Call C from Android using JNI with only a C shared library and header file

Question

Android Studio 0.3.1

Hello,

I have the following library written in C but don't have the source code for it only the libapp_module.so

libapp_module.so: ELF 32-bit LSB shared object, ARM, version 1 (SYSV), dynamically linked (uses shared libs), not stripped

I have the header file for this library:

#ifdef __cplusplus
extern "C" {
#endif

#ifdef _WIN32
#define LIB_API(type) __declspec(dllexport) type
#else 
#define LIB_API(type) type
#endif

LIB_API(int) module_init();

#ifdef __cplusplus
}
#endif

The problem is the exception when I try and call the function from my Android App:

D/dalvikvm﹕ Added shared lib /data/app-lib/com.sunsystems.snaptest-1/libapp_module.so 0x416f3d88
D/dalvikvm﹕ No JNI_OnLoad found in /data/app-lib/com.sunsystems.snaptest-1/libapp_module.so 0x416f3d88, skipping init
W/dalvikvm﹕ No implementation found for native Lcom/sunsystems/snaptest/App_Module;.module_init:()I
UnsatisfiedLinkError: Native method not found: com.sunsystems.snaptest.App_Module.module_init:()I

This is what I am doing

I have created a Android App and I want to call that module_init() from my App so I have created a class called App_Module.java

public class App_Module {
    /* Load native C libraries */
    static {
        System.loadLibrary("app_module");
    }

    public native static int module_init();
}

And I used JNI like this in my root of the project:

javah -jni -classpath build/classes/debug -d jni/ com.sunsystems.snaptest.App_Module

Which generated the following header interface:

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_sunsystems_snaptest_App_Module */

#ifndef _Included_com_sunsystems_snaptest_App_Module
#define _Included_com_sunsystems_snaptest_App_Module
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_sunsystems_snaptest_App_Module
 * Method:    module_init
 * Signature: ()I
 */
JNIEXPORT jint JNICALL Java_com_sunsystems_snaptest_App_Module_module_1init
  (JNIEnv *, jclass);

#ifdef __cplusplus
}
#endif
#endif

Then in my Android App I load the library in the above App_Module class and call it like this:

App_Module.module_init()

So I guess it cannot find symbol inside the libapp_module.so library.

Many thanks for any suggestions,

@joni : I followed you and @Joni. But I receive the error java.lang.UnsatisfiedLinkError: module_init. Can u help me solve problem? — Huy Tower
– Huy Tower, Commented Mar 21, 2014 at 9:42

Joni · Accepted Answer · 2013-10-25 13:55:51Z

1

You have to implement the native method and make it call the function in your library. For example:

#include "header_file_for_your_library.h"

#include "com_sunsystems_snaptest_App_Module.h"

JNIEXPORT jint JNICALL Java_com_sunsystems_snaptest_App_Module_module_1init(JNIEnv *env, jclass klass) {
    return module_init();
}

Just about any tutorial on JNI or Android NDK will have further details.

answered Oct 25, 2013 at 13:55

Joni

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1 Comment

Huy Tower Over a year ago

I followed you and @Joni. But I receive the error java.lang.UnsatisfiedLinkError: module_init. Can u help me solve problem?

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