Working with Modules

# 20 Working with Modules

- Modules Introduction
- Basics
- The SWIG runtime code
- External access to the runtime
- A word of caution about static libraries
- References
- Reducing the wrapper file size

## 20.1 Modules Introduction

Each invocation of SWIG requires a module name to be specified. The module name is used to name the resulting target language extension module. Exactly what this means and what the name is used for depends on the target language, for example the name can define a target language namespace or merely be a useful name for naming files or helper classes. Essentially, a module comprises target language wrappers for a chosen collection of global variables/functions, structs/classes and other C/C++ types.

The module name can be supplied in one of two ways. The first is to specify it with the special `%module` directive. This directive must appear at the beginning of the interface file. The general form of this directive is:

    %module(option1="value1", option2="value2", ...) modulename

where the modulename is mandatory and the options add one or more optional additional features. Typically no options are specified, for example:

    %module mymodule

The second way to specify the module name is with the `-module` command line option, for example `-module mymodule`. If the module name is supplied on the command line, it overrides the name specified by the `%module` directive.

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 together 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.

This chapter describes the problem of using SWIG in programs where you want to create a collection of modules. Each module in the collection is created via separate invocations of SWIG.

## 20.2 Basics

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.

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

    // File: base.h
    class base {
    public:
      int foo();
    };

    // File: base_module.i
    %module base_module

    %{
    #include "base.h"
    %}
    %include "base.h"

    // File: derived_module.i
    %module derived_module

    %{
    #include "base.h"
    %}

    %import "base_module.i"

    %inline %{
    class derived : public base {
    public:
      int bar();
    };
    %}

To create the wrapper properly, module `derived_module` needs to know about the `base` class and that its interface is covered in another module. The line `%import "base_module.i"` lets SWIG know exactly that. Often the `.h` file is passed to `%import` instead of the `.i`, which unfortunately doesn't work for all language modules. For example, Python requires the name of module that the base class exists in so that the proxy classes can fully inherit the base class's methods. Typically you will get a warning when the module name is missing, eg:

    derived_module.i:8: Warning 401: Base class 'base' ignored - unknown module name for base. Either
    import
    the appropriate module interface file or specify the name of the module in the %import directive.

It is sometimes desirable to import the header file rather than the interface file and overcome the above warning. For example in the case of the imported interface being quite large, it may be desirable to simplify matters and just import a small header file of dependent types. This can be done by specifying the optional `module` attribute in the `%import` directive. The `derived_module.i` file shown above could be replaced with the following:

    // File: derived_module.i
    %module derived_module

    %{
    #include "base.h"
    %}

    %import(module="base_module") "base.h"

    %inline %{
    class derived : public base {
    public:
      int bar();
    };

Note that "base_module" is the module name and is the same as that specified in `%module` in `base_module.i` as well as the `%import` in `derived_module.i`.

Another issue to beware 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.

## 20.3 The SWIG runtime code

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.

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.

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.

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.

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.

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.

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.

## 20.4 External access to the runtime

As described in The run-time type checker, the functions `SWIG_TypeQuery`, `SWIG_NewPointerObj`, and others sometimes need to be called. Calling these functions from a typemap is supported, since the typemap code is embedded into the `_wrap.c` 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, SWIG can be run in a different mode via `-external-runtime` to generate the run-time instead of the normal mode of processing an input interface file. For example:

    $ swig -python -external-runtime <filename>

The filename argument is optional and if it is not passed, then the default filename will be something like `swigpyrun.h`, 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 `SWIG_TypeQuery`, `SWIG_NewPointerObj`, `SWIG_ConvertPtr` and others. The exact argument parameters for these functions might differ between language modules; please check the language module chapters for more information.

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).

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.

## 20.5 A word of caution about static libraries

When working with multiple SWIG modules, you should take care not to use static libraries. For example, if you have a static library `libfoo.a` 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 NOT 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.

## 20.6 References

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.

## 20.7 Reducing the wrapper file size

Using multiple modules with the `%import` 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.

-fcompact
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.

-fvirtual
This command line option will remove the generation of superfluous virtual method wrappers. Consider the following inheritance hierarchy:

    struct Base {
      virtual void method();
      ...
    };

    struct Derived : Base {
      virtual void method();
      ...
    };

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

%feature("compactdefaultargs")
This feature can reduce the number of wrapper methods when wrapping methods with default arguments. The section on default arguments discusses the feature and its limitations.