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I have a struct:

struct some_struct_s {

   int arg1;

   int arg2;

};

I have a C function:

int func(some_struct_s *output);

Both are %included into my SWIG file.

I want some_struct_s *output to be treated like an output parameter. Python example:

int_val, some_struct_output = func()

"Output parameters" is covered in the manual for POD-types (sec 10.1.3), but not for non-POD types.

How do I tell SWIG I want some_struct_s *output to be an output parameter?

From the documentation:

11.5.7 "argout" typemap

The "argout" typemap is used to return values from arguments. This is most commonly used to write wrappers for C/C++ functions that need to return multiple values. The "argout" typemap is almost always combined with an "in" typemap---possibly to ignore the input value....

Here's a complete example for your code (no error checking for brevity):

%module test



// Declare an input typemap that suppresses requiring any input and

// declare a temporary stack variable to hold the return data.

%typemap(in,numinputs=0) some_struct_s* (some_struct_s tmp) %{

    $1 = &tmp;

%}



// Declare an output argument typemap.  In this case, we'll use

// a tuple to hold the structure data (no error checking).

%typemap(argout) some_struct_s* (PyObject* o) %{

    o = PyTuple_New(2);

    PyTuple_SET_ITEM(o,0,PyLong_FromLong($1->arg1));

    PyTuple_SET_ITEM(o,1,PyLong_FromLong($1->arg2));

    $result = SWIG_Python_AppendOutput($result,o);

%}



// Instead of a header file, we'll just declare this code inline.

// This includes the code in the wrapper, as well as telling SWIG

// to create wrappers in the target language.

%inline %{



struct some_struct_s {

   int arg1;

   int arg2;

};



int func(some_struct_s *output)

{

    output->arg1 = 1;

    output->arg2 = 2;

    return 0;

}



%}

Demo below. Note that the int return value of zero as well as the output parameter as a tuple are returned as a list.

>>> import test

>>> test.func()

[0, (1, 2)]

If you don't want typemaps, you can also inject code to create the object and return it to hide it from the user:

%module test



%rename(_func) func; // Give the wrapper a different name



%inline %{



struct some_struct_s {

   int arg1;

   int arg2;

};



int func(struct some_struct_s *output)

{

    output->arg1 = 1;

    output->arg2 = 2;

    return 0;

}



%}



// Declare your interface

%pythoncode %{

def func():

    s = some_struct_s()

    r = _func(s)

    return r,s

%}

Demo:

>>> import test

>>> r,s=test.func()

>>> r

0

>>> s

<test.some_struct_s; proxy of <Swig Object of type 'some_struct_s *' at 0x000001511D70A880> >

>>> s.arg1

1

>>> s.arg2

2

You can make the typemap language agnostic if you carefully select SWIG macros:

%module test



%typemap(in,numinputs=0) struct some_struct_s *output %{

    $1 = malloc(sizeof(struct some_struct_s));

%}



%typemap(argout) struct some_struct_s* output {

    %append_output(SWIG_NewPointerObj($1,$1_descriptor,1));

}



%inline %{



struct some_struct_s {

   int arg1;

   int arg2;

};



int func(struct some_struct_s *output)

{

    output->arg1 = 1;

    output->arg2 = 2;

    return 0;

}



%}

Demo:

>>> import test

>>> r,s=test.func()

>>> r

0

>>> s

<test.some_struct_s; proxy of <Swig Object of type 'some_struct_s *' at 0x000001DD0425A700> >

>>> s.arg1

1

>>> s.arg2

2