Android NDK: read file from assets inside of shared library

Question

In my app I have to pass a file from assets folder to shared library. I cannot do it with use of jni right now. I'm using precompiled shared library in my project, in which I have hardcoded path to my file, but I'm getting error "No such file or directory". So in my .apk file I have .so file in libs/armeabi-v7a folder and my file in /assets folder.

I have tried to do it like this:

char *cert_file = "/assets/cacert.cert";
av_strdup(cert_file);

And some other paths, but it doesn't work.

Is it possible at all?

Answer 1

You can simply use the AAssetManager class in C++.

Basically you need to:

• During the init of you library get a pointer on: AAssetManager* assetManager

• Use it to read your file:

  // Open your file
  AAsset* file = AAssetManager_open(assetManager, filePath, AASSET_MODE_BUFFER);
  // Get the file length
  size_t fileLength = AAsset_getLength(file);
  
  // Allocate memory to read your file
  char* fileContent = new char[fileLength+1];
  
  // Read your file
  AAsset_read(file, fileContent, fileLength);
  // For safety you can add a 0 terminating character at the end of your file ...
  fileContent[fileLength] = '\0';
  
  // Do whatever you want with the content of the file
  
  // Free the memoery you allocated earlier
  delete [] fileContent;
  

You can find the official ndk documentation here.

Edit: To get the AAssetManager object:

• In a native activity, you main function as a paramater android_app* app, you just need to get it here: app->activity->assetManager
• If you have a Java activity, you need so send throught JNI an instance of the java object AssetManager and then use the function AAssetManager_fromJava()

Answer 2

Your assets are packaged into your apk, so you can't refer to them directly during runtime like in the sample code you provided. You have 2 options here:

• Process the assets as an input stream, using

Context.getAssets().open('cacert.cert')

• Copy out your asset to a local file in your files dir, and then reference the filename of the copied file.

Answer 3

Building off of the answer by @Sistr , I used getBuffer along with AASSET_MODE_BUFFER (instead of AAsset_read).

However, I was trying to write the buffer to an ofstream. I got signal crashes using the <<operator:

ofstream myStream(<some file>, ios_base::binary);
auto buffer = AAsset_getBuffer(asset);
myStream << buffer;

I presume this is because the buffer pointer points to the asset without a NULL character at the end (I was reading a text asset; Android source code). Using the write function worked:

ofstream myStream(<some file>, ios_base::binary);
auto buffer = AAsset_getBuffer(asset);
myStream.write((char *)buffer, AAsset_getLength(asset));

This is because the <<operator does not have to call strlen to find out how many characters to write since it is explicitly given by AAsset_getLength.