Question

I’m trying to use SWIG to create a Octave function. But even the most basic example code seems to fail before I even get the chance to get it into octave. I am not sure if I should be concerned that swig creates a extension type for the C++ source file (.cxx) it generates that mkoctfile doesn’t recognize (it only takes .C .cpp .cc, which seems like allot for swig to mess up that much and create what seems like the ONE extension for C++ it doesn’t support).

Any help from someone with more experience with this would be greatly appreciated!

The steps I have done are as follows:

swig -octave swig_test.i
mv swig_test_wrap.cxx swig_test_wrap.cpp <--- This is necessary because mkoctfile doesn't recognize the cxx type c++ code that swig generates
mkoctfile swig_test_wrap.cpp

This results in 4 errors consistantly:

swig_test_wrap.cpp:1449:24: error: invalid covariant return type for 'virtual Octave_map octave_swig_type::map_value() const'
/usr/include/octave-3.4.0/octave/../octave/ov-base.h:560:22: error:   overriding 'virtual octave_map octave_base_value::map_value() const'
swig_test_wrap.cpp:1657:24: error: invalid covariant return type for 'virtual Octave_map octave_swig_ref::map_value() const'
/usr/include/octave-3.4.0/octave/../octave/ov-base.h:560:22: error:   overriding 'virtual octave_map octave_base_value::map_value() const'

The source code of my outlandishly basic swig_test.c

int fact(int n) {
  if (n <= 1) return 1;
  else return n*fact(n-1);
}

int my_mod(int x, int y) {
   return (x%y);
}

Then the code of my wrapper inteface file swig_test.i

%module swig_test
%{
  extern int fact(int n);
  extern int my_mod(int x, int y);
%}

 extern int fact(int n);
 extern int my_mod(int x, int y);

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UPDATE: May 9th 2011

So I still have not found a solution to this, and am starting to wonder if maybe these programs are out of date? The documentation most certainly is. Just as an example: the instructions say

 swig -octave swig_test.i -o swig_test_wrap.cxx

now that will certainly not work regardless, because mkoctfile wont take type cxx as stated before. Also, this command just is physically written wrong. As typed above, it returns the error.

 swig error : unrecognized option example.i
 use swig -help for available options

The command SHOULD be entered as:

 swig -octave -o swig_test_wrap.cpp swig_test.i

That WILL generate the swig_test_wrap.cpp file just as advertised. I would have thought there would be a -i option for input file in the argv of swig, but hey, now that I know that order matters here, someone must have just not updated the documentation when they changed something about how the function works.

So now, after running this command I have my swig_test_wrap.cpp file. Next I take that and try to execute

mkoctfile swig_test_wrap.cpp swig_test.c

Again, I get the same error as above: “invalid covariant return type” etc, however I also DO get a file swig_test.o out of the process. Just for fun, I then ran

mkoctfile swig_test.o

And lo and behold, this DOES generate a file called swig_test.oct. However when I went into octave and tried to load the file by running

octave:1>swig_test

I get the response error: ‘swig_test’ undefined near line 1 column 1

So as far as I can tell, I’m right back to square one. Anyone have any ideas?

Answer 1

So it turns out that this issue is related to the version of octave that I was using. Octave version 3.4.0 doesn’t seem to work with swig yet. I got help on source-forge and once i downgraded to version 3.2.4-r3 it works perfectly.

And it is a known bug that you have to use:

swig -octave -o WRAPFILE.cpp INPUT.i

and its a problem with mkoctfile that you have to use the .cpp extension, since they SHOULD accept the .cxx extension as it is a valid extension for C++ files.