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<H1><a name="Modules"></a>15 Working with Modules</H1>
<!-- INDEX -->
<div class="sectiontoc">
<ul>
<li><a href="#Modules_nn1">Basics</a>
<li><a href="#Modules_nn2">The SWIG runtime code</a>
<li><a href="#external_run_time">External access to the runtime</a>
<li><a href="#Modules_nn4">A word of caution about static libraries</a>
<li><a href="#Modules_nn5">References</a>
<li><a href="#Modules_nn6">Reducing the wrapper file size</a>
</ul>
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<!-- INDEX -->



<p>
When first working with SWIG, users commonly start by creating a
single module.  That is, you might define a single SWIG interface that
wraps some set of C/C++ code.  You then compile all of the generated
wrapper code into a module and use it.   For large applications, however,
this approach is problematic---the size of the generated wrapper code
can be rather large.  Moreover, it is probably easier to manage the
target language interface when it is broken up into smaller pieces.
</p>

<p>
This chapter describes the problem of using SWIG in programs
where you want to create a collection of modules.
</p>

<H2><a name="Modules_nn1"></a>15.1 Basics</H2>


<p>
The basic usage case with multiple modules is when modules do not have
cross-references (ie. when wrapping multiple independent C APIs). In that case,
swig input files should just work out of the box - you simply create multiple
wrapper .cxx files, link them into your application, and insert/load each in the
scripting language runtime as you would do for the single module case.
</p>

<p>
A bit more complex is the case in which modules need to share information.
For example, when one module extends the class of the another by deriving from
it:
</p>

<div class="code"><pre>
%module base

%inline %{
class base {
public:
       int foo(void);
};
%}
</pre></div>
&nbsp;
<div class="code"><pre>
%module derived

%import "base.i"

%inline %{
class derived : public base {
public:
       int bar(void);
};
%}
</pre></div>

<p>To create the wrapper properly, module <tt>derived</tt> needs to know the
<tt>base</tt> class and that it's interface is covered in another module. The
line <tt>%import "base.i"</tt> lets SWIG know exactly that. The common mistake here is
to <tt>%import</tt> the <tt>.h</tt> file instead of the <tt>.i</tt>, which sadly won't do the trick. Another issue
to take care of is that multiple dependent wrappers should not be linked/loaded
in parallel from multiple threads as SWIG provides no locking - for more on that
issue, read on.</p>

<H2><a name="Modules_nn2"></a>15.2 The SWIG runtime code</H2>


<p>
Many of SWIG's target languages generate a set of functions commonly known as
the "SWIG runtime." These functions are primarily related to the runtime type
system which checks pointer types and performs other tasks such as proper
casting of pointer values in C++. As a general rule, the statically typed target
languages, such as Java, use the language's built in static type checking and
have no need for a SWIG runtime. All the dynamically typed / interpreted
languages rely on the SWIG runtime.
</p>

<p>
The runtime functions are private to each SWIG-generated module. That is, the
runtime functions are declared with "static" linkage and are visible only to the
wrapper functions defined in that module. The only problem with this approach is
that when more than one SWIG module is used in the same application, those
modules often need to share type information. This is especially true for C++
programs where SWIG must collect and share information about inheritance
relationships that cross module boundaries.
</p>

<p>
To solve the problem of sharing information across modules, a pointer to the
type information is stored in a global variable in the target language
namespace. During module initialization, type information is loaded into the
global data structure of type information from all modules.
</p>

<p>
There are a few trade offs with this approach. This type information is global
across all SWIG modules loaded, and can cause type conflicts between modules
that were not designed to work together. To solve this approach, the SWIG
runtime code uses a define SWIG_TYPE_TABLE to provide a unique type table. This
behavior can be enabled when compiling the generated _wrap.cxx or _wrap.c file
by adding -DSWIG_TYPE_TABLE=myprojectname to the command line argument.
</p>

<p>
Then, only modules compiled with SWIG_TYPE_TABLE set to myprojectname will share
type information. So if your project has three modules, all three should be
compiled with -DSWIG_TYPE_TABLE=myprojectname, and then these three modules will
share type information. But any other project's types will not interfere or
clash with the types in your module.
</p>

<p>
Another issue relating to the global type table is thread safety. If two modules
try and load at the same time, the type information can become corrupt. SWIG
currently does not provide any locking, and if you use threads, you must make
sure that modules are loaded serially. Be careful if you use threads and the
automatic module loading that some scripting languages provide. One solution is
to load all modules before spawning any threads, or use SWIG_TYPE_TABLE to
separate type tables so they do not clash with each other.
</p>

<p>
Lastly, SWIG uses a #define SWIG_RUNTIME_VERSION, located in Lib/swigrun.swg and
near the top of every generated module. This number gets incremented when the
data structures change, so that SWIG modules generated with different versions
can peacefully coexist. So the type structures are separated by the
(SWIG_TYPE_TABLE, SWIG_RUNTIME_VERSION) pair, where by default SWIG_TYPE_TABLE
is empty. Only modules compiled with the same pair will share type information.
</p>

<H2><a name="external_run_time"></a>15.3 External access to the runtime</H2>


<p>As described in <a href="Typemaps.html#runtime_type_checker">The run-time type checker</a>,
the functions <tt>SWIG_TypeQuery</tt>, <tt>SWIG_NewPointerObj</tt>, and others sometimes need
to be called.  Calling these functions from a typemap is supported, since the typemap code
is embedded into the <tt>_wrap.c</tt> file, which has those declarations available.  If you need
to call the SWIG run-time functions from another C file, there is one header you need
to include.  To generate the header that needs to be included, run the following command:

<div class="shell"><pre>
$ swig -python -external-runtime &lt;filename&gt;
</pre></div>

<p>The filename argument is optional and if it is not passed, then the default filename will
be something like <tt>swigpyrun.h</tt>, depending on the language.  This header file should
be treated like any of the other _wrap.c output files, and should be regenerated when the
_wrap files are.  After including this header, your code will be able to call <tt>SWIG_TypeQuery</tt>,
<tt>SWIG_NewPointerObj</tt>, <tt>SWIG_ConvertPtr</tt> and others.  The exact argument parameters
for these functions might differ between language modules; please check the language module chapters
for more information.</p>

<p>Inside this header the functions are declared static and are included inline into the file,
and thus the file does not need to be linked against any SWIG libraries or code (you might still
need to link against the language libraries like libpython-2.3).  Data is shared between this
file and the _wrap.c files through a global variable in the scripting language.  It is also
possible to copy this header file along with the generated wrapper files into your own package,
so that you can distribute a package that can be compiled without SWIG installed (this works
because the header file is self-contained, and does not need to link with anything).</p>

<p>
This header will also use the -DSWIG_TYPE_TABLE described above, so when
compiling any code which includes the generated header file should define the
SWIG_TYPE_TABLE to be the same as the module whose types you are trying to
access.
</p>

<H2><a name="Modules_nn4"></a>15.4 A word of caution about static libraries</H2>


<p>
When working with multiple SWIG modules, you should take care not to use static
libraries.  For example, if you have a static library <tt>libfoo.a</tt> and you link a collection
of SWIG modules with that library, each module will get its own private copy of the library code inserted
into it. This is very often <b>NOT</b> what you want and it can lead to unexpected or bizarre program
behavior. When working with dynamically loadable modules, you should try to work exclusively with shared libraries.
</p>

<H2><a name="Modules_nn5"></a>15.5 References</H2>


<p>
Due to the complexity of working with shared libraries and multiple modules, it might be a good idea to consult
an outside reference.  John Levine's "Linkers and Loaders" is highly recommended.
</p>

<H2><a name="Modules_nn6"></a>15.6 Reducing the wrapper file size</H2>


<p>
Using multiple modules with the <tt>%import</tt> directive is the most common approach to modularising large projects.
In this way a number of different wrapper files can be generated, thereby avoiding the generation of a single large wrapper file.
There are a couple of alternative solutions for reducing the size of a wrapper file through the use of command line options and features.
</p>

<p>
<b>-fcompact</b><br>
This command line option will compact the size of the wrapper file without changing the code generated into the wrapper file.
It simply removes blank lines and joins lines of code together.
This is useful for compilers that have a maximum file size that can be handled.
</p>

<p>
<b>-fvirtual</b><br>
This command line option will remove the generation of superfluous virtual method wrappers.
Consider the following inheritance hierarchy:
</p>

<div class="code">
<pre>
struct Base {
  virtual void method();
  ...
};

struct Derived : Base {
  virtual void method();
  ...
};
</pre>
</div>

<p>
Normally wrappers are generated for both methods, whereas this command line option will suppress the generation of a wrapper for <tt>Derived::method</tt>.
Normal polymorphic behaviour remains as <tt>Derived::method</tt> will still be called should you have
a <tt>Derived</tt> instance and call the wrapper for <tt>Base::method</tt>.
</p>

<p>
<b>%feature("compactdefaultargs")</b><br>
This feature can reduce the number of wrapper methods when wrapping methods with default arguments. The section on <a href="SWIGPlus.html#SWIGPlus_default_args">default arguments</a> discusses the feature and its limitations.
</p>

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