Using swig c++ to python with no access to library source

Question

I'm new to swig and tried scouring the documentation for an answer but I either missed it or didn't recognize the answer when I saw it.

I have a large C++ library from a third party for which I have no source code except the header. I need to access a few of the APIs from python and several of the APIs have function prototypes similar to:

int foo(int& a, int& b, int& c);

I've created a .i file that looks like:

%module myWrapper 
%{   
  #include libraryHeader.h
%}
extern int foo(int& m1, int& m2, int& bf);

This goes through swig, the g++ compiler and linker with no problems. I can import the module into python but when I call it I get the following error:

TypeError: in method 'foo', argument 1 of type 'int &'

I've declared 3 int variables like m1 = 0; m2 = 0; bf = 0 so I am passing something in. Is there a way to do this using the typemaps.i library or explicitly using typemaps? The documentation seems a bit vague on the subject.

Thanks in advance, Paul

Answer 1

SWIG needs hints where parameters are not simply inputs. These hints are provided by typemaps. You either need to define typemaps or use pre-defined ones (seetypemaps.i ).

Here's an example:

%module test

%{
    int foo(int& a, int& b, int& c)
    {
        int ret = a + b + c;
        a = 10;
        b = 20;
        c = 30;
        return ret;
    }
%}

%include <typemaps.i>
int foo(int& INOUT, int& INOUT, int& INOUT);

With the INOUT typemap, Python can pass integers into a function with for int* or int& , and the return value will be a tuple of the original return value and any output arguments. Compiling the SWIG result above and using in Python looks like this:

>>> import test
>>> test.foo(1,2,3)
[6, 10, 20, 30]

You can update variables with code like this:

>>> a,b,c = 1,2,3
>>> r,a,b,c = test.foo(a,b,c)
>>> r
6
>>> a,b,c
(10, 20, 30)

Note you can also %apply existing typemaps to types so if using %include "libraryHeader.h" instead of directly declaring functions you can use the following to generically apply the INOUT typemap to all int& parameters:

%apply int &INOUT { int& };
%include "libraryHeader.h"

Answer 2

I've answered my own question with the help of others on this thread and stubbornness. I created a .i file that looks like:

%module myWrapper

%include "typemaps.i"

%{
 #include "libraryHeader.h"
 typedef unsigned int MY_RESULT;
%}

typedef unsigned int MY_RESULT;
extern MY_RESULT MyGetVersion(int& OUTPUT, int& OUTPUT, int& OUTPUT, int& 
OUTPUT);
extern MY_RESULT MyGetDeviceDriverVersion(int deviceType, int& OUTPUT, int& 
OUTPUT, int& OUTPUT, int& OUTPUT);

It swigs, compiles and links cleanly and a very simple python test script returns the expected results. The script looks like:

#!/usr/bin/python3

import myWrapper

ret = 0    # function return value
maj = 0    # major version
min = 0    # minor version
bugFix = 0 # bug fix number
build = 0  # build

#
# Because of the typemap defined in myWrapper.i, we don't need to supply function arguments. 
# The results are returned by the function call as seen below...
#
ret, maj, min, bugFix, build = myWrapper.MyGetVersion()

#
# See what we got 
#
print('function return: '+str(ret)+' Maj: '+str(maj)+' Min: '+str(min)+' BugFix:'+str(bugFix)+' Build: '+str(build))

#
# This call returns the same information that the call above does but adds an input argument
# that specifies the Device Catagory. 0 is PCI
#
ret, maj, min, bugFix, build = myWrapper.MyGetDeviceDriverVersion(0)

print('function return: '+str(ret)+' Maj: '+str(maj)+' Min: '+str(min)+' BugFix: '+str(bugFix)+' Build: '+str(build))

The result of running this script provides the expected results:

function return: 0 Maj: 5 Min: 31 BugFix: 0 Build: 109
function return: 0 Maj: 4 Min: 26 BugFix: 4 Build: 253

Thank you all very much for the help and I hope that this answer can help someone else.

Paul