CNI and interface methods

[Stephen Kell <srk31@srcf.ucam.org>]

Hi,

I'm using CNI for the first time and have hit a query. What exactly is 
and isn't supposed to work when invoking interface methods through 
*object* references (from C++, so I mean pointers), and passing these 
references around?

The CNI chapter in the gcj manual says a lot about interface references, 
but not about the case of using object references where that object's 
class implements one or more interfaces. One thing that would appear to 
be worth mentioning is that clearly, passing them to methods wanting an 
interface reference won't compile without some sort of cast, since the 
gcjh-generated headers don't encode the subtyping relationship between 
interfaces and their implementing classes. I'm guessing a simple static 
cast isn't the right thing to do though, because I'm getting segfaults 
when I try.

Specifically, I'm CNI-calling from C++ to Java code which, in turn, 
makes a call through an interface reference that I previously passed in 
to the Java object's constructor (i.e. I passed in an *object* 
reference, again from the C++ code, which required a cast as I just 
described). I get a segfault during the calling sequence of the 
interface call made by the Java code. The fault occurs just before the 
call to _Jv_LookupInterfaceMethodIdx.

(This is all on a fresh build of vanilla gcc 4.3.3, on i686 Linux, by 
the way.)

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0xb7f5e8f0 (LWP 26704)]
0x080828ad in cakeJavaParser.toplevel()cakeJavaParser$toplevel_return () 
at cakeJavaParser.java:145
145             retval.start = input.LT(1);
Current language:  auto; currently java
(gdb) bt
#0  0x080828ad in 
cakeJavaParser.toplevel()cakeJavaParser$toplevel_return ()
    at cakeJavaParser.java:145
#1  0x0805a4e2 in cake::request::process (this=@bffd0c18) at cake.cpp:33
#2  0x0805a7da in main (argc=2, argv=@bffd0cd4) at main.cpp:52

(Possibly significant: in earlier test runs, with a previous build of my 
code, I was getting the segfault *during* the call to 
_Jv_LookupInterfaceMethodIdx, with iface->ioffsets being null when it 
probably shouldn't. I can't reproduce that now for some reason.)

The extent of my CNI code is

	JvCreateJavaVM(NULL);
	JvAttachCurrentThread(NULL, NULL);
	JvInitClass(&java::lang::System::class$);

and shortly afterwards

	org::antlr::runtime::ANTLRInputStream *stream = new org::antlr::runtime::ANTLRInputStream(in_file);
	cakeJavaLexer *lexer = new cakeJavaLexer((org::antlr::runtime::CharStream*) stream);
    org::antlr::runtime::CommonTokenStream *tokenStream = new org::antlr::runtime::CommonTokenStream((org::antlr::runtime::TokenSource*) lexer);
    cakeJavaParser *parser = new cakeJavaParser((org::antlr::runtime::TokenStream*) tokenStream);
    parser->toplevel();
	
where the last statement triggers the segfault. The toplevel() method is 
Java code; it immediately calls a method on lexer (the object I passed 
earlier) and it's this call whose dispatch fails.

Should this code work?  I thought about class initialization, but having 
read the docs, it wouldn't appear that I need to add any JvInitClass 
calls manually in this case. 

There's a tarball at 
<http://www.cl.cam.ac.uk/~srk31/private/cake-snapshot-20090402.tar.bz2> 
which should be sufficient to reproduce the above (just do 'make test'), 
unless I've forgotten to include something. Let me know if there's any 
more details that would be useful.

Thanks for reading! Any answers or ideas appreciated,

Stephen.