GCJ and JNI

["Jay H. Hartley" <maven at jay dot hartley dot name>]

I've been trying to use GCJ to ahead-of-time compile a Java app and have
encountered a runtime problem. The app uses a custom JNI library for serial
port access, and loading it is the source of my current trouble. I'm trying
to find out if the current version of GCJ can do what I want. If it does,
I'd appreciate guidance in tracking down the source of my problem. Now the
details...

The application is intended to run on an embedded system (ARM chip, Linux
kernel 2.4.26, glibc 2.3.2), so it's a cross-compile. We have a commercial
JVM option available, but I'm exploring GCJ as a possible alternative.

I used Dan Kegel's Crosstool to set up my build environment with GCC 4.1.0,
targeting arm-unknown-linux, glibc 2.3.2. This was recommended by the
hardware vendor, and it's been working great for C/C++ components of the
system.

I successfully compiled and ran a simple hello-world Java app, but a more
complex test app that uses the JNI library gives me an error:

$ ./serialTerminal -c /dev/ttyS1 -b 9600 -p N -d 8 -s 1
os.name="Linux"  os.arch="armv4l"
pc : [<2aadb414>]    lr : [<2b30d924>]    Tainted: P
sp : 7fffefb0  ip : 2bb14498  fp : 7ffff314
r10: 2aae4214  r9 : 00000001  r8 : 7f5fedbc
r7 : 7f5ff09c  r6 : 00000000  r5 : 7f5fedd8  r4 : 00000078
r3 : 00000000  r2 : 0000003c  r1 : 7f5ff09c  r0 : 7f5fedbc
Flags: Nzcv  IRQs on  FIQs on  Mode USER_32  Segment user
Control: C000317F  Table: 011F8000  DAC: 00000015
Error operating serial terminal: java.lang.NullPointerException
java.lang.NullPointerException
   <<No stacktrace available>>
Exiting...

The os.name/os.arch output line and the "Error operating..." output are in
app code. The rest appears to be dumped to root as part of shared library
loading.

The first line is generated just before "System.loadLibrary(libName)". The
line just after loadLibrary() is not executed, so the NPE pretty clearly
comes from the System call. In digging through the GCJ class source, I don't
see any opportunity for a NullPointerException in the Java, so I think it's
being thrown from the native code. I've used the -g flag to turn on line
tracing and such, so I assume the lack of stacktrace is due to the native
throw. 

The Status page for GCJ (http://gcc.gnu.org/java/status.html) indicates that
file I/O for ARM is not yet supported, and only lists "Bare Metal ARM," not
"GNU/Linux ARM." On the other hand, it references GCC 3.2, so I'm not sure
how up-to-date the list is.

I haven't found a gdb binary for this target that was built by CrossTool,
though there are some gdb-6.4 files and headers. I googled several old posts
about adding gdb support to CrossTool, but no clear docs. Regardless of
whether GCJ supports this platform, I'd appreciate any guidance on
building/getting a gdb I can use.

Thanks in advance,

Jay

--------
Jay H. Hartley, Ph.D.
Senior Software Engineer/Architect
Modius, Inc.