Calling legacy C library functions.

Question

How do you call legacy C functions with JNI. This is code that I have access to the libraries and header files but not to the source code. I have written, run and understand how to write native C code for the JNI but have yet to find information on how to call an existing function. I JUST DON'T GET IT! I hope I am not overlooking the obvious and apologize if this is the case. A simple example would be most helpful and apreciated.
Thanks much,
Darrin

Answer 1

Below I am using the simple example from sun and altering it to call legacy code to print to the screen "HelloWorld".
// HelloWorld.c
class HelloWorld {
public native void displayHelloWorld();
static {
System.loadLibrary("hello");
public static void main(String[] args) {
new HelloWorld().displayHelloWorld();
//HelloWorld.h
/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class HelloWorld */
#ifndef IncludedHelloWorld
#define IncludedHelloWorld
#ifdef __cplusplus
extern "C" {
#endif
* Class: HelloWorld
* Method: displayHelloWorld
* Signature: ()V
JNIEXPORT void JNICALL Java_HelloWorld_displayHelloWorld
(JNIEnv *, jobject);
#ifdef __cplusplus
#endif
#endif
//HelloWorldImp.c
//Altered to call legacy code that prints "HelloWorld"
#include <jni.h>
#include "HelloWorld.h"
#include <stdio.h>
#include "LegacyC.h"
JNIEXPORT void JNICALL
Java_HelloWorld_displayHelloWorld(JNIEnv *env, jobject obj)
PrintHello( );
return;
//LegacyC.h
//The legacy C code header file
void PrintHello( );
//LegacyCode.c
//The legacy C source code
# include <stdio.h>
void PrintHello( )
printf("HelloWorld\n");
I have tried this and it does not work.
This is just to illustrate what I am trying to do, in reality I do not have the source to the legacy only the library.
Very frustrating.
Could you alter and repost this code to make it work.
Thank you,
Darrin