JNI wrapper complaining about undefined references

Adrijan 07/27/2017. 0 answers, 39 views
android android c android-ndk android-ndk cmake jni

I'm trying to use the TagLib library in an Android app. As far as I know, I have to write a JNI wrapper for it. I wanted to try something simple - read the title from the tag.

I wrote the following code in Java:

package developer.rogan.taglib;

import android.support.annotation.NonNull;
import android.util.Log;

public class TagJNI {

    private static final String TAG = "TagJNI";

    private String filename;
    private String title;

    public TagJNI(@NonNull String filename) {
        this.filename = filename;
    }

    public String getTitle() {
        Log.d(TAG, "getTitle, filename = " + filename);
        this.title = taglibGetTitle(filename);
        return title;
    }

    private native String taglibGetTitle(String filename);
}

Android Studio then gave me the option to automatically generate the function in native code (the file is called tagjni.c). I added some of my own code to add functionality:

#include <jni.h>
#include "../../../../../../Development/C++/taglib-1.11.1/bindings/c/tag_c.h"

TagLib_File *file;
TagLib_Tag *tag;
char *title;

JNIEXPORT jstring JNICALL
Java_developer_rogan_taglib_TagJNI_taglibGetTitle(JNIEnv *env, jobject instance,
                                                  jstring filename_) {

    taglib_set_strings_unicode(JNI_FALSE);
    const char *filename = (*env)->GetStringUTFChars(env, filename_, 0);

    file = taglib_file_new(filename);
    tag = taglib_file_tag(file);
    title = taglib_tag_title(tag);
    taglib_file_free(file);
    (*env)->ReleaseStringUTFChars(env, filename_, filename);
    return (*env)->NewStringUTF(env, title);
}

There's something going on with the include statement too. I can write #include and then the other include says that it's unused. However, when I delete it, none of the variables are recognised anymore. When I hover over the errors, I get the option to add the #include.

When I try to run the app, I get the following error (and some more similar ones): undefined reference to 'taglib_set_strings_unicode'

Does this mean that the function I'm calling cannot be found? I had a lot of issues setting the library up and finally got it to work somehow. As I started writing the JNI wrapper, Android Studio complained about the .c file not being included in the project. After looking at the NDK web page I figured I have to create one root CMakeLists.txt and reference the other two (one for TagLib and one for JNI wrapper). I know very little about C and CMakeLists. This is what I came up with:

cmake_minimum_required(VERSION 3.4.1)

add_subdirectory( # Specifies the directory of the CMakeLists.txt file.
                  /home/adrijan/Development/C++/taglib-1.11.1
                  # Specifies the directory for the build outputs.
                  /home/adrijan/devel/tagtest/taglib/src/main/cpp )
add_library( taglib SHARED IMPORTED )

add_subdirectory( # Specifies the directory of the CMakeLists.txt file.
                  /home/adrijan/devel/tagtest/taglib/src/main/jni
                  # Specifies the directory for the build outputs.
                  /home/adrijan/devel/tagtest/taglib )

TagLib already includes a CMakeLists so I just pointed to it. For the JNI wrapper, I placed CMakeLists.txt inside the directory where the code is:

cmake_minimum_required(VERSION 3.4.1)

# Specifies a library name, specifies whether the library is STATIC or
# SHARED, and provides relative paths to the source code. You can
# define multiple libraries by adding multiple add.library() commands,
# and CMake builds them for you. When you build your app, Gradle
# automatically packages shared libraries with your APK.

add_library( # Specifies the name of the library.
             jni-taglib

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             tagjni.c )

# Specifies a path to native header files.
include_directories(tagjni.h)

It seems to me that I'm missing something really obvious here. I believe it has to do something with the way I set up CMake, but I can't figure it out.

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