15 Working with Modules

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.

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

15.1 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.

This can present a problem with threads. 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.

15.2 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, run the following command:

$ 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).

15.3 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.

15.4 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.

15.5 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.

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.

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.

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.