Re: [Swig-user] Java, typemaps, and argout?

[William S F. <ws...@fu...>]

J. Tang wrote:
> I have read the SWIG manual, but am stumped on how to do the following:
> 
> I have the C function:
>           void recv_data(char **data, int *len);
> 
> that reads stuff and puts data into the location pointed to by '*data'
> and the read size to '*len'.  I want to write a Java wrapper that
> returns an instance of byte[] instead of java.lang.String.
> 
> The best I could come up with is:
> 
> jByteArray fake_recv_data() {
>         char *data = NULL;
>         int len = 0;
>         jobject jresult = NULL;
>         recv_data(self->sipc, &data, &len);
>         jresult = (*jenv)->NewByteArray(jenv, len);
>         (*jenv)->SetByteArrayRegion(jenv, jresult, 0, len, data);
>         return jresult;
> };
>         
> I believe that I need a native function that will change the output from
> a parameter to a function return value.  However, I do not know how to
> access jenv from within the native function.  What is the recommended
> SWIG way of doing what I need?
> 
Java SWIG doesn't support turning a parameter into a return type. 
Instead you have to think of some way to pass the parameter as a Java 
reference, so that the JNI code can modify it and those modifications 
can be seen from within Java.

> I am sure that there is some magic via %typemaps possible, but I am
> thoroughly stumped.  '%typemap(argout)' does not seem to change the
> return type from void to a jByteArray; '%typemap(out) void' does not
> either.
> 
argout can be used to do this in the scripting languages only.

> Thanks for any assistance for this seemingly trivial task.
> 
> [*] I am aware that Java does not really have out parameters.
> 
See the Examples/java/typemap example for ideas on how to make an output 
parameter possible in Java. It uses the various.i library for changing 
the behaviour of char *. You should be able to use the same ideas for 
char **.


William