How to access value of struct that only has pointer typedef with SWIG/Python

Question

I am creating a Python wrapper for an existing code base with a DLL and header. I am using SWIG.

The code base has multiple struct pointer defs like so:

typedef struct session* session_t;

In this example, session is not defined anywhere else. All other functions in the DLL take session_t as an input. If I try and reference session anywhere in SWIG generated code I get compile errors that session is not defined. I need a way to access the value of session_t in Python code.

I've tried using cpointer.i and pointer_functions macro, but this doesn't work since it assumes the input type is not a pointer.

I thought if I could redefine the struct:

typedef struct Session {} session, *session_t;

This could work, but I cant find a way to do this with SWIG.

Is there a way to access the value of a struct pointer with SWIG if it's only definition is a typdef struct pointer?

Edit: possible fix is the SWIG %ignore directive. Might be able to ignore the original struct def and replace it in the .i file.

Solved(ish): I marked Mark Tolonen's answer as correct since it has a lot of useful relevant information. The API I'm trying to wrap has a lot of deleted constructors and behaviors locking down how it's used. I ended up writing a C++ wrapper in it's own namespace then telling Swig to ignore everything except my namespace. I then only use standard or my own types to interface with Swig/Python.

A possible workaround is to complete the structure declaration, e.g. put struct session {}; somewhere in the header file. This way, you must also ensure that you never allocate a struct session by yourself. — minmaxavg
– minmaxavg, Commented Sep 15, 2018 at 1:35
You need a factory function like session_t session_open(void) which returns a new instance of session_t (like e.g. fopen). — Henri Menke
– Henri Menke, Commented Sep 15, 2018 at 6:42
I anticipate that you have some function, which creates a session* and perhaps other functions, which use session*. Take a lot at this answer, stackoverflow.com/questions/32478649/… The base class can be replaced with a simple typedef to make it more C like — Jens Munk
– Jens Munk, Commented Sep 16, 2018 at 17:29

Mark Tolonen · Accepted Answer · 2018-09-16 18:09:07Z

2

SWIG works very well with opaque types. Here's a full example where I've provided the source for a test.dll that uses an opaque typedef. The structure is only defined within the .c file and SWIG has no knowledge of the definition. If you are still having a problem update your question with a reproducible example. I suspect you've used session instead of session_t in some typemap, but without a reproducible example I can only show that it works without a definition.

test.c

#define API_EXPORTS
#include <stdlib.h>
#include "test.h"

struct session {
    int a;
};

API session_t alloc(void) {
    session_t s = malloc(sizeof(session_t));
    if(s)
        s->a = 123;
    return s;
}

API int get_a(session_t s) {
    return s->a;
}

API void release(session_t s) {
    free(s);
}

test.h

#ifdef API_EXPORTS
#   define API __declspec(dllexport)
#else
#   define API __declspec(dllimport)
#endif

typedef struct session* session_t;

API session_t alloc(void);
API int get_a(session_t s);
API void release(session_t session);

test.i

%module test
%{
#include "test.h"
%}

%include <windows.i>
%include "test.h"

test session

Python 2.7.14 (v2.7.14:84471935ed, Sep 16 2017, 20:19:30) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import test
>>> s = test.alloc()
>>> test.get_a(s)
123
>>> test.release(s)
>>>

edited Sep 16, 2018 at 18:09

answered Sep 16, 2018 at 17:50

Mark Tolonen

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4 Comments

Henri Menke Over a year ago

I had this in mind as well, but I think OP is asking how to get a session_t without an alloc function, i.e. something like for C++ classes where you can get an instance using test.Class().

Mark Tolonen Over a year ago

@Henri A DLL that uses an opaque pointer without a way to obtain one? That makes no sense, but the OP is free to clarify.

Geordie Over a year ago

Yeah this is a combination of less than idea DLL src and me not being a C expert. Since session_t is a pointer does sizeof(session_t) give the size of a pointer or the structure it's pointing to?

Mark Tolonen Over a year ago

@Geordie sizeof(session_t) gives the size of the pointer. I would expect some API of the DLL to be something like OpenSession() and return a session_t somehow, similar to my demo with alloc().

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