Re: Converting a JAR into a gcj-statically-linked SO

[Andrew Haley <aph at redhat dot com>]

Matthijs van de Water writes:

 > I've been working with GCJ (4.2) to convert a JAR library into a Linux
 > shared object for an embedded system in such a way that it does not
 > depend on libgcj.
 > Until now, I'm failing at producing the right commandline options to
 > get this to link properly.
 > 
 > Let me explain what I'm trying to do in some more detail:
 > I have a JAR file which is in fact a library. I want to use this
 > library on an embedded Linux system and make calls to it from a native
 > application written in C/C++.
 > There are _no_ other processes on the system that could benefit from
 > libgcj.so, even if I had 33MB of space on my rootfs laying around
 > (which I don't), so I want to statically link libgcj to the resulting
 > native library. The API of the resulting library will totally hide
 > anything Java related, so I was expecting to be able to link it to my
 > application without needing libgcj.
 > 
 > I've been reading similar requests on the list and the static linking
 > WiKi page, but until now I've failed to get this to link (let alone
 > work).
 > 
 > My commandline is:
 > gcj -save-temps -shared -fPIC -Wl,-Bsymbolic MyJavaLib.jar -o libmyjava.so
 > gcc -o libmynative.so -shared -fPIC native_lib.cc MyJavaLib.o -lstdc++
 > -shared-libgcc -Wl,-non_shared -lgcj -Wl,-call_shared -lsupc++
 > -Wl,--as-needed -lz -lgcc_s -lpthread -lc -lm -ldl -Wl,--no-as-needed
 > 
 > This produces a native SO (libmynative.so) which (according to readelf
 > -d) does not depend on libgcj. So far so good.
 > 
 > Unfortunately, I cannot get a simple test.cc program which uses the
 > libmynative API (without any Java dependencies) to link. It always
 > complains about missing references to java in libmynative.so.
 > gcc -o test test.cc -lstdc++ -L . -lmynative

Tell us what messages *exactly*.

 > Could anyone give me some pointers in the right direction?
 > If I add "-lgcj" to the test program gcc line it does compile, but
 > obviously depends on libgcj. Also, the resulting program does not work
 > (crashes with a SEGV). When using only shared libraries, the test
 > program works fine.
 > 
 > Is what I try to do even feasible?
 > 
 > I'm currently testing on my desktop (Ubuntu), but I'll be moving to an
 > ARM embedded system if I get this working.

This is tricky to do, but not impossible.  David Daney has some
experience in this area.

If you to produce a simple complete test case that we can try it'll be
much easier to help you.

Andrew.

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