How do I convert to type SWIGTYPE_p_void in SWIG generated Java bindings?

Question

I am developing some SWIG-generated Java bindings for a C library. The library contains functions that take parameters of type void *. On the C side these would

Answer 1

The simplest solution is to use SWIG's to make a type that wraps an array of float and an array of int and any other types you care about. These can be converted to SWIGTYPE_p_float etc. trivially using the cast() member function. The problem is that this can't automatically be converted to a SWIGTYPE_p_void from within Java. You could in theory call:

new SWIGTYPE_p_void(FloatArray.getCPtr(myfloatarr));

but for various reasons (not least it's cumbersome) that's less than ideal. (It also has issues with memory ownership and garbage collection).

So instead I defined an interface:

public interface VoidPtr {
  public long asVoidPtr();
}

That we can make the wrapped version of your library take as input and the array classes FloatArray, IntArray etc. implement for us.

This ends up with the module file:

%module test

%include 

%typemap(javainterfaces) FloatArray "VoidPtr"
%typemap(javainterfaces) IntArray "VoidPtr"

%typemap(javacode) FloatArray %{
  public long asVoidPtr() {
    return getCPtr(this);    
  }
%}

%typemap(javacode) IntArray %{
  public long asVoidPtr() {
    return getCPtr(this);
  }
%}

%array_class(float, FloatArray);
%array_class(int, IntArray);

%typemap(jstype) void *arr "VoidPtr"
%typemap(javain) void *arr "$javainput.asVoidPtr()"

void foo(void *arr);

Which modifies void *arr to be treated as our VoidPtr type and automatically calls the asVoidPtr() method. You could use typemap copying or macros to make this less repetitive. (Note, there's a possible issue with premature garbage collection that might need to be addressed here depending on how you planned to use this)

This allows us to write code like:

public class run {
  public static void main(String[] argv) {
    FloatArray arr = new FloatArray(100);
    test.foo(arr);    
  }
}

I think this is the easiest, cleanest solution. There are several other ways you could solve this though:

1. It's also possible to write some code that would take an actual Java array rather than just the SWIG array_class and implement this interface by calling a JNI function to obtain the underlying pointer. You'd have to write a version of this for every primitive type though, just like the above.

   An interface file could then look something like:

   %module test
   
   %{
   void foo(void *arr);
   %}
   
   %typemap(in,numinputs=0) JNIEnv *env "$1 = jenv;"
   
   %rename(foo) fooFloat;
   %rename(foo) fooInt;
   %inline %{
   void fooFloat(JNIEnv *env, jfloatArray arr) {
     jboolean isCopy;
     foo((*env)->GetFloatArrayElements(env, arr, &isCopy));
     // Release after call with desired semantics
   }
   
   void fooInt(JNIEnv *env, jintArray arr) {
     jboolean isCopy;
     foo((*env)->GetIntArrayElements(env, arr, &isCopy));
     // Release after call
   }
   %}
   
   void foo(void *arr);
   

   Which then gives you overloads of foo which take float[] and int[] as well as SWIGTYPE_p_void.

2. You could use a trick with a union:

   %inline %{
     union Bodge {
       void *v;
       float *f;
       int *i;
     };
   %}
   

   although this is considered bad form, it does generate you a Java interface that can be used to convert from SWIGTYPE_p_int to SWIGTYPE_p_void.

3. I think it's possible to make FloatArray inherit from SWIGTYPE_p_void, something like the following compiled but untested code:

   %module test
   
   %include 
   
   %typemap(javabase) FloatArray "SWIGTYPE_p_void"
   %typemap(javabody) FloatArray %{
     private long swigCPtr; // Minor bodge to work around private variable in parent
     private boolean swigCMemOwn;
     public $javaclassname(long cPtr, boolean cMemoryOwn) {
       super(cPtr, cMemoryOwn);
       this.swigCPtr = SWIGTYPE_p_void.getCPtr(this);
       swigCMemOwn = cMemoryOwn;
     }
   %}
   
   %array_class(float, FloatArray);
   
   void foo(void *arr);
   

   This duplicates the pointer on the Java side, but nothing changes that (currently) in either the void pointer or array classes so that's not as big a problem as it first seems. (You could also make it protected in the base class with an alternative typemap I think, or use a modified version of carrays.i that gets swigCPtr via the getCPtr function instead)