Question

I have a very simple agent, basically just the required Agent_OnLoad method signature.

If I compile it with g++.

 g++ -g -fno-strict-aliasing -fPIC -fno-omit-frame-pointer -W -Wall  -Wno-unused -Wno-parentheses  -I. -I../agent_util -I/home/user/apps/Genuitec/Common/binary/com.sun.java.jdk.linux.x86_1.6.0.013//include -I/home/user/apps/Genuitec/Common/binary/com.sun.java.jdk.linux.x86_1.6.0.013//include/linux  -c -o ../src/testagent.o -DMAX_THREADS=1000 -DJVMTI_TYPE=1 ../src/testagent.c

and create a shared lib and run a test on the agent

LD_LIBRARY_PATH=`pwd` /home/mnc/apps/Genuitec/Common/binary/com.sun.java.jdk.linux.x86_1.6.0.013//bin/java -agentlib:testagent -version

I get an error

Error occurred during initialization of VM
Could not find agent library on the library path or in the local directory: testagent
make: *** [test] Error 1

If I compile as using the following command i.e. compile as C, it works ok.

gcc -Wl,-soname=calltracer.so -g -fno-strict-aliasing -fPIC -fno-omit-frame-pointer -W -Wall  -Wno-unused -Wno-parentheses  -I. -I../agent_util -I/home/user/apps/Genuitec/Common/binary/com.sun.java.jdk.linux.x86_1.6.0.013//include -I/home/user/apps/Genuitec/Common/binary/com.sun.java.jdk.linux.x86_1.6.0.013//include/linux  -c -o ../src/testagent.o -DMAX_THREADS=1000 -DJVMTI_TYPE=1 ../src/testagent.c

and then create a shred lib and test it

LD_LIBRARY_PATH=`pwd` /home/user/apps/Genuitec/Common/binary/com.sun.java.jdk.linux.x86_1.6.0.013//bin/java -agentlib:testagent -version
java version "1.6.0_13"
Java(TM) SE Runtime Environment (build 1.6.0_13-b03)
Java HotSpot(TM) Server VM (build 11.3-b02, mixed mode)

it works ok.

The problem is the code I have, is cpp code for the actual method and not c. Can an agent be created using c++ code? I suspect it is but I don’t know what I’m doing wrong.

Here’s the source for my test agent. Cant get much more simple then this.

/*testagent.c*/
#include "jni.h"
#include "jvmti.h"


JNIEXPORT jint JNICALL Agent_OnLoad(JavaVM *vm, char *options, void *reserved)
{
return 0;
}

/* Agent_OnUnload() is called last */
JNIEXPORT void JNICALL
Agent_OnUnload(JavaVM *vm)
{
}

This works ok when compiled as a c file

here's the output from the nm command
0000046c T Agent_OnLoad
00000476 T Agent_OnUnload
00001f18 a _DYNAMIC
00001ff4 a _GLOBAL_OFFSET_TABLE_
         w _Jv_RegisterClasses
00001f08 d __CTOR_END__
00001f04 d __CTOR_LIST__
00001f10 d __DTOR_END__
00001f0c d __DTOR_LIST__
000004d4 r __FRAME_END__
00001f14 d __JCR_END__
00001f14 d __JCR_LIST__
0000200c A __bss_start
         w __cxa_finalize@@GLIBC_2.1.3
00000480 t __do_global_ctors_aux
000003b0 t __do_global_dtors_aux
00002008 d __dso_handle
         w __gmon_start__
00000467 t __i686.get_pc_thunk.bx
0000200c A _edata
00002014 A _end
000004b8 T _fini
00000348 T _init
0000200c b completed.7021
00002010 b dtor_idx.7023
00000430 t frame_dummy

Here’s the another version, I added your suggestion of the extern "C" but I had the same result as before, the library could not be found.

/*testagent.c*/
#include "jni.h"
#include "jvmti.h"

extern "C" {
    JNIEXPORT jint JNICALL Agent_OnLoad(JavaVM *vm, char *options, void *reserved)
    {
        return 0;
    }
}

/* Agent_OnUnload() is called last */
JNIEXPORT void JNICALL
Agent_OnUnload(JavaVM *vm)
{
}

Here’s the output from the nm command

000004bc T Agent_OnLoad
000004c6 T Agent_OnUnload
0000200c d DW.ref.__gxx_personality_v0
00001f18 a _DYNAMIC
00001ff4 a _GLOBAL_OFFSET_TABLE_
         w _Jv_RegisterClasses
00001f08 d __CTOR_END__
00001f04 d __CTOR_LIST__
00001f10 d __DTOR_END__
00001f0c d __DTOR_LIST__
00000594 r __FRAME_END__
00001f14 d __JCR_END__
00001f14 d __JCR_LIST__
00002010 A __bss_start
         w __cxa_finalize@@GLIBC_2.1.3
000004d0 t __do_global_ctors_aux
00000400 t __do_global_dtors_aux
00002008 d __dso_handle
         w __gmon_start__
         U __gxx_personality_v0
000004b7 t __i686.get_pc_thunk.bx
00002010 A _edata
00002018 A _end
00000508 T _fini
0000039c T _init
00002010 b completed.7021
00002014 b dtor_idx.7023
00000480 t frame_dummy

The trace from the nm commands are slightly different but they both include the Agent_OnLoad.

This is the command line used to create the shared lib in both cases.

cc -g -fno-strict-aliasing -fPIC -fno-omit-frame-pointer -W -Wall  -Wno-unused -Wno-parentheses  -I. -I../agent_util -I/home/user/apps/Genuitec/Common/binary/com.sun.java.jdk.linux.x86_1.6.0.013//include -I/home/user/apps/Genuitec/Common/binary/com.sun.java.jdk.linux.x86_1.6.0.013//include/linux  -Wl,-soname=libtestagent.so -static-libgcc -mimpure-text  -shared -o libtestagent.so ../src/testagent.o -lc

out put from ldd, NOT working case (g++)

ldd libtestagent.so 
linux-gate.so.1 =>  (0x00d96000)
libc.so.6 => /lib/tls/i686/cmov/libc.so.6 (0x0019a000)
/lib/ld-linux.so.2 (0x005ee000)

output from ldd, working case (gcc)

ldd libtestagent.so 
linux-gate.so.1 =>  (0x00544000)
libc.so.6 => /lib/tls/i686/cmov/libc.so.6 (0x00908000)
/lib/ld-linux.so.2 (0x003a2000)

Been using linux for about 15 years never knew you could do LD_DEBUG=all, very useful.
Here’s the interesting output

2689:   symbol=__gxx_personality_v0;  lookup in file=/home/mnc/apps/Genuitec/Common/binary/com.sun.java.jdk.linux.x86_1.6.0.013//bin/java [0]
      2689: symbol=__gxx_personality_v0;  lookup in file=/lib/tls/i686/cmov/libpthread.so.0 [0]
      2689: symbol=__gxx_personality_v0;  lookup in file=/home/mnc/apps/Genuitec/Common/binary/com.sun.java.jdk.linux.x86_1.6.0.013/bin/../jre/lib/i386/jli/libjli.so [0]
      2689: symbol=__gxx_personality_v0;  lookup in file=/lib/tls/i686/cmov/libdl.so.2 [0]
      2689: symbol=__gxx_personality_v0;  lookup in file=/lib/tls/i686/cmov/libc.so.6 [0]
      2689: symbol=__gxx_personality_v0;  lookup in file=/lib/ld-linux.so.2 [0]
      2689: symbol=__gxx_personality_v0;  lookup in file=/home/mnc/apps/Genuitec/Common/binary/com.sun.java.jdk.linux.x86_1.6.0.013/jre/lib/i386/server/libjvm.so [0]
      2689: symbol=__gxx_personality_v0;  lookup in file=/lib/tls/i686/cmov/libm.so.6 [0]
      2689: symbol=__gxx_personality_v0;  lookup in file=/home/mnc/apps/javacalltracer/Calltracer/jvmti/libtestagent.so [0]
      2689: symbol=__gxx_personality_v0;  lookup in file=/lib/tls/i686/cmov/libc.so.6 [0]
      2689: symbol=__gxx_personality_v0;  lookup in file=/lib/ld-linux.so.2 [0]
      2689: /home/mnc/apps/javacalltracer/Calltracer/jvmti/libtestagent.so: error: symbol lookup error: undefined symbol: __gxx_personality_v0 (fatal)
      2689: 
      2689: file=/home/mnc/apps/javacalltracer/Calltracer/jvmti/libtestagent.so [0];  destroying link map
Error occurred during initialization of VM

I did a search on stackoverflow regarding this, a post suggested adding a global for this symbol
so I’ve added __gxx_personality_v0 as a
void *__gxx_personality_v0;

and now the JVM found the library, when compiling with g++.

java,#gcc

Answer 1

You didn’t show us contents of testagent.c.

My guess is that you are being bitten by C++ name mangling, either because you didn’t use JNIEXPORT when defining Agent_OnLoad(), or because JNIEXPORT doesn’t include extern "C" on your platform.

Surrounding your definition of Agent_OnLoad() with extern "C" { and } should be all you need.

You can verify whether name mangling is in fact your problem by executing

nm libtestagent.so | grep Agent_OnLoad

and comparing the result for working (gcc) and broken (g++) versions.

Update:

Ok, so C++ name mangling wasn’t it. You next step should be finding out why dynamic linker is failing to dlopen("libtestagent.so"). You can do that by prefixing your command with LD_DEBUG=all, collecting the working and non-working output (output will be huge), and looking for the differences.

Update 2:

I’ve added __gxx_personality_v0 as a void *__gxx_personality_v0;

That’s not quite the correct way to fix the problem, and will likely cause you problems later when you start using actual C++ in the agent.

The correct way to fix the problem is to link the library with g++, and not gcc. That will add a dependency on libstdc++.so.6, which defines __gxx_personality_v0 and a bunch of other stuff your C++ code will need.