Re: GCJ .jar to .so with native method

[Joe Hoffert <jhoffert at dre dot vanderbilt dot edu>]

Hi, Andrew.

On Tue, 2007-12-04 at 18:13 +0000, Andrew Haley wrote:
> Joe Hoffert writes:
> 
>  > You're right that using gcj or g++ to link doesn't matter. The
>  > difference with what I was doing before was the compiler/linker
>  > options.
> 
> Can you let us know what in our instructions didn't give you the
> information you needed?  It might help us improve our documentation.

I didn't see anything in gcj.pdf that talks about conflicting compiler
or linker options. The options that gave me trouble are the
-fvisibility=hidden and -fvisibility-inlines-hidden. When I take these
out the program compiles and links. I think I'll be fine without these
options.

Now the program is aborting with the first "Java" call. I'm simply
calling a method specified by the gcjh generated header file. The code
is aborting from initializeClass:

(gdb) where
#0  0x00110402 in __kernel_vsyscall ()
#1  0x0084bfa0 in raise () from /lib/libc.so.6
#2  0x0084d8b1 in abort () from /lib/libc.so.6
#3  0x02da0d38 in _Jv_Throw () from /usr/lib/libgcj.so.8rh
#4  0x02ddc888 in java::lang::Class::initializeClass ()
   from /usr/lib/libgcj.so.8rh
#5  0x02dddd36 in _Jv_InitClass () from /usr/lib/libgcj.so.8rh
#6  0x00145a44 in
multishot.examples.RicochetApp.initialize(java.lang.String,
java.lang.String, int, java.lang.String)multishot.examples.RicochetApp
()
   from /home/jhoffert/Ricochet/trimmed/c
++-java/linux-jdk1.5/libricochet.so
#7  0x08048ec2 in main (argc=4, argv=0xbfca39e4) at
RicochetAppMain.cpp:95


I also saw in the documentation (i.e., chapter 12, section 12.7 of
gcj.pdf) that the C++ compiler doesn't invoke extra code to make sure a
class is initialized. The method I'm calling is static. I added code to
try and initialize the class:

  JvInitClass(&multishot::examples::RicochetApp::class$); // this is
line 87

 but the program aborts in that call:

(gdb) where
#0  0x02da2610 in _Jv_Linker::verify_class ()
from /usr/lib/libgcj.so.8rh
#1  0x02da4762 in _Jv_Linker::wait_for_state ()
from /usr/lib/libgcj.so.8rh
#2  0x02ddc6b4 in java::lang::Class::initializeClass ()
   from /usr/lib/libgcj.so.8rh
#3  0x02dddd36 in _Jv_InitClass () from /usr/lib/libgcj.so.8rh
#4  0x0804908d in JvInitClass (cls=0x804a690)
    at /usr/lib/gcc/i386-redhat-linux/4.1.2/../../../../include/c
++/4.1.2/gcj/cni.h:30
#5  0x08048f7e in main (argc=4, argv=0xbfe54394) at
RicochetAppMain.cpp:87


Is there I'm not doing when calling JvCreateJavaVM? I called this
initially with an empty JvVMInitArgs argument and the program aborted so
I added the -D option for the .jar file. This seems redundant to me but
I wasn't sure what else to try. This argument is what I pass when I'm
using the invocation API.

I appreciate your time but don't want to waste it. Please point me to
documentation, examples, or tutorials if you don't have time to keep
helping me.

Thanks!

-Joe

>  > One more question. Is there any advantage to converting a .jar
>  > library to a .so library and using it as opposed to simply using
>  > the original .jar file with the Java invocation API and the JNI to
>  > call back to C++ from Java? I thought I would get a performance
>  > increase with using the .so library but perhaps there are no
>  > substantive differences between the two approaches.
> 
> You will probably get a performance increase, yes.  In particular,
> calling native code from compiled code is very quick.
> 
> Andrew.
>