Accessing C constants (titles) from JNI (Java Native Interface)

How do I access constants defined in C header files from the Java side (through JNI)?

I have a header file C_ header. C library of H:

// I'll try to get these values in java
#define PBYTES 64
#define SBYTES 128
#define SGBYTES 128

Then I have this java code libbind_ jni. java：

package libbind;

public class libbind_jni {
static{
    try {
        System.load("libbind_jni.so");
    } catch (UnsatisfiedLinkError e) {
        System.err.println("Native code library Failed to load.\n" + e);
        System.exit(1);
    }
}

public static native String[] pubm(String seed);

Then in libbind_ jni. Run javah on Java and I generate JNI header file JNI_ header. h：

#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     libbind_libbind_jni
 * Method:    pubm
 * Signature: (Ljava/lang/String;)[Ljava/lang/String;
 */
JNIEXPORT jobjectArray JNICALL Java_lib_f_1jni_pubm
  (jnienv *,jclass,jstring);

Then I put JNI in the library for JNI_ source. C wrote a small C code:

#include "jni_header.h"
#include "c_header.h"
#include <stdlib.h>

JNIEXPORT jobjectArray JNICALL Java_libbind_libbind_1jni_pubm
  (jnienv *env,jclass cls,jstring jniSeed){
    unsigned char vk[PBYTES];
    unsigned char sk[SBYTES];
    <whatever...>
}

– here you compile the libbind. Java library so

And Java source code J_ source. java：

import libbind.libbind_jni;

public class test {

    /* trying to access constants from library (libbind.so) */
    System.out.println(libbind_jni.PBYTES);
    System.out.println(libbind_jni.SBYTES);
    System.out.println(libbind_jni.SGBYTES);

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


$javac test.java

test.java:5: cannot find symbol
symbol  : variable PBYTES
location: class libbind.libbind_jni
    System.out.println(libbind_jni.PBYTES);

So the question is: how do I access these definitions / constants in the C header file from Java?

thank you.

Solution

The simple answer is, you can't You must define the same value in Java

public static final int PBYTES = 64;
public static final int SBYTES = 128;
public static final int SGBYTES = 128;

Alternatively, you can define some native methods that return these values However, such macros cannot be accessed from Java

public static native int PBYTES();
public static native int SBYTES();
public static native int SGBYTES();