38 SWIG and MzScheme/Racket

This section contains information on SWIG's support of Racket, formally known as MzScheme.

38.1 Creating native structures

Example interface file:

/* define a macro for the struct creation */
%define handle_ptr(TYPE, NAME)
%typemap(argout) TYPE *NAME{
  Scheme_Object *o = SWIG_NewStructFromPtr($1, $*1_mangle);

%typemap(in, numinputs=0) TYPE *NAME (TYPE temp) {
  $1 = &temp;

/* setup the typemaps for the pointer to an output parameter cntrs */
handle_ptr(struct diag_cntrs, cntrs);

Then in scheme, you can use regular struct access procedures like

        ; suppose a function created a struct foo as
        ; (define foo (make-diag-cntrs (#x1 #x2 #x3) (make-inspector))
        ; Then you can do
        (format "0x~x" (diag-cntrs-field1 foo))
        (format "0x~x" (diag-cntrs-field2 foo))

38.2 Simple example

A few examples are available in the Examples/mzscheme directory. The code and log of a session using SWIG below should help getting started.

C header file:

// example.h
int fact(int n);

C source code:

// File: example.c
#include "example.h"

int fact(int n) {
  if (n < 0) { /* This should probably return an error, but this is simpler */
    return 0;
  if (n == 0) {
    return 1;
  else {
    /* testing for overflow would be a good idea here */
    return n * fact(n-1);

SWIG interface file:

/* File: example.i */
%module example

#include "example.h"

int fact(int n);

The session below using the above files is on an OS X machine, but the points to be made are more general. On OS X, libtool is the tool which creates libraries, which are named .dylib, rather than .so on other unixes, or .dll on Windows.

% swig -mzscheme -declaremodule example.i
% gcc -c -m32 -o example.o example.c # force 32-bit object file (mzscheme is 32-bit only)
% libtool -dynamic -o libexample.dylib example.o # make it into a library
% ls # what've we got so far?
example.c example.o
example.h example_wrap.c
example.i libexample.dylib*
% mzc --cgc --cc example_wrap.c # compile the wrapping code
% LDFLAGS="-L. -lexample" mzc --ld example_wrap.dylib example_wrap.o # ...and link it
% mzscheme -e '(path->string (build-path "compiled" "native" (system-library-subpath)))'
% mkdir -p compiled/native/i386-macosx/3m # move the extension library to a magic place
% mv example_wrap.dylib compiled/native/i386-macosx/3m/example_ss.dylib
% mzscheme
Welcome to MzScheme v4.2.4 [3m], Copyright (c) 2004-2010 PLT Scheme Inc.
> (require "example.ss")
> (fact 5)
> ^D
% echo 'It works!'

Some points of interest:

38.3 External documentation

See the C API for more description of using the mechanism for adding extensions. The main documentation is here.

Tip: mzc's --vv option is very useful for debugging the inevitable library problems you'll encounter.