36 SWIG and C as the target language

This chapter describes SWIG's support for creating ANSI C wrappers. This module has a special purpose and thus is different from most other modules.

NOTE: this module is still under development.

36.1 Overview

SWIG is normally used to provide access to C or C++ libraries from target languages such as scripting languages or languages running on a virtual machine. SWIG performs analysis of the input C/C++ library header files from which it generates further code. For most target languages this code consists of two layers; namely an intermediary C code layer and a set of language specific proxy classes and functions on top of the C code layer. We could also think of C as just another target language supported by SWIG. The aim then is to generate a pure ANSI C interface to the input C or C++ library and hence the C target language module.

With wrapper interfaces generated by SWIG, it is easy to use the functionality of C++ libraries inside application code written in C. This module may also be useful to generate custom APIs for a library, to suit particular needs, e.g. to supply function calls with error checking or to implement a "design by contract".

Flattening C++ language constructs into a set of C-style functions obviously comes with many limitations and inconveniences. All data and functions become global. Manipulating objects requires explicit calls to special functions. We are losing the high level abstraction and have to work around it.

Known C++ Shortcomings in Generated C API:

• Enums with a context like class or namespace are broken
• Global variables are not supported
• Qualifiers are stripped
• Vararg functions are not supported.

36.2 Preliminaries

36.2.1 Running SWIG

Consider the following simple example. Suppose we have an interface file like:

/* File: example.i */
%module test
%{
#include "stuff.h"
%}
int fact(int n);

To build a C module (C as the target language), run SWIG using the -c option :

$ swig -c example.i

The above assumes C as the input language. If the input language is C++ add the -c++ option:

$ swig -c++ -c example.i

Note that -c is the option specifying the target language and -c++ controls what the input language is.

This will generate an example_wrap.c file or, in the latter case, example_wrap.cxx file, along with example_wrap.h (the same extension is used in both C and C++ cases for the last one). The names of the files are derived from the name of the input file by default, but can be changed using the -o and -oh options common to all language modules.

The xxx_wrap.c file contains the wrapper functions, which perform the main functionality of SWIG: each of the wrappers translates the input arguments from C to C++, makes calls to the original functions and marshals C++ output back to C data. The xxx_wrap.h header file contains the declarations of these functions as well as global variables.

36.2.2 Command line options

The following table list the additional command line options available for the C module. They can also be seen by using:

$ swig -c -help

36.2.3 Compiling a dynamic module

The next step is to build a dynamically loadable module, which we can link to our application. This can be done easily, for example using the gcc compiler (Linux, MinGW, etc.):

$ swig -c example.i
$ gcc -c example_wrap.c
$ gcc -shared example_wrap.o -o libexample.so

Or, for C++ input:

$ swig -c++ -c example.i
$ g++ -c example_wrap.cxx
$ g++ -shared example_wrap.o -o libexample.so

Now the shared library module is ready to use. Note that the name of the generated module is important: is should be prefixed with lib on Unix, and have the specific extension, like .dll for Windows or .so for Unix systems.

36.2.4 Using the generated module

The simplest way to use the generated shared module is to link it to the application code during the compilation stage. The process is usually similar to this:

$ gcc runme.c -L. -lexample -o runme

This will compile the application code (runme.c) and link it against the generated shared module. Following the -L option is the path to the directory containing the shared module. The output executable is ready to use. The last thing to do is to supply to the operating system the information of location of our module. This is system dependant, for instance Unix systems look for shared modules in certain directories, like /usr/lib, and additionally we can set the environment variable LD_LIBRARY_PATH (Unix) or PATH (Windows) for other directories.

36.3 Basic C wrapping

Wrapping C functions and variables is obviously performed in a straightforward way. There is no need to perform type conversions, and all language constructs can be preserved in their original form. However, SWIG allows you to enhance the code with some additional elements, for instance using check typemap or %extend directive.

36.3.1 Functions

For each C function declared in the interface file a wrapper function with a prefix, required to make its name different from the original one, is created. The prefix for the global functions is module_, i.e. the name of the SWIG module followed by underscore, by default. If -namespace option is used, the prefix corresponding to the given fixed namespace is used instead. If nspace feature is used, the prefix corresponding to the namespace in which the function is defined is used -- note that, unlike with -namespace option, this prefix can be different for different functions. The wrapper function performs a call to the original function, and returns its result.

For example, for function declaration in the module mymath:

int gcd(int x, int y);

The output is simply:

int mymath_gcd(int arg1, int arg2) {
  int result;
  result = gcd(arg1,arg2);
  return result;
}

Now one might think, what's the use of creating such functions in C? The answer is, you can apply special rules to the generated code. Take for example constraint checking. You can write a "check" typemap in your interface file:

%typemap(check) int POSITIVE {
  if ($1 <= 0)
    fprintf(stderr, "Expected positive value in $name.\n");
}

int gcd(int POSITIVE, int POSITIVE);

And now the generated result looks like:

int _wrap_gcd(int arg1, int arg2) {
  {
    if (arg1 <= 0)
      fprintf(stderr, "Expected positive value in gcd.\n");
  }
  {
    if (arg1 <= 0)
      fprintf(stderr, "Expected positive value in gcd.\n");
  }
  int result;
  result = gcd(arg1,arg2);
  return result;
}

This time calling gcd with negative value argument will trigger an error message. This can save you time writing all the constraint checking code by hand.

36.3.2 Variables

Wrapping variables comes also without any special issues. All global variables are directly accessible from application code. There is a difference in the semantics of struct definition in C and C++. When handling C struct, SWIG simply rewrites its declaration. In C++ struct is handled as class declaration.

You can still apply some of the SWIG features when handling structs, e.g. %extend directive. Suppose, you have a C struct declaration:

typedef struct {
  int x;
  char *str;
} my_struct;

You can redefine it to have an additional fields, like:

%extend my_struct {
  double d;
};

In application code:

struct my_struct ms;
ms.x = 123;
ms.d = 123.123;

36.4 Basic C++ wrapping

The main reason of having the C module in SWIG is to be able to access C++ from C. In this chapter we will take a look at the rules of wrapping elements of the C++ language.

By default, SWIG attempts to build a natural C interface to your C/C++ code.

C++ Type	SWIG C Translation
Class Example	Empty structure Example
Public, mutable member variable Foo Example::foo	Example_foo_get(Example *e); Example_foo_set(Example *e, Foo *f);
Public, immutable member variable Foo Example::bar	Example_foo_get(Example *e);

This section briefly covers the essential aspects of this wrapping.

36.3.3 Enums

C enums and unscoped C++ enums are simply copied to the generated code and both the enum itself and its elements keep the same name as in the original code unless -namespace option is used or nspace feature is enabled, in which case the prefix corresponding to the specified namespace is used.

For scoped C++11 enums, the enum name itself is used as an additional prefix.

36.4.1 Classes

Consider the following example. We have a C++ class, and want to use it from C code.

class Circle {
public:
  double radius;

  Circle(double r) : radius(r) { };
  double area(void);
};

What we need to do is to create an object of the class, manipulate it, and finally, destroy it. SWIG generates C functions for this purpose each time a class declaration is encountered in the interface file.

The first two generated functions are used to create and destroy instances of class Circle. Such instances are represented on the C side as pointers to special structs, called SwigObj. They are all "renamed" (via typedef) to the original class names, so that you can use the object instances on the C side using pointers like:

Circle *circle;

The generated functions make calls to class' constructors and destructors, respectively. They also do all the necessary things required by the SWIG object management system in C.

Circle * Circle_new(double r);
void Circle_delete(Circle * self);

The class Circle has a public variable called radius. SWIG generates a pair of setters and getters for each such variable:

void Circle_radius_set(Circle * self, double radius);
double Circle_radius_get(Circle * self);

For each public method, an appropriate function is generated:

double Circle_area(Circle * self);

You can see that in order to use the generated object we need to provide a pointer to the object instance (struct Circle in this case) as the first function argument. In fact, this struct is basically wrapping pointer to the "real" C++ object.

Our application code could look like this:

  Circle *c = Circle_new(1.5);
  printf("radius: %f\narea: %f\n", Circle_radius_get(c), Circle_area(c));
  Circle_delete(c);

After running this we'll get:

radius: 1.500000
area: 7.068583

Backend Developer Documentation

Typemaps

C Typemaps, a Code Generation Walkthrough

The Interface

%module example

%inline
%{
  class SomeClass{};
  template <typename T> class SomeTemplateClass{};
  SomeClass someFunction(SomeTemplateClass<int> &someParameter, int simpleInt);
%}

%template (SomeIntTemplateClass) SomeTemplateClass<int>;

// wrapper header file
typedef struct SwigObj_SomeClass SomeClass;

SomeClass * SomeClass_new();

void SomeClass_delete(SomeClass * carg1);
        
SomeClass* someFunction(SomeIntTemplateClass* carg1, int carg2);
        
        
typedef struct SwigObj_SomeIntTemplateClass SomeIntTemplateClass;
        
SomeIntTemplateClass * SomeIntTemplateClass_new();
        
void SomeIntTemplateClass_delete(SomeIntTemplateClass * carg1);

The Wrapper

SWIGEXPORTC SwigObj * module_someFunction(SwigObj * carg1, int carg2) {
  SomeClass * cppresult;
  SomeTemplateClass< int > *arg1 = 0 ;
  int arg2 ;
  SwigObj * result;
  
  {
    if (carg1)
    arg1 = (SomeTemplateClass< int > *) carg1->obj;
    else
    arg1 = (SomeTemplateClass< int > *) 0;
  }
  arg2 = (int) carg2;
  {
    const SomeClass &_result_ref =  someFunction(*arg1,arg2);cppresult = (SomeClass*) &_result_ref;
  }
  {
    result = SWIG_create_object(cppresult, SWIG_STR(SomeClass));
  }
  return result;
}

1. Prototype
2. C return value variable
3. Local variables equal to the called C++ function's parameters
4. [C++ return value variable]
5. Assignment (extraction) of wrapper parameters to local parameter copies
6. [Contract (e.g. constraints) checking]
7. C++ function call
8. [Exception handling]
9. [Assignment to C++ return value]
10. Assignment to C return value

ctype                        ctype            ctype
---------                    ---------        ---
SwigObj * module_someFunction(SwigObj * carg1, int carg2);

ctype
---------
SwigObj * result;

SomeTemplateClass< int > *arg1 = 0 ;
int arg2 ;

cppouttype
-----------
SomeClass * cppresult;

{
  if (carg1)
  arg1 = (SomeTemplateClass< int > *) carg1->obj;
  else
  arg1 = (SomeTemplateClass< int > *) 0;
}
arg2 = (int) carg2;

At this point we are ready to call the C++ function with our parameters.

{
  const SomeClass &_result_ref =  someFunction(*arg1,arg2);cppresult = (SomeClass*) &_result_ref;
}

{
  result = SWIG_create_object(cppresult, SWIG_STR(SomeClass));
}

return result;

The Proxy

// wrapper header file
typedef struct SwigObj_SomeClass SomeClass;

SomeClass * SomeClass_new();

void SomeClass_delete(SomeClass * carg1);

SomeClass* someFunction(SomeIntTemplateClass* carg1, int carg2);


typedef struct SwigObj_SomeIntTemplateClass SomeIntTemplateClass;

SomeIntTemplateClass * SomeIntTemplateClass_new();

void SomeIntTemplateClass_delete(SomeIntTemplateClass * carg1);

36.5 Exception handling