Convert a member of structure of type signed char * to byte array in Java (byte[]) using SWIG

Question

I'm trying to convert a member of structure of type signed char * to byte array in Java. I've the following structure:

typedef struct {
    signed char * content;
    int contentLength;
} Foo;

I've tried this:

%typemap(jni) signed char *content [ANY] "jbyteArray"
%typemap(jtype) signed char *content [ANY] "byte[]"
%typemap(jstype) signed char *content [ANY] "byte[]"
%typemap(javaout) signed char *content [ANY] {
    return $jnicall;
}
%typemap(memberin) int contentLength [ANY] {
    int length=0;
    $1 = &length;
}

%typemap(out) signed char * content [ANY] {
    $result = JCALL1(NewByteArray, jenv, length);
    JCALL4(SetByteArrayRegion, jenv, $result, 0, length, $1);
}

But there isn't a result. The method getContent of the Foo have the following signature:

SWIGTYPE_p_signed_char getContent();

I want this method to returns byte[]. Is there a solution?

Answer 1

That's pretty close to what you want. You don't want the [ANY] since the size of the array is not "fixed" in C (it's specified by an int, but that's not part of its type).

You can make your typemap work with:

%module test

%typemap(jni) signed char *content "jbyteArray"
%typemap(jtype) signed char *content "byte[]"
%typemap(jstype) signed char *content "byte[]"
%typemap(javaout) signed char *content {
    return $jnicall;
}

%typemap(out) signed char * content {
    $result = JCALL1(NewByteArray, jenv, arg1->contentLength);
    JCALL4(SetByteArrayRegion, jenv, $result, 0, arg1->contentLength, $1);
}

// Optional: ignore contentLength;
%ignore contentLength;

%inline %{
typedef struct {
    signed char * content;
    int contentLength;
} Foo;
%}

I might be missing something here, but I can't see a better way of getting hold of the "self" pointer from within an out typemap than this - arg$argnum doesn't work and neither does $self. There aren't any other typemaps that get applied to this function that would help.

(Note you probably also want to write a memberin for signed char * content or make it immutable. I'd be tempted to %ignore the contentLength member entirely too).