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> 60my $tmp = 10; 61print "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> 92static void 93function_foo (int foo) 94{ 95} 96 97int 98main (int argc, 99 char *argv[]) 100{ 101 function_foo (10); 102 103 return 0; 104} 105 106push $0xa 107call 0x80482f4 <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> 186
