# SWIG Internals Manual

Thien-Thi Nguyen
ttn@glug.org

David M. Beazley
beazley@cs.uchicago.edu

$Header: /cvsroot/swig/SWIG/Doc/Devel/internals.html,v 1.7 2004/08/12 20:53:26 wsfulton Exp $

(Note : This is a work in progress.)

## Table of Contents

- 1. Introduction
  - 1.1 Directory Guide
  - 1.2 Overall Program Flow
- 2. DOH
  - 2.1 Motivation and Background
  - 2.2 Basic Types
  - 2.3 Creating, Copying and Destroying Objects
  - 2.4 A Word About Mutability and Copying
  - 2.5 Strings
  - 2.6 Lists
  - 2.7 Hash Tables
  - 2.8 Files
  - 2.9 Void Objects
  - 2.10 Utility Functions
- 3. Types and Typemaps
- 4. Parsing
- 5. Difference Between SWIG 1.1 and SWIG 1.3
- 6. Plans for SWIG 2.0
- 7. C/C++ Wrapper Support Functions
- 8. Symbol Naming Guidelines for Generated C/C++ Code
- 9. Debugging SWIG

## 1. Introduction

This document details SWIG internals: architecture and sometimes implementation. The first few sections concentrate on data structures, interfaces, conventions and code shared by all language targets. Subsequent sections focus on a particular language.

The audience is assumed to be SWIG developers (who should also read the SWIG Engineering Manual before starting to code).

### 1.1 Directory Guide

Doc | HTML documentation. If you find a documentation bug, please let us know.
Examples | This subdir tree contains examples of using SWIG w/ different scripting languages, including makefiles. Typically, there are the "simple" and "matrix" examples, w/ some languages offering additional examples. The GIFPlot example has its own set of per-language subdirectories. See the README more index.html file in each directory for more info. [FIXME: Ref SWIG user manual.]
Lib | These are the `.i` (interface) files that form the SWIG installed library. Language-specific files are in subdirectories (for example, guile/typemaps.i). Each language also has a `.swg` file implementing runtime type support for that language. The SWIG library is not versioned.
Misc | Currently this subdir only contains file `fileheader`. See the Engineering Manual for more info.
Source | SWIG source code is in this subdir tree. Directories marked w/ "(*)" are used in building the `swig` executable.
DOH (*) | C library providing memory allocation, file access and generic containers. Result: libdoh.a
Experiment | [TODO]
Include (*) | Configuration .h files
LParse | Parser (lex / yacc) files and support [why not (*)?!]
Modules | [TODO]
Modules1.1 (*) | Language-specific callbacks that does actual code generation (each language has a .cxx and a .h file). Result: libmodules11.a
Preprocessor (*) | SWIG-specialized C/C++ preprocessor. Result: libcpp.a
SWIG1.1 (*) | Parts of SWIG that are not language-specific, including option processing and the type-mapping system. Result: libswig11.a. Note: This directory is currently being phased out.
SWIG1.3 | [TODO] [funny, nothing here is presently used for swig-1.3]. This directory might turn into a compatibility interface between SWIG1.3 and the SWIG1.1 modules.
Swig (*) | This directory contains the new ANSI C core of the system and contains generic functions related to types, file handling, scanning, and so forth.
Tools | Libtool support and the mkdist.py script.
Win | This improperly-named (spit spit) subdir only has README.txt.

### 1.2 Overall Program Flow

Here is the general control flow and where under subdir `Source` to look for code:

- `Modules1.1/swigmain.cxx:main()` is the program entry point. It parses the language-specifying command-line option (for example, `-java`), creating a new language-specific wrapping object (each language is a C++ class derived from base class `Language`). This object and the command-line is passed to `SWIG_main()`, whose return value is the program exit value.
- `SWIG1.1/main.cxx:SWIG_main()` is the "real" main. It initializes the preprocessor and typemap machinery, defines some preprocessor symbols, locates the SWIG library, processes common command-line options, and then calls the language-specific command-line parser. From here there are three paths: "help", "checkout" and everything else.
  - In "help" mode, clean up open files and exit.
  - In "checkout" mode, copy specified files from the SWIG library to the current directory. Errors cause error messages but no non-lcoal exits.
  - Otherwise, do wrapping: determine output file name(s), define some preprocessor symbols and run the preprocessor, initialize the interface-definition parser, set up the typemap for handling new return strings, and finally do the language-specific parse (by calling the language object's `parse()` method), which creates output files by side-effect.
  Afterwards, remove temporary files, and clean up. If the command-line included `-freeze`, go into an infinite loop; otherwise return the error count.
- The language-specific `parse()` (and all other language-specific code) lives in `Modules1.1/foo.{h,cxx}` for language Foo. Typically, `FOO::parse()` calls `FOO::headers()` and then the global function `yyparse()`, which uses the callbacks registered by `SWIG_main()` above.

## 2. DOH

DOH is a collection of low-level objects such as strings, lists, and hash tables upon which the rest of SWIG is built. The name 'DOH' unofficially stands for "Dave's Object Hack", but it's also a good expletive to use when things don't work (as in "SWIG core dumped---DOH!").

### 2.1 Motivation and Background

The development of DOH is influenced heavily by the problems encountered during earlier attempts to create a C++ based version of SWIG2.0. In each of these attempts (over a 3 year period), the resulting system always ended up growing into a collossal nightmare of large inheritance hierarchies and dozens of specialized classes for different types of objects (functions, variables, constants, etc.). The end result was that the system was tremendously complicated, difficult to understand, difficult to maintain, and fairly inflexible in the grand scheme of things.

DOH takes a different approach to tackling the complexity problem. First, rather than going overboard with dozens of types and class definitions, DOH only defines a handful of simple yet very useful objects that are easy to remember. Second, DOH uses dynamic typing---one of the features that make scripting languages so useful and which make it possible to accomplish things with much less code. Finally, DOH utilizes a few coding tricks that allow it to perform a limited form of function overloading for certain C datatypes (more on that a little later).

The key point to using DOH is that instead of thinking about code in terms of highly specialized C data structures, just about everything ends up being represented in terms of a just a few datatypes. For example, structures are replaced by DOH hash tables whereas arrays are replaced by DOH lists. At first, this is probably a little strange to most C/C++ programmers, but in the long run in makes the system extremely flexible and highly extensible. Also, in terms of coding, many of the newly DOH-based subsystems are less than half the size (in lines of code) of the earlier C++ implementation.

### 2.2 Basic Types

The following built-in types are currently provided by DOH:

- String. A string of characters with automatic memory management and high-level operations such as string replacement. In addition, strings support file I/O operations that make it possible to use them just about anyplace a file can be used.

- List. A list of arbitrary DOH objects (of possibly mixed types).

- Hash. A hash table that maps a set of string keys to a set of arbitrary DOH objects. The DOH version of an associative array for all of you Perl fans.

- File. A DOH wrapper around the C FILE * structure. This is provided since other objects sometimes want to behave like files (strings for instance).

- Void. A DOH wrapper around an arbitrary C pointer. This can be used if you want to place arbitrary C data structures in DOH lists and hash tables.

Due to dynamic typing, all of the objects in DOH are represented by pointers of type `DOH *`. Furthermore, all objects are completely opaque--that means that the only way to access the internals of an object is through a well-defined public API. For convenience, the following symbolic names are sometimes used to improve readability:

- `DOHString *`. A String object.
- `DOHList *`. A list object.
- `DOHHash *`. A hash object.
- `DOHFile *`. A file object.
- `DOHVoid *`. A void object.
- `DOHString_or_char *`. A DOH String object or a raw C "char *".

It should be stressed that all of these names are merely symbolic aliases to the type `DOH *` and that no compile-time type checking is performed (of course, a runtime error may occur if you screw up).

### 2.3 Creating, Copying, and Destroying Objects

The following functions can be used to create new DOH objects

- `NewString(DOHString_or_char *value)`
  Create a new string object with contents initially set to value. value can be either a C string or a DOH string object.

- `NewStringf(char *fmt, ...)`
  Create a new string object with contents initially set to a formatted string. Think of this as being sprintf() combined with an object constructor.

- `NewList()`
  Create a new list object that is initially empty.

- `NewHash()`
  Create a new hash object that is initially empty.

- `NewFile(DOHString_or_char *filename, char *mode)`
  Open a file and return a file object. This is a wrapper around the C `fopen()` library call.

- `NewFileFromFile(FILE *f)`
  Create a new file object given an already opened `FILE *` object.

- `NewVoid(void *obj, void (*del)(void *))`
  Create a new DOH object that is a wrapper around an arbitrary C pointer. `del` is an optional destructor function that will be called when the object is destroyed.

Any object can be copied using the `Copy()` function. For example:

>     DOH *a, *b, *c, *d;
>     a = NewString("Hello World");
>     b = NewList();
>     c = Copy(a);         /* Copy the string a */
>     d = Copy(b);         /* Copy the list b */

Copies of lists and hash tables are shallow. That is, their contents are only copied by reference.

Objects can be deleted using the `Delete()` function. For example:

>     DOH *a = NewString("Hello World");
>     ...
>     Delete(a);              /* Destroy a */

All objects are referenced counted and given a reference count of 1 when initially created. The `Delete()` function only destroys an object when the reference count reaches zero. When an object is placed in a list or hash table, it's reference count is automatically increased. For example:

>     DOH *a, *b;
>     a = NewString("Hello World");
>     b = NewList();
>     Append(b,a);         /* Increases refcnt of a to 2 */
>     Delete(a);           /* Decreases refcnt of a to 1 */
>     Delete(b);           /* Destroys b, and destroys a */

Should it ever be necessary to manually increase the reference count of an object, the DohIncref() function can be used:

>     DOH *a = NewString("Hello");
>     DohIncref(a);

### 2.4 A Word About Mutability and Copying

All DOH objects are mutable regardless of their current reference count. For example, if you create a string and then create a 1000 references to it (in lists and hash tables), changes to the string will be reflected in all of the references. Therefore, if you need to make any kind of local change, you should first make a copy using the Copy() function. Caveat: when copying lists and hash tables, elements are copied by reference.

### 2.5 Strings

The DOH String type is perhaps the most flexible object. First, it supports a variety of string-oriented operations. Second, it supports many of the same operations as lists. Finally, strings provide file I/O operations that allow them to be used interchangably with DOH file objects. [ TODO ]

### 2.6 Lists

[ TODO ]

### 2.7 Hash tables

[ TODO ]

### 2.8 Files

[ TODO ]

### 2.9 Void objects

[ TODO ]

### 2.10 Utility functions

[ TODO ]

## 3. Types and Typemaps

Revised: Dave Beazley (8/14/00)

The representation and manipulation of types is currently in the process of being reorganized and (hopefully) simplified. The following list describes the current set of functions that are used to manipulate datatypes. These functions are different than in SWIG1.1 and may change names in the final SWIG1.3 release.

- `SwigType_str(SwigType *t, char *name)`.
  This function produces the exact string representation of the datatype `t`. `name` is an optional parameter that specifies a declaration name. This is used when dealing with more complicated datatypes such as arrays and pointers to functions where the output might look something like "`int (*name)(int, double)`".

- `SwigType_lstr(SwigType *t, char *name)`.
  This function produces a string representation of a datatype that can be safely be assigned a value (i.e., can be used as the "lvalue" of an expression). To do this, qualifiers such as "const", arrays, and references are stripped away or converted into pointers. For example:

  >     Original Datatype              lstr()
  >     ------------------             --------
  >     const char *a                  char *a
  >     double a[20]                   double *a
  >     double a[20][30]               double *a
  >     double &a                      double *a

  The intent of the lstr() function is to produce local variables inside wrapper functions--all of which must be reassignable types since they are the targets of conversions from a scripting representation.

- `SwigType_rcaststr(SwigType *t, char *name)`.
  This function produces a string that casts a type produced by the `lstr()` function to the type produced by the `str()` function. You might view it as the inverse of lstr(). This function only produces output when it needs to (when str() and lstr() produce different results). Furthermore, an optional name can be supplied when the cast is to be applied to a specific name. Examples:

  >     Original Datatype             rcaststr()
  >     ------------------            ---------
  >     char *a
  >     const char *a                 (const char *) name
  >     double a[20]                  (double *) name
  >     double a[20][30]              (double (*)[30]) name
  >     double &a                     (double &) *name

- `SwigType_lcaststr(SwigType *t, char *name)`.
  This function produces a string that casts a type produced by the `str()` function to the type produced by the `lstr()` function. This function only produces output when it needs to (when str() and lstr() produce different results). Furthermore, an optional name can be supplied when the cast is to be applied to a specific name.

  >     Original Datatype             lcaststr()
  >     ------------------            ---------
  >     char *a
  >     const char *a                 (char *) name
  >     double a[20]                  (double *) name
  >     double a[20][30]              (double *) name
  >     double &a                     (double *) &name

- `SwigType_manglestr(SwigType *t)`.
  Produces a type-string that is used to identify this datatype in the target scripting language. Usually this string looks something like "`_p_p_double`" although the target language may redefine the output for its own purposes. Normally this function strips all qualifiers, references, and arrays---producing a mangled version of the type produced by the `lstr()` function.

The following example illustrates the intended use of the above functions when creating wrapper functions using shorthand pseudocode. Suppose you had a function like this:

>     int foo(int a, double b[20][30], const char *c, double &d);

Here's how a wrapper function would be generated using the type generation functions above:

>     wrapper_foo() {
>        lstr("int","result")
>        lstr("int","arg0")
>        lstr("double [20][30]", "arg1")
>        lstr("const char *", "arg2")
>        lstr("double &", "arg3")
>        ...
>        get arguments
>        ...
>        result = (lcaststr("int"))  foo(rcaststr("int","arg0"),
>                                    rcaststr("double [20][30]","arg1"),
>                                    rcaststr("const char *", "arg2"),
>                                    rcaststr("double &", "arg3"))
>        ...
>     }

Here's how it would look with the corresponding output filled in:

>     wrapper_foo() {
>        int      result;
>        int      arg0;
>        double  *arg1;
>        char    *arg2;
>        double  *arg3;
>        ...
>        get arguments
>        ...
>        result = (int) foo(arg0,
>                           (double (*)[30]) arg1,
>                           (const char *) arg2,
>                           (double &) *arg3);
>        ...
>     }

Notes:

- For convenience, the string generation functions return a "`char *`" that points to statically allocated memory living inside the type library. Therefore, it is never necessary (and it's an error) to free the pointer returned by the functions. Also, if you need to save the result, you should make a copy of it. However, with that said, it is probably worth nothing that these functions do cache the last 8 results. Therefore, it's fairly safe to make a handful of repeated calls without making any copies.

[TODO]

## 4. Parsing

[TODO]

## 5. Difference Between SWIG 1.1 and SWIG 1.3

[TODO]

## 6. Plans for SWIG 2.0

[TODO]

## 7. The C/C++ Wrapping Layer

Added: Dave Beazley (July 22, 2000)

When SWIG generates wrappers, it tries to provide a mostly seamless integration with the original code. However, there are a number of problematic features of C/C++ programs that complicate this interface.

- Passing and returning structures by value. When used, SWIG converts all pass-by-value functions into wrappers that pass by reference. For example:

  >     double dot_product(Vector a, Vector b);

  gets turned into a wrapper like this:

  >     double wrap_dot_product(Vector *a, Vector *b) {
  >          return dot_product(*a,*b);
  >     }

  Functions that return by value require a memory allocation to store the result. For example:

  >     Vector cross_product(Vector *a, Vector *b);

  become

  >     Vector *wrap_cross_product(Vector *a, Vector *b) {
  >        Vector *result = (Vector *) malloc(sizeof(Vector));
  >        *result = cross_product(a,b);
  >        return result;
  >     }

  Note: If C++ is being wrapped, the default copy constructor is used instead of malloc() to create a copy of the return result.

- C++ references. C++ references are handled exactly the same as pass/return by value except that a memory allocation is not made for functions that return a reference.

- Qualifiers such as "const" and "volatile". SWIG strips all qualifiers from the interface presented to the target language. Besides, what in the heck is "const" in Perl anyways?

- Instance Methods. Method invocations are handled as a function call in which a pointer to the object (the "this" pointer) appears as the first argument. For example, in the following class:

  >     class Foo {
  >     public:
  >         double bar(double);
  >     };

  The "bar" method is wrapped by a function like this:

  >     double Foo_bar(Foo *self, double arg0) {
  >        return self->bar(arg0);
  >     }

- Structure/class data members. Data members are handled by creating a pair of wrapper functions that set and get the value respectively. For example:

  >     struct Foo {
  >         int x;
  >     };

  gets wrapped as follows:

  >     int Foo_x_get(Foo *self) {
  >         return self->x;
  >     }
  >     int Foo_x_set(Foo *self, int value) {
  >         return (self->x = value);
  >     }

- Constructors. Constructors for C/C++ data structures are wrapped by a function like this:

  >     Foo *new_Foo() {
  >         return new Foo;
  >     }

  Note: For C, new objects are created using the calloc() function.

- Destructors. Destructors for C/C++ data structures are wrapper like this:

  >     void delete_Foo(Foo *self) {
  >         delete self;
  >     }

  Note: For C, objects are destroyed using free().

The creation of wrappers and various type transformations are handled by a collection of functions found in the file `Source/Swig/cwrap.c`.

- `char *Swig_clocal(DataType *t, char *name, char *value)`
  This function creates a string containing the declaration of a local variable with type `t`, name `name`, and default value `value`. This local variable is stripped of all qualifiers and will be a pointer if the type is a reference or user defined type.

- `DataType *Swig_clocal_type(DataType *t)`
  Returns a type object corresponding to the type string produced by the Swig_clocal() function.

- `char *Swig_clocal_deref(DataType *t, char *name)`
  This function is the inverse of the `clocal()` function. Given a type and a name, it produces a string containing the code needed to cast/convert the type produced by `Swig_clocal()` back into it's original type.

- `char *Swig_clocal_assign(DataType *t, char *name)`
  Given a type and name, this produces a string containing the code (and an optional cast) needed to make an assignment from the real datatype to the local datatype produced by `Swig_clocal()`. Kind of the opposite of deref().

- `int Swig_cargs(Wrapper *w, ParmList *l)`
  Given a wrapper function object and a list of parameters, this function declares a set of local variables for holding all of the parameter values (using Swig_clocal()). Returns the number of parameters. In addition, this function sets the local name of each parameter which can be retrieved using the `Parm_Getlname()` function.

- `void Swig_cresult(Wrapper *w, DataType *t, char *resultname, char *decl)`
  Generates the code needed to set the result of a wrapper function and performs all of the needed memory allocations for ANSI C (if necessary). `t` is the type of the result, `resultname` is the name of the result variable, and `decl` is a string that contains the C code which produces the result.

- `void Swig_cppresult(Wrapper *w, DataType *t, char *resultname, char *decl)`
  Generates the code needed to set the result of a wrapper function and performs all of the needed memory allocations for C++ (if necessary). `t` is the type of the result, `resultname` is the name of the result variable, and `decl` is a string that contains the C code which produces the result.

- `Wrapper *Swig_cfunction_wrapper(char *fname, DataType *rtype, ParmList *parms, char *code)`
  Create a wrapper around a normal function declaration. `fname` is the name of the wrapper, `rtype` is the return type, `parms` are the function parameters, and `code` is a string containing the code in the function body.

- `Wrapper *Swig_cmethod_wrapper(char *classname, char *methodname, DataType *rtype, DataType *parms, char *code)`

- `char *Swig_cfunction_call(char *name, ParmList *parms)` This function produces a string containing the code needed to call a C function. The string that is produced contains all of the transformations needed to convert pass-by-value into pass-by-reference as well as handle C++ references. Produces a string like "name(arg0, arg1, ..., argn)".

Here is a short example showing how these functions could be used. Suppose you had a C function like this:

>     double dot_product(Vector a, Vector b);

Here's how you might write a really simple wrapper function

>     ParmList *l = ... parameter list of the function ...
>     DataType *t = ... return type of the function ...
>     char     *name = ... name of the function ...
>     Wrapper *w = NewWrapper();
>     Printf(w->def,"void wrap_%s() {\n", name);

>     /* Declare all of the local variables */
>     Swig_cargs(w, l);

>     /* Convert all of the arguments */
>     ...

>     /* Make the function call and declare the result variable */
>     Swig_cresult(w,t,"result",Swig_cfunction(name,l));

>     /* Convert the result into whatever */
>     ...

>     Printf(w->code,"}\n");
>     Wrapper_print(w,out);

The output of this would appear as follows:

>     void wrap_dot_product() {
>         Vector *arg0;
>         Vector *arg1;
>         double  result;

>         ...
>         result = dot_product(*arg0, *arg1);
>         ...
>     }

Notice that the `Swig_cargs()`, `Swig_cresult()`, and `Swig_cfunction()` functions have taken care of the type conversions for the `Vector` type automatically.

Notes:

- The intent of these functions is to provide consistent handling of function parameters and return values so that language module writers don't have to worry about it too much.

- These functions may be superceded by features in the new typemap system which provide hooks for specifying local variable declarations and argument conversions.

## 8. Symbol Naming Guidelines for Generated C/C++ Code

The C++ standard (ISO/IEC 14882:1998(E)) states:

>     17.4.3.1.2 Global names [lib.global.names]

>     1 Certain sets of names and function signatures are always reserved to the implementation:

>         * Each name that contains a double underscore (__) or begins with an underscore followed
>           by an upper case letter (2.11) is reserved to the implementation for any use.
>         * Each name that begins with an underscore is reserved to the implementation for use as
>           a name in the global namespace.165)

>         165) Such names are also reserved in namespace ::std (17.4.3.1). [back to text]

When generating code it is important not to generate symbols that might clash with the code being wrapped. It is tempting to flout the standard or just use a symbol which starts with a single underscore followed by a lowercase letter in order to avoid name clashes. However even these legal symbols can also clash with symbols being wrapped. The following guidelines should be used when generating code in order to meet the standard and make it highly unlikely that symbol clashes will occur:

For C++ code that doesn't attempt to mangle a symbol being wrapped (for example SWIG convenience functions):

- Put symbols in the `Swig` namespace, for example class `Swig::Director`. Qualify using the `Swig` namespace whenever the symbol is referenced, even within the `Swig` namespace, for example `new Swig::Director()` not `new Director()`.
- Use `swig_` as a prefix for all member variables and member functions that are involved in an inheritance chain with wrapped classes, for example `Swig::Director::swig_get_up()` and `bool Swig::Director::swig_up`.
- Alternatively class names can be prefixed with `Swig` in the global namespace for example `template<class T> class SwigValueWrapper`.

For code compiled as C or C++ that doesn't attempt to mangle a symbol being wrapped (for example SWIG convenience functions):

- Use `SWIG_` as a prefix for structures for example `SWIG_JavaExceptions_t`.
- Use `SWIG_` as a prefix for global functions for example `SWIG_TypeRegister`.
- Use `SWIG_` as a prefix for macros for example `#define SWIG_PY_INT 1`

For code compiled as C or C++ that attempts to mangle a wrapped symbol:

- Use `SWIGxxx` or `Swigxxx` as a prefix where xxx is chosen which would make `SWIGxxx`/`Swigxxx` a unique symbol in the global namespace, for example `class SwigDirectorFoo` when wrapping `class Foo`. Don't use a trailing underscore for the prefix as this may generate a double underscore when wrapping a symbol which starts with a single underscore.

In the past SWIG has generated many symbols which flout the standard especially double underscores. In fact they may not all be rooted out yet, so please fix them when you see them.

## 9. Debugging SWIG

Warning. Debugging SWIG is for the very patient.

The DOH types are all typedefined to void. Consequently, it is impossible for debuggers to extract any information about DOH objects. Most debuggers will be able to display useful variable information when an object is cast to the appropriate type. Below are some tips for displaying some of the DOH objects. Be sure to compile with compiler optimisations turned off before attempting the casts shown in a debugger window else they are unlikely to work. Even displaying the underlying string in a String* doesn't work straight off in all debuggers due to the multiple definition of String as a struct and a void.

Below are a list of common SWIG types. With each is the cast that can be used in the debugger to extract the underlying type information and the underlying char * string.

- String *s;
(String *)((DohBase *)s)->data
The underlying char * string can be displayed with
((String *)((DohBase *)s)->data)->str

- SwigType *t;
(String *)((DohBase *)t)->data
The underlying char * string can be displayed with
((String *)((DohBase *)t)->data)->str

- String_or_char *sc;
Either
((String *)((DohBase *)sc)->data)->str
or
(char *)sc
will work depending on whether the underlying type is really a String * or char *.

Copyright (C) 1999-2004 SWIG Development Team.