Question

I’ve got some C code that I want to expose to Python. It has a calling convention like this:

int add(int a, int b, int *err)

where the return value would be (a+b) or whatever, but if something went wrong, then I would get an error code in *err. I want to wrap this function so that it behaves like this, from the Python perspective:

def add(a,b):
    if something_bad:
        raise RuntimeError("something bad")
    return a+b

This should be easy, right? But I’m not finding it so.

Here is something that I have that works, but look out for the myerr3 kludge:

%module myswig
%feature("autodoc","1");

%{
int add(int a, int b, int *err){
    if(a < 0)*err = 1;
    if(b < 0)*err = 2;
    return a+b;
}

char *err_string(int err){
    switch(err){
    case 1:return "first argument was less than 0";
    case 2:return "second argument was less than 0";
    default:return "unknown error";
    }
}
%}

%typemap(in,numinputs=0) int *err (int myerr = 0){
    $1 = &myerr;
};

%exception{
    $action
    if(myerr3 != 0){
        PyErr_SetString(PyExc_RuntimeError,err_string(myerr3));
        return NULL;
    }
};

int add(int a, int b, int *err);

This behaves as it should, eg with

import myswig
print "add(1,1) = "
print myswig.add(1,1)
# prints '2'

print "add(1,-1) = "
print myswig.add(1,-1)
# raises an exception

# we never get here...
print "here we are"

but I can’t really use this solution, because if I have another function like

int add(int a, int b, int c, int *err)

then my myerr3 kludge will break down.

What’s the better way to solve this problem, without changing the calling convention of the C code?

Answer 1

From Karl Wette, via the swig-user mailing list:

You could modify your “in” typemap by moving the declaration of
“myerr” inside the typemap:

%typemap(in,numinputs=0, noblock=1) int *err {
   int myerr = 0;
   $1 = &myerr;
};

So long as there’s only one “int *err” argument in each function, this
should be fine. You can then use “myerr” directly without the argument
number.

This seems to be exactly the right solution, no kludges required. Thanks Karl!