Loading a dynamic C shared library using JNI which also loads another shared library

Question

Using JNI on Java eclipse (Linux), I am loading a dynamic shared library called first.so. So far all goes well. The problems being, that first.so also loads a dynamic library called second.so.

When running the program I am getting many "undefined symbol" errors regarding symbols located in second.so.

It seems that a library loaded using JNI cannot load other C libraries on run-time because we are in a Java environment. Is my assumption correct? Do I need special compile flags to compile the first.so library or is it special arguments to tell eclipse that it will be trying to load an .so during runtime?

Thanks in advance!

Answer 1

It seems that a library loaded using JNI cannot load other C libraries on run-time because we are in a Java environment. Is my assumption correct?

No.

How is libsecond.so used by libfirst.so? Is it a linked dependency, or is it loaded by dlopen?

I've found that something like:

static {
    System.loadLibrary("second");
    System.loadLibrary("first");
}

in the class that uses the JNI usually works.

EDIT: Now that I know how you are loading libsecond.so this worked for me:

Test.java

public class Test {

    public static void main (String args[]) {
        test();
    }

    private native static void test();

    static {
        System.loadLibrary("first");
    }
} 

first.c -- The sole translation unit of libfirst.so

#include <jni.h>
#include "Test.h"
#include <dlfcn.h>
#define LIBNAME "libsecond.so"

#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     Test
 * Method:    test
 * Signature: ()V
 */
JNIEXPORT void JNICALL
Java_Test_test(JNIEnv *env , jclass cls)
{
  void* h;
  void (*sym)(void);

  h = dlopen(LIBNAME, RTLD_LAZY|RTLD_GLOBAL);
  if (h) {
    printf("dlopen " LIBNAME " worked\n");
    sym = (void (*)(void))dlsym(h,"second");
    sym();
  } else {
    printf("dlopen " LIBNAME " failed\n");
  }
}

#ifdef __cplusplus
}
#endif

second.c -- The sole translation unit of libsecond.so

#include <stdio.h>

void
second(void)
{
  printf("hello from second\n");
}

Makefile

CFLAGS=-fPIC

all : libfirst.so libsecond.so 

libsecond.so : second.o
        $(CC) -shared -Wl,-soname,libsecond.so.0 -o $@ $^ -lc

libfirst.so : first.o
        $(CC) -shared -Wl,-soname,libfirst.so.0 -o $@ $^ -ldl -lc

clean:
        rm -f *.o *.so

Test.h can be produced by javah Test. Note that libfirst.so and libsecond.so are not linked together.