Calling a native C Library, from C++ code -

Question

Hi all! please bear with me as i am not sure if my question was specific enough but here goes nothing.
i have a native function that i set up in Java. i use JNI to create my C header file(stub) . all that is good and well but inside my C++ code i want to be able to call a separate C++ function that calls a separate C DLL(library). it seems to me that java shouldn't care about this but evidently it does. i can give you the code and see what you think instead of me trying to explain it all. I am using Linux and g++.
my first idea is that somehow java still needs to know about the C DLL but i dont understand why since i am not calling any functions from the C Library from Java, but rather in my C++ code. please look at the code below. thanks!
//THIS IS THE .java File *********************
public class HelloWorld
public native double getPopulation(double w, double x, double y, double z);
static
   System.loadLibrary("populationLib");
        public static void main(String[] args)
                HelloWorld hello = new HelloWorld();
                double value = hello.getPopulation(-89.3, 30.1, -89.6, 30.8);
                System.out.println("value returned is: " + value);
}now i perform javac HelloWorld.java which creates HelloWorld.class.
now i perform javah -jni HelloWorld which creates my HelloWorld.h C stub.
//THIS IS the HelloWorld.h C Stub*******************
/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
#include <shapefil.h>  //*NOTE - i had to add this on my own for the C Library that i want to access
/* Header for class HelloWorld */
#ifndef _Included_HelloWorld
#define _Included_HelloWorld
#ifdef __cplusplus
extern "C" {
#endif
* Class:     HelloWorld
* Method:    getPopulation
* Signature: (DDDD)D
JNIEXPORT jdouble JNICALL Java_HelloWorld_getPopulation
  (JNIEnv *, jobject, jdouble, jdouble, jdouble, jdouble);
double setPopulation(double, double, double, double); //*NOTE - also i had to add this function because it is the separate C++ function that i mentioned at the beginning. this function will try to call the C library functions from shapefil.h.
#ifdef __cplusplus
#endif
#endif
//this is HelloWorld.cpp, my C++ code to match the C Stub that was created earlier***********
#include <iostream>
#include "HelloWorld.h"
using namespace std;
JNIEXPORT double JNICALL Java_HelloWorld_getPopulation(JNIEnv *env, jobject obj, jdouble w, jdouble x,
                                                                                  jdouble y, jdouble z)
  jdouble pop;
  pop = setPopulation(w, x, y, z);
  return (jfloat)pop;
double setPopulation(double x1, double y1, double x2, double y2)
  SHPHandle mySHPhandle = NULL;
  DBFHandle myDBFhandle = NULL;
  SHPObject *mySHPobject = NULL;
  DBFFieldType stuff;
  int field_count;
  int record_count;
  //**NOTE** - these 4 calls below access the functions in the object file that was created through the shapefile C library
  mySHPhandle = SHPOpen("file1.shp", "rb");
  myDBFhandle = DBFOpen("file2.dbf", "rb");
  field_count = DBFGetFieldCount(myDBFhandle);
  record_count = DBFGetRecordCount(myDBFhandle);
  cout << "in set population: x1: " << x1 << " y1: " << y1 << " x2: " << x2 << " y2: " << y2 << endl;
  return 10.0;
}now i need to compile and create the shared library for Java to use:
g++ -shared -I/include files needed -L/library files needed HelloWorld.cpp -o libpopulationLib.so
file successfully created.
now i must run Java against it to see if it runs:
$Java HelloWorld
Exception in thread "main" java.lang.UnsatisfiedLinkError: /home/user/work/iga/src/population/libpopulationLib.so: /home/user/work/iga/src/population/libpopulationLib.so: undefined symbol: SHPOpen
at java.lang.ClassLoader$NativeLibrary.load(Native Method)
at java.lang.ClassLoader.loadLibrary0(ClassLoader.java:1751)
at java.lang.ClassLoader.loadLibrary(ClassLoader.java:1676)
at java.lang.Runtime.loadLibrary0(Runtime.java:823)
at java.lang.System.loadLibrary(System.java:1030)
at HelloWorld.<clinit>(HelloWorld.java:35)
As you can see it cannot see the function SHPOpen from the external C library i needed to call. I am fairly new to Java and therefore really new to JNI but what surprises me is that Java even cares about or needs to know what i do outside of the original native function that i declared at the beginning.
In any event, what do i need to do to solve this?
thanks so much for your time and patience and help in advance..
Edited by: usmsci on Feb 20, 2008 8:56 AM

Answer 1

Names a mangled in C++.
They are not mangled in C.
You can't mix C and C++ without dealing with that.
I don't see anything in your code that deals with that.