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<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="id-1.3.2.6"></a>Exporting a C API</h2></div></div></div>
<p>
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.
</p>
<p>
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 trigger a call to the target C function with the argument list.
</p>
<p>
For the sake of discussion, here is a sample C function and the associated 32 bit x86
assembly code generated by GCC on a Linux computer:
</p>
<div class="informalexample">
<table class="listing_frame" border="0" cellpadding="0" cellspacing="0">
<tbody>
<tr>
<td class="listing_lines" align="right"><pre>1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16</pre></td>
<td class="listing_code"><pre class="programlisting"><span class="keyword">static</span><span class="normal"> </span><span class="type">void</span>
<span class="function">function_foo</span><span class="normal"> </span><span class="symbol">(</span><span class="type">int</span><span class="normal"> foo</span><span class="symbol">)</span>
<span class="cbracket">{</span>
<span class="cbracket">}</span>
<span class="type">int</span>
<span class="function">main</span><span class="normal"> </span><span class="symbol">(</span><span class="type">int</span><span class="normal"> argc</span><span class="symbol">,</span>
<span class="normal"> </span><span class="type">char</span><span class="normal"> </span><span class="symbol">*</span><span class="normal">argv</span><span class="symbol">[])</span>
<span class="cbracket">{</span>
<span class="normal"> </span><span class="function">function_foo</span><span class="normal"> </span><span class="symbol">(</span><span class="number">10</span><span class="symbol">);</span>
<span class="normal"> </span><span class="keyword">return</span><span class="normal"> </span><span class="number">0</span><span class="symbol">;</span>
<span class="cbracket">}</span>
<span class="normal">push $</span><span class="number">0xa</span>
<span class="normal">call </span><span class="number">0x80482f4</span><span class="normal"> </span><span class="symbol"><</span><span class="normal">function_foo</span><span class="symbol">></span></pre></td>
</tr>
</tbody>
</table>
</div>
<p>
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
<code class="function">function_foo</code>. As you can see, C function calls are implemented by
GCC as native function calls (this is probably the fastest implementation possible).
</p>
<p>
Now, let's say we want to call the C function <code class="function">function_foo</code> from
a Python program. To do this, the Python interpreter needs to:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>Find where the function is located. This probably means finding the binary generated by the C compiler
which exports this function.</p></li>
<li class="listitem"><p>Load the code of the function in executable memory.</p></li>
<li class="listitem"><p>Convert the Python parameters to C-compatible parameters before calling
the function.</p></li>
<li class="listitem"><p>Call the function with the right calling convention.</p></li>
<li class="listitem"><p>Convert the return values of the C function to Python-compatible
variables to return them to the Python code.</p></li>
</ul></div>
<p>
</p>
<p>
The process described above is pretty complex and there are a lot of ways to make it entirely automatic
and transparent to C and Python programmers:
</p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem"><p>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 Python functions which delegate work to
C functions.</p></li>
<li class="listitem"><p>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.</p></li>
<li class="listitem"><p>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 <span class="emphasis"><em>dynamic type</em></span>
<a href="#ftn.id-1.3.2.6.6.1.3.1.2" class="footnote" name="id-1.3.2.6.6.1.3.1.2"><sup class="footnote">[1]</sup></a>
library is then used by special generic glue code to automatically convert function parameters and
function calling conventions between different runtime domains.</p></li>
</ul></div>
<p>
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.
</p>
<div class="figure">
<a name="id-1.3.2.6.6.2"></a><p class="title"><b>Figure 1. </b></p>
<div class="figure-contents"><div class="mediaobject" align="center"><img src="glue.png" align="middle"></div></div>
</div>
<p><br class="figure-break">
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.
</p>
<p>
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-language interoperability.
</p>
<div class="footnotes">
<br><hr style="width:100; text-align:left;margin-left: 0">
<div id="ftn.id-1.3.2.6.6.1.3.1.2" class="footnote"><p><a href="#id-1.3.2.6.6.1.3.1.2" class="para"><sup class="para">[1] </sup></a>
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.
</p></div>
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