first use which forces a type on the variable.
56 The interpreter also often provides a lot of automatic conversions from one type to the other. For example,
57 in Perl, a variable which holds an integer can be automatically converted to a string given the
58 required context:
59 <informalexample><programlisting>
60 my $tmp = 10;
61 print "this is an integer converted to a string:" . $tmp . "\n";
62 </programlisting></informalexample>
63 Of course, it is also often possible to explicitly specify conversions when the default conversions provided
64 by the language are not intuitive.
65 </para>
66  
67 </sect1>
68  
69 <sect1>
70 <title>Exporting a C API</title>
71  
72 <para>
73 C APIs are defined by a set of functions and global variables which are usually exported from a
74 binary. C functions have an arbitrary number of arguments and one return value. Each function is thus
75 uniquely identified by the function name and the set of C types which describe the function arguments
76 and return value. The global variables exported by the API are similarly identified by their name and
77 their type.
78 </para>
79  
80 <para>
81 A C API is thus merely defined by a set of names to which a set of types are associated. If you know the
82 function calling convention and the mapping of the C types to the machine types used by the platform you
83 are on, you can resolve the name of each function to find where the code associated to this function
84 is located in memory, and then construct a valid argument list for the function. Finally, all you have to
85 do is trigger a call to the target C function with the argument list.
86 </para>
87  
88 <para>
89 For the sake of discussion, here is a sample C function and the associated 32 bit x86
90 assembly code generated by GCC on a Linux computer:
91 <informalexample><programlisting>
92 static void
93 function_foo (int foo)
94 {
95 }
96  
97 int
98 main (int argc,
99 char *argv[])
100 {
101 function_foo (10);
102  
103 return 0;
104 }
105  
106 push $0xa
107 call 0x80482f4 &lt;function_foo>
108 </programlisting></informalexample>
109 The assembly code shown above is pretty straightforward: the first instruction pushes
110 the hexadecimal value 0xa (decimal value 10) as a 32-bit integer on the stack and calls
111 <function>function_foo</function>. As you can see, C function calls are implemented by
112 GCC as native function calls (this is probably the fastest implementation possible).
113 </para>
114  
115 <para>
116 Now, let's say we want to call the C function <function>function_foo</function> from
117 a Python program. To do this, the Python interpreter needs to:
118 <itemizedlist>
119 <listitem><para>Find where the function is located. This probably means finding the binary generated by the C compiler
120 which exports this function.</para></listitem>
121 <listitem><para>Load the code of the function in executable memory.</para></listitem>
122 <listitem><para>Convert the Python parameters to C-compatible parameters before calling
123 the function.</para></listitem>
124 <listitem><para>Call the function with the right calling convention.</para></listitem>
125 <listitem><para>Convert the return values of the C function to Python-compatible
126 variables to return them to the Python code.</para></listitem>
127 </itemizedlist>
128 </para>
129  
130 <para>
131 The process described above is pretty complex and there are a lot of ways to make it entirely automatic
132 and transparent to C and Python programmers:
133 <itemizedlist>
134 <listitem><para>The first solution is to write by hand a lot of glue code, once for each function exported or imported,
135 which does the Python-to-C parameter conversion and the C-to-Python return value conversion. This glue code is then
136 linked with the interpreter which allows Python programs to call Python functions which delegate work to
137 C functions.</para></listitem>
138 <listitem><para>Another, nicer solution is to automatically generate the glue code, once for each function exported or
139 imported, with a special compiler which
140 reads the original function signature.</para></listitem>
141 <listitem><para>The solution used by GLib is to use the GType library which holds at runtime a description of
142 all the objects manipulated by the programmer. This so-called <emphasis>dynamic type</emphasis>
143 <footnote>
144 <para>
145 There are numerous different implementations of dynamic type systems: all C++
146 compilers have one, Java and .NET have one too. A dynamic type system allows you
147 to get information about every instantiated object at runtime. It can be implemented
148 by a process-specific database: every new object created registers the characteristics
149 of its associated type in the type system. It can also be implemented by introspection
150 interfaces. The common point between all these different type systems and implementations
151 is that they all allow you to query for object metadata at runtime.
152 </para>
153 </footnote>
154 library is then used by special generic glue code to automatically convert function parameters and
155 function calling conventions between different runtime domains.</para></listitem>
156 </itemizedlist>
157 The greatest advantage of the solution implemented by GType is that the glue code sitting at the runtime domain
158 boundaries is written once: the figure below states this more clearly.
159 <figure>
160 <mediaobject>
161 <imageobject> <!-- this is for HTML output -->
162 <imagedata fileref="glue.png" format="PNG" align="center"/>
163 </imageobject>
164 <imageobject> <!-- this is for PDF output -->
165 <imagedata fileref="glue.jpg" format="JPG" align="center"/>
166 </imageobject>
167 </mediaobject>
168 </figure>
169  
170 Currently, there exist at least Python and Perl generic glue code which makes it possible to use
171 C objects written with GType directly in Python or Perl, with a minimum amount of work: there
172 is no need to generate huge amounts of glue code either automatically or by hand.
173 </para>
174  
175 <para>
176 Although that goal was arguably laudable, its pursuit has had a major influence on
177 the whole GType/GObject library. C programmers are likely to be puzzled at the complexity
178 of the features exposed in the following chapters if they forget that the GType/GObject library
179 was not only designed to offer OO-like features to C programmers but also transparent
180 cross-language interoperability.
181 </para>
182  
183 </sect1>
184  
185 </chapter>