CODEWITHSUNDEEP
swig
c12
c12

Reputation: 9827

SWIG Convert unsigned char* to 20 byte buffer Java structure

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);

Upvotes: 0

Views: 1313

Answers (1)

Flexo - Save the data dump
Flexo - Save the data dump

Reputation: 88711

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.

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 ...

Upvotes: 1

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