SWIG-generated Go wrapper mishandling(?) pointers to pointers to structs

[mark mellar]

(Cross posting with SO https://stackoverflow.com/questions/61258160/swig-generated-go-wrapper-mishandling-pointers-to-pointers-to-structs)

Hi folks,

We're integrating a Go program with a 3rd party C api, but are getting some odd results when working with pointers to pointers to structs. A model of the header file looks a bit like this...

//mytest.h 

typedef struct Books {
 char title[50];
 char author[50];
 char subject[100];
 int book_id;
} 

Book; extern void initbook(Book** b);

With a model implementation like this...

//mytest.c 
#include <stdlib.h> 
#include <string.h> 
#include <stdio.h> 

typedef struct Books {
 char title[50];
 char author[50];
 char subject[100];
 int book_id; 
} Book; 

void initbook(Book** b){ *b = malloc(sizeof(Book));
 strcpy( (*b)->title, "A Book Title"); 
}

We've tried wrapping the API with a very simple SWIG file which looks like this...

%module mytest %{
 #include "./mytest.h" 
%} 
%include "./mytest.h"

And call it from a Go main function like this...

//main.go 

package main 
import "fmt" 
import "mytest" 
func main() {
 bookPtr := mytest.NewBook()
 mytest.Initbook(bookPtr)
 fmt.Printf("title: '%s'\n", bookPtr.GetTitle()) 
}

However, we've found when we run this we the value being returned from GetTitle() is garbled....

> go run ../main.go 
title: '??'

Digging in to the generated C code we found that editing the Book constructor to return Book** rather than Book*, and then modifying the GetTitle function to receive a Book** instead of Book* things started working as expected.

Any hints on how we might configure SWIG to generate a version of the code which works for us. without us having to hack what it generates?

Thanks!

Mark

[Ian Lance Taylor]

Since this is a C API, I suggest that you use cgo. https://blog.golang.org/cgo.

If you want to use SWIG, you'll likely need to use directives like
%gotype to teach SWIG how the C type should be represented in Go. It
seems clear that SWIG is getting confused, as from what you wrote
above it seems that the Go code ought to be mytest.Initbook(&bookPtr).

Ian