-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. -
- - --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, but this module is actually also capable of generating C++ wrappers defined completely inline using the C functions, thus wrapping the original C++ library API in another, similar C++ API. Contrary to the natural initial reaction, this is far from being completely pointless, as wrapping C++ API in this way avoids all problems due to C++ ABI issues, e.g. it is now possible to use the original C++ API using a different C++ compiler, or a different version of the same compiler, or even the same compiler, but with different compilation options affecting the ABI. The C++ wrapper API is not identical to the original one, but strives to be as close to it as possible. -
- --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. -
- --The following table list the additional command line options available for the C module. They can also be seen by using: -
- --$ swig -c -help -
| C specific options | -|
|---|---|
| -namespace <nspace> | -Generate wrappers with the prefix based on the provided namespace, e.g. if the option value is outer::inner, the prefix outer_inner_ will be used. Notice that this is different from using SWIG nspace feature, as it applies the the prefix to all the symbols, regardless of the namespace they were actually declared in. Notably, this allows to export instantiations of templates defined in the std namespace, such as std::vector, using a custom prefix rather than std_. | -
| -nocxx | -Don't generate C++ wrappers, even when -c++ option is used. See C++ Wrappers section for more details. | -
| -noexcept | -generate wrappers with no support of exception handling; see Exceptions chapter for more details | -
-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. -
- --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. -
- --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. -
- --It is also possible to output arbitrary additional code into the generated header by using %insert directive with cheader section, e.g. -
-%insert("cheader") %{
-#include "another.h"
-%}
--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. -
- --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; -
-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); - | -
-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. -
- - --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 -
| Typemap | -Used for | -
|---|---|
| ctype | -Provides types used for the C API and
- Typecasts wrapper functions return values in proxy functions
-
- MyClass *MyClass_new(void) {
- return (MyClass *)MyClass_new();
- }
-
- |
-
| in | -Mapping of wrapper functions parameters to local C++ variables
-
-
- SwigObj* MyClass_do(SwigObj *carg1) {
- SomeCPPClass *arg1 = 0;
- if (carg1)
- arg1 = (SomeCPPClass*)carg1->obj
- else
- arg1 = 0;
- }
- |
-
| out | -Assigns wrapped function's return value to a dedicated return variable, packaging it into SwigObj if necessary | -
| cppouttype | -Type of the result variable used for the return value if the wrapped function is a C++ function - | -
| cxxintype | -Defines the type for the parameters of C++ wrapper functions corresponding to this type. By default is the same as ctype, but may sometimes be different to make the functions more convenient to use. For example, ctype for std::string is const char*, but cxxintype typemap for it is std::string const&, i.e. even though the C++ string passed as a raw pointer via C API, the C++ wrapper still accepts a C++ string. If this typemap is defined, cxxin should normally be defined as well. If it is not defined, ctype is used. - | -
| cxxouttype | -Similar to cxxintype, but is used for the function return values and together with cxxout typemap. Also defaults to ctype if not defined. - | -
| cxxin | -Defines how to transform cxxintype value to ctype - | -
| cxxout | -Defines how to transform ctype value returned by a function to cxxouttype - | -
| cxxcode | -May contain arbitrary code that will be injected in the declaration of the C++ wrapper class corresponding to the given type. Ignored for non-class types. The special variable $cxxclassname is replaced with the name of the class inside this typemap expansion and $cclassptrname is replaced with the name of the pointer type used to represent the class in C wrapper functions. - | -
-%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); -
-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;
-}
--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; -
-// 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); -
-Any call to a C++ function may throw an exception, which cannot be caught by C code. Instead, the special SWIG_CException_get_pending() function must be called to check for this. If it returns a non-null pointer, SWIG_CException_msg_get() can be called to retrieve the error message associated with the exception. Finally, SWIG_CException_reset_pending() must be called to free the exception object and reset the current pending exception. Note that exception handling is much simpler when using C++, rather than C, wrappers, see sections 36.6.2. -
- --When -c++ command line option is used (and -nocxx one is not), the header file generated by SWIG will also contain the declarations of C++ wrapper functions and classes mirroring the original API. All C++ wrappers are fully inline, i.e. don't need to be compiled separately, and are always defined inside the namespace (or nested namespaces) specified by -namespace command-line option or the namespace with the same name as the SWIG module name if this option is not specified. -
- --C++ wrappers try to provide a similar API to the original C++ API being wrapped, notably any class Foo in the original API appears as a class with the same name in the wrappers namespace, and has the same, or similar, public methods. A class Bar deriving from Foo also derives from it in the wrappers and so on. There are some differences with the original API, however. Some of them are due to fundamental limitations of the approach used, e.g.: -
-Exception handling in C++ is more natural, as the exceptions are re-thrown when using C++ wrappers and so can be caught, as objects of the special SWIG_CException type, using the usual try/catch statement. The objects of SWIG_CException class have code() and msg() methods, with the latter returning the error message associated with the exception. -
- --If necessary, a custom exception type may be used instead of SWIG_CException. To do this, a custom implementation of swig_check() function, called to check for the pending exception and throw the corresponding C++ exception if necessary, must be provided and SWIG_swig_check_DEFINED preprocessor symbol must be defined to prevent the default implementation of this function from being compiled: -
-
-%insert(cxxheader) %{
-#ifndef SWIG_swig_check_DEFINED
-#define SWIG_swig_check_DEFINED 1
-
-#include
-
-class Exception : public std::runtime_error {
-public:
- explicit Exception(const char* msg) : std::runtime_error{msg} {}
-};
-
-inline void swig_check() {
- if (auto* swig_ex = SWIG_CException_get_pending()) {
- Exception const e{SWIG_CException_msg_get(swig_ex)};
- SWIG_CException_reset_pending();
- throw e;
- }
-}
-
-template T swig_check(T x) {
- swig_check();
- return x;
-}
-
-#endif // SWIG_swig_check_DEFINED
-%}
-