Question

I have a C function (composeKey) that has an input unsigned char* parameter (“key”). On the C side, “key” needs to be an empty 20 byte buffer structure. I’m assuming that a Java short array with a size of 20 would be the correct Java structure to pass composeKey’s “key” parameter, but I’m unsure. Maybe a byte[20] is what i need. If that is correct, what SWIG Interface file modification is needed to generate the composeKey Java method with a short[] as input for the “key” parameter?

C Function:
int composeKey(const void* secret, int secret_len, unsigned char* key, int length);

Answer 1

Solution to your specific problem

Java doesn’t really distinguish between short[20] and (e.g.) short[21] in its type system. You can do something that’s pretty sensible quite simply though, by making the fact that the length of key is always 20 obvious to SWIG:

%module test

%include "arrays_java.i"

int func(unsigned char key[20]);

This can work even without changing the actual signature of your function directly – SWIG can wrap that, but have the wrapped code call a function that still takes unsigned char* quite sensibly:

%module test

%{
#include "header.h"
// fine even if it's func(unsigned char *key) in the header.
%}

%include "arrays_java.i"
int func(unsigned char key[20]);

If you call func from Java with an inappropriately sized array you’ll get an IndexOutOfBoundsException exception thrown for you automatically by the code that SWIG generates.

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General solution

In this specific case "arrays_java.i" provides a suitable typemap already. In the more general case this works by providing SWIG with a typemap for unsigned char [ANY] (literally write ANY in SWIG typemap syntax). This then gets instantiated for specific values in place of ANY (sort of like a template in C++), you can then access the specific value of ANY in your typemap using $1_size and supply code that the sizes gets filled in to look (some JNI details omitted for brevity) roughly like:

if (GetArrayLength(arg) != $1_size) {
    // Code to throw a Java Exception ...

Which then in the generated wrapper becomes:

if (GetArrayLength(arg) != 20) {
    // Code to throw a Java Exception ...