(Note : This is a work in progress.)
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.
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.
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.
Each file should include a short abstract, license information and a SVN revision tag 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 http://www.swig.org/legal.html.
*
* xxx.c
*
* This file defines ...
* ----------------------------------------------------------------------------- */
static char cvs[] = "$Id$";
#include "swig.h"
/* Declarations */
typedef struct {
int x, y;
} Foo;
...
/* Private Declarations (used only in this file) */
static int avariable;
...
/* Functions */
...
The SVN revision tag should be placed into a static string as shown
above mangled with the name of the file.
This adds the revision information to the SWIG executable and
makes it possible to extract version information from a raw binary
(sometimes useful in debugging).
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.
/* Simple bottom-up program */
#include <stdio.h>
int foo(int x, int y) {
/* Implement foo */
...
}
int bar() {
...
foo(i,j);
...
}
...
int main(int argc, char **argv) {
...
bar();
...
}
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.
/* -------------------------------------------------------------------------
* Swig_add_directory()
*
* Adds a directory to the SWIG search path.
* ------------------------------------------------------------------------- */
void
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.
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.Preprocessor_define() Swig_add_directory()
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.
Separate words in a constant should be separated by underscores as with functions.#define SWIG_TOKEN_LPAREN 1
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;
typedef struct Foo {
int line;
} Foo;
It is better to hide the implementation of Foo and provide an
function-call interface like this:
Although this results in worse performance, there are many practical reasons for doing this. The most important reason is that it allows you to change the internal representation of Foo without breaking all of the other modules or having to recompile the entire universe after making your changes.typedef struct Foo Foo; extern int Foo_getline(Foo *f); extern void Foo_setline(Foo *f, int line);
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 svn access.