Question

I’m using Cent OS, SWIG 1.3 and I’ve tested to compile the sample Java example from SWIG examples. It consists from:

example.c

/* A global variable */
double Foo = 3.0;

/* Compute the greatest common divisor of positive integers */
int gcd(int x, int y) {
  int g;
  g = y;
  while (x > 0) {
    g = x;
    x = y % x;
    y = g;
  }
  return g;
}

example.i

%module example

extern int gcd(int x, int y);
extern double Foo;

Then I use the command:

swig -java example.i

Then I compile the generated example_wrap.c with:

gcc -c example_wrap.c -I/usr/java/jdk1.6.0_24/include -I/usr/java/jdk1.6.0_24/include/linux

And I have the following errors:

example_wrap.c: In function ‘Java_exampleJNI_Foo_1set’:
example_wrap.c:201: error: ‘Foo’ undeclared (first use in this function)

Is the example.i file wrong or I don’t accomplish something? Or this is a bug in SWIG? Is there a workaround?

Answer 1

You’ve told SWIG that the function and globals will be declared, but you need to make sure that declaration is visible in the generated wrapper code. (You also probably got a warning about an implicit declaration of gcd, if you didn’t use a higher warning setting for gcc)

The solution is to make that declaration visible, the simplest way is:

%module example

%{
// code here is passed straight to example_wrap.c unmodified
extern int gcd(int x, int y);
extern double Foo;
%}

// code here is wrapped:
extern int gcd(int x, int y);
extern double Foo;

Personally I’d add an example.h file with those declarations in and the make the module file:

%module example

%{
// code here is passed straight to example_wrap.c unmodified
#include "example.h"
%}

// code here is wrapped:
%include "example.h"

with a corresponding include in example.c for good measure.

An alternative style to write this would be:

%module example

%inline %{
  // Wrap and pass through to example_wrap.c simultaneously
  extern int gcd(int x, int y);
  extern double Foo;  
%}

But normally I’d only recommend using %inline for cases where what you’re wrapping is specific to the process of wrapping and not a general part of the library you want to wrap.