Access C constants (header) from JNI (Java Native Interface)

c, constants, header, java, java-native-interface

Solution

Simple answer is that you can't. You'll have to define the same values in Java.

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

Or you can define some native methods that return these values. But the macros as such cannot be accessed from Java.

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

Problem

how do I access constants defined in the C header file from Java side (through JNI)? I have a C library with this header file c_header.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 running javah at libbind_jni.java 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 write a little C code for JNI library jni_source.c: ``` #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 goes compiling of C library for java -- libbind.so And the JAVA source 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 C header file from Java? Thanks.

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