fopen/fread APK Assets from NativeActivity on Android

Question

I have only been able to find solutions dated 2010 and earlier. So I wanted to see if there was a more up-to-date stance on this.

I'd like to avoid using Java and purely use C++, to access files (some less-or-more than 1MB) stored away in the APK. Using AssetManager means I can't access files like every other file on every other operating system (including iOS).

If not, is there a method in C++ where I could somehow map fopen/fread to the AssetManager APIs?

Eleaseeleatic

Since assets aren't files, the answer is likely "no". Assets are entries in the ZIP archive that is the APK. – Brevity

Technically, since an APK is nothing more than a fancy zip file it is perfectly possible to access any file anywhere in the APK as if it were a zip file in c++. It does however require some java, but only to get your apk install location. I know for a fact reeading is not an issue but I never tried writing to an APK. – Cydnus

Answer 1

I actually found pretty elegant answer to the problem and blogged about it here.

The summary is:

• The AAssetManager API has NDK bindings. This lets you load assets from the APK.
• It is possible to combine a set of functions that know how to read/write/seek against anything and disguise them as a file pointer (FILE*).
• If we create a function that takes an asset name, uses AssetManager to open it, and then disguises the result as a FILE* then we have something that's very similar to fopen.
• If we define a macro named fopen we can replace all uses of that function with ours instead.

My blog has a full write up and all the code you need to implement in pure C. I use this to build lua and libogg for Android.

Gilgamesh

for file size: size_t file_size (FILE *fp) { android_seek(fp, 0L, SEEK_END); size_t sz = ftell(fp); android_seek(fp, 0L, SEEK_SET); return sz; } – Hype

Answer 2

Short answer

No. AFAIK mapping fread/fopen in C++ to AAssetManager is not possible. And if were it would probably limit you to files in the assets folder. There is however a workaround, but it's not straightforward.

Long Answer

It IS possible to access any file anywhere in the APK using zlib and libzip in C++. Requirements : some java, zlib and/or libzip (for ease of use, so that's what I settled for). You can get libzip here: http://www.nih.at/libzip/

libzip may need some tinkering to get it to work on android, but nothing serious.

Step 1 : retrieve APK location in Java and pass to JNI/C++

String PathToAPK;
ApplicationInfo appInfo = null;
PackageManager packMgmr = parent.getPackageManager();
try {
    appInfo = packMgmr.getApplicationInfo("com.your.application", 0);
} catch (NameNotFoundException e) {
    e.printStackTrace();
    throw new RuntimeException("Unable to locate APK...");
}

PathToAPK = appInfo.sourceDir;

Passing PathToAPK to C++/JNI

JNIEXPORT jlong JNICALL Java_com_your_app(JNIEnv *env, jobject obj, jstring PathToAPK)
{
    // convert strings
    const char *apk_location = env->GetStringUTFChars(PathToAPK, 0);

    // Do some assigning, data init, whatever...
    // insert code here

    //release strings
    env->ReleaseStringUTFChars(PathToAPK, apk_location);

    return 0;
}

Assuming that you now have a std::string with your APK location and you have zlib on libzip working you can do something like this:

if(apk_open == false)
{
    apk_file = zip_open(apk_location.c_str(), 0, NULL);

    if(apk_file == NULL)
    {
        LOGE("Error opening APK!");
        result = ASSET_APK_NOT_FOUND_ERROR;
    }else
    {
        apk_open = true;
        result = ASSET_NO_ERROR;
    }
}

And to read a file from the APK:

if(apk_file != NULL){
    // file you wish to read; **any** file from the APK, you're not limited to regular assets
    const char *file_name = "path/to/file.png";

    int file_index;
    zip_file *file;
    struct zip_stat file_stat;

    file_index = zip_name_locate(apk_file, file_name, 0);

    if(file_index == -1)
    {
        zip_close(apk_file);
        apk_open = false;

        return;
    }

    file = zip_fopen_index(apk_file, file_index, 0);
    if(file == NULL)
    {
        zip_close(apk_file);
        apk_open = false;

        return;
    }

    // get the file stats
    zip_stat_init(&file_stat);
    zip_stat(apk_file, file_name, 0, &file_stat);
    char *buffer = new char[file_stat.size];

    // read the file
    int result = zip_fread(file, buffer, file_stat.size);
    if(result == -1)
    {
        delete[] buffer;
        zip_fclose(file);

        zip_close(apk_file);
        apk_open = false;

        return;
    }

    // do something with the file
    // code goes here

    // delete the buffer, close the file and apk
    delete[] buffer;
    zip_fclose(file);

    zip_close(apk_file);
    apk_open = false;

Not exactly fopen/fread but it gets the job done. It should be pretty easy to wrap this to your own file reading function to abstract the zip layer.

Cydnus

libz is provided by Android toolchain these days - would there be any dangers in using it over libzip? Are they even different/same? – Eleaseeleatic

None at all. I use the android provided libz as well; libzip is just a library that sits on top libz to make reading/writing a little easier. – Cydnus

One quick fix you need delete[] not delete on the buffer. – Subarid