SWIG Engineering Manual

David Beazley

(Note : This is a work in progress.)

Table of Contents

1. Introduction

The purpose of this document is to describe various coding conventions and organizational aspects for SWIG developers. The idea for this document is largely borrowed from John Ousterhout's Tcl/Tk Engineering Manual. It is not my intent to overly managerial about matters--rather I'm hoping to make life a little less chaotic for everyone.

First a little background: SWIG was started in 1995 as a one-person project and continued in this mode of operation until about 1998. Most of this development was driven by ideas submitted by early SWIG users as opposed to being motivated by a grand design. As a result, the code ended up being a pretty horrible C++ coding disaster. A mostly working disaster perhaps, but a disaster nonetheless.

With that said, the primary goal of future SWIG development is to reengineer the original system, fix most of its inherent design flaws, and to produce what I hope will become a highly extensible and modular interface compiler framework. To this do this, there are a few critical areas of work. First, I want to restructure SWIG as a collection of loosely coupled modules written in either ANSI C or an scripting language. Second, I want the system to be minimalistic in its use of data structures and interconnections. The primary reason for this is that the fewer data structures there are, the less users will have to remember. This will also make the system more accessible to non-experts. Finally, I want to reevaluate the whole idea of a SWIG module is and expand the definition to include just about anything from parsers, preprocessors, optimizers, interface editors, and code generators.

The rest of this document outlines a few general rules of how code should be developed within the SWIG project. These rules are primarily drawn from my own experience developing software and observing the practices of other successful projects.

2. Programming Languages and Libraries

All SWIG modules must be written in either ANSI C or one of the scripting languages for which SWIG can generate an interface (e.g., Perl, Python, or Tcl). C++ is currently being used to write SWIG modules, but it is only being utilized to avoid working with a lot of pointers to functions. Advanced C++ features like namespaces, templates, and overloading should not be used..

Module writers should make every attempt to use only those functions described in the POSIX.1 standard. This includes most of the functions contained the Kernighan and Ritchie C programming book. Use of operating system dependent functionality such as socket libraries should always be included inside a conditional compilation block so that it can be omitted on problematic platforms. If you are unsure about a library call, check the man page or contact Dave.

3. The Source Directory and Module Names

All SWIG modules are contained within the "Source" directory. Within this directory, each module is placed into its own subdirectory. The name of this subdirectory should exactly match the name of the module. For example, if you are creating a module called "Tcl", all of your files should be placed in a directory "Tcl".

When choosing a module name, please pick a name that is not currently in use. As a general convention, the first letter of a module name is capitalized such as "Perl". Alternatives such as "perl" or "PERL" should be avoided. In certain instances, the first two letters may be capitalized as in "CParse." The exact usage of this is somewhat inconsistent and isn't terribly important--just make sure the first letter is capitalized. Also, module names should not start with numbers, include underscores or any other special non-alphanumeric characters.

5. File Structure

Each file in a module should be given a filename that is all lowercase letters such as "parser.c", not "Parser.c" or "PARSER.c". Please note that filenames are case-insensitive on Windows so this convention will prevent you from inadvertently creating two files that differ in case-only.

Each file should include a short abstract and license information like this:

/* -----------------------------------------------------------------------------
 * This file is part of SWIG, which is licensed as a whole under version 3 
 * (or any later version) of the GNU General Public License. Some additional
 * terms also apply to certain portions of SWIG. The full details of the SWIG
 * license and copyrights can be found in the LICENSE and COPYRIGHT files
 * included with the SWIG source code as distributed by the SWIG developers
 * and at https://www.swig.org/legal.html.
 * xxx.c
 * This file defines ...
 * ----------------------------------------------------------------------------- */

#include "swig.h"

/* Declarations */
typedef struct {
   int x, y;
} Foo;


/* Private Declarations (used only in this file) */
static int  avariable;


/* Functions */

As a general rule, files start to get unmanageable once they exceed about 2000 lines. Files larger than this should be broken up into multiple files. Similarly, you should avoid the temptation to create many small files as this increases compilation time and makes the directory structure too complicated.

6. Bottom-Up Design

Within each source file, the preferred organization is to use what is known as "bottom-up" design. Under this scheme, lower-level functions appear first and the highest level function appears last. The easy way to remember is that the "main" function of your module should always appear last in the source file. For example:
/* Simple bottom-up program */
#include <stdio.h>

int foo(int x, int y) {
    /* Implement foo */

int bar() {

int main(int argc, char **argv) {
This choice of design is somewhat arbitrary however it has a number of benefits particular to C. In particular, a bottom-up design generally eliminates the need to include forward references--resulting in cleaner code and fewer compilation errors.

7. Functions

All functions should have a function header that gives the function name and a short description like this:
/* -------------------------------------------------------------------------
 * Swig_add_directory()
 * Adds a directory to the SWIG search path.
 * ------------------------------------------------------------------------- */

Swig_add_directory(DOH *dirname) {

In the function declaration, the return type and any specifiers (extern or static) should appear on a separate line followed by the function name and arguments as shown above. The left curly brace should appear on the same line as the function name.

Function declarations should NOT use the pre-ANSI function declaration syntax. The ANSI standard has been around long enough for this to be a non-issue.

8. Naming Conventions

The following conventions are used to name various objects throughout SWIG.


Functions should consist of the module name and the function name separated by an underscore like this:
In general, the module name should match the name of the module subdirectory and the function name should be in all lowercase with words separated by underscores.

Structures and Types

If your module defines new structures, the structure name should include the name of the module and the name of the structure appended together like this:
typedef struct SwigScanner {
} SwigScanner;

typedef struct LParseType {
} LParseType;
In this case, both the name of the module and the type should be capitalized. Also, whenever possible, you should use the "typedef struct Name { ... } Name" form when defining new data structures.

Global Variables

Global variables should be avoided if at all possible. However, if you must use a global variable, please prepend the module name and use the same naming scheme as for functions.


Constants should be created using #define and should be in all caps like this:
#define   SWIG_TOKEN_LPAREN  1
Separate words in a constant should be separated by underscores as with functions.

Structure members

Structure members should be in all lower-case and follow the same word-separation convention as for function names. However, the module name does not have to be included. For example:
typedef struct SwigScanner {
  DOH           *text;           /* Current token value */
  DOH           *scanobjs;       /* Objects being scanned */
  DOH           *str;            /* Current object being scanned */
  char          *idstart;        /* Optional identifier start characters */
  int            next_token;     /* Next token to be returned */
  int            start_line;     /* Starting line of certain declarations */
  int            yylen;          /* Length of text pushed into text */
  DOH           *file;           /* Current file name */
} SwigScanner;

Static Functions and Variables

Static declarations are free to use any naming convention that is appropriate. However, most existing parts of SWIG use lower-case names and follow the same convention as described for functions.

9. Visibility

Modules should keep the following rules in mind when exposing their internals:

10. Miscellaneous Coding Guidelines

These are largely covered in the main documentation in the Extending.html file.

11. Git Tagging Conventions

Use git tag to declare some set of file revisions as related in some symbolic way. This eases reference, retrieval and manipulation of these files later. At the moment (2001/01/16 14:02:53), the conventions are very simple; let's hope they stay that way!

There are two types of tags, internal (aka personal) and external. Internal tags are used by SWIG developers primarily, whereas external tags are used when communicating with people w/ anonymous git access.

That's all there is to it. Some example tags:
Copyright (C) 1999-2004 SWIG Development Team.