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!

Is `second.so` another JNI library or just an ordinary C library used only by `first.so`? If it's a JNI library, I think you might need to load that from Java with `System.loadLibrary()` instead of from C with `dlopen()`, although I don't know for sure. — Adam Rosenfield, Feb 07 '14 at 02:40

ldav1s · Answer 1 · 2014-02-07T19:31:49.920

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.

thanks for the quick response. libsecond.so is loaded by dlopen. — Ori Gil, Feb 07 '14 at 03:47
Seems to be quite enough just to load libraries in correct order, as shown at the top of the response (`System.loadLibrary("second"); System.loadLibrary("first");`) — pevik, Mar 30 '16 at 12:25

score 0 · Answer 2 · edited May 23 '17 at 12:07

0

All usual rules apply to dopen() from a library loaded from Java. Particularly, you check your LD_LIBRARY_PATH, rpath, runpath, etc. See also dlopen failed：cannot open shared object file: No such file or directory

edited May 23 '17 at 12:07

Community

1
1

answered Feb 07 '14 at 07:43

Alex Cohn

56,089
9
113
307

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

2 Answers2