Building shared libraries (.so) to use TF Lite C++ API in Android applications

When following Android quick start guide for building the C++ shared libraries to use their APIs on Android apps, I experienced many Bazel build errors, such as does not contain a toolchain for cpu 'arm64-v8a' when building tensorflow lite. After 2 days looking for a solution, I finally managed to make it work. The quick start guide was not informative enough, I had to configure somethings before executing the simple

bazel build -c opt --config=android_arm //tensorflow/lite:libtensorflowlite.so
# or
bazel build -c opt --config=android_arm64 //tensorflow/lite:libtensorflowlite.so

commands listed on it. This repo shows more detailed instructions to build the libraries for using C++ API on Android.

If you don't care about the process, just go ahead and get the generated libraries (for 32bit armeabi-v7a and 64bit arm64-v8a) in the folder generated-libs.

Step by step guide to build TF Lite C++ libraries

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• Step 1: Install Bazel

• Step 2: Clone the TensorFlow repo

git clone https://github.com/tensorflow/tensorflow
cd ./tensorflow/

• Step 3: Configure Android build Before running the bazel build ... command, you need to configure the build process. Do so by executing

./configure

The configure file is at the root of the tensorflow directory, which you cd to at Step 2. Now you have to input some configurations on the command line:

$ ./configure
You have bazel 3.7.2-homebrew installed.
Please specify the location of python. [Default is /Library/Developer/CommandLineTools/usr/bin/python3]: /Users/cuongvng/opt/miniconda3/envs/style-transfer-tf-lite/bin/python

First is the location of python, because ./configure executes the .configure.py file. Choose the location that has Numpy installed, otherwise the later build will fail. Here I point it to the python executable of a conda environment.

Next,

Found possible Python library paths:
  /Users/cuongvng/opt/miniconda3/envs/style-transfer-tf-lite/lib/python3.7/site-packages
Please input the desired Python library path to use.  Default is [/Users/cuongvng/opt/miniconda3/envs/style-transfer-tf-lite/lib/python3.7/site-packages]

I press Enter to use the default site-packages, which contains necessary libraries to build TF.

Next,

Do you wish to build TensorFlow with ROCm support? [y/N]: N
No ROCm support will be enabled for TensorFlow.

Do you wish to build TensorFlow with CUDA support? [y/N]: N
No CUDA support will be enabled for TensorFlow.

Do you wish to download a fresh release of clang? (Experimental) [y/N]: N
Clang will not be downloaded.

Please specify optimization flags to use during compilation when bazel option "--config=opt" is specified [Default is -Wno-sign-compare]: 

Key in as showed above, on the last line type Enter. Then it asks you whether to configure ./WORKSPACE for Android builds, type y to add configurations.

Would you like to interactively configure ./WORKSPACE for Android builds? [y/N]: y
Searching for NDK and SDK installations.

Please specify the home path of the Android NDK to use. [Default is /Users/cuongvng/library/Android/Sdk/ndk-bundle]: /Users/cuongvng/Library/Android/sdk/ndk/21.1.6352462

That is the home path of the Android NDK (version 21.1.6352462) on my local machine. Note that when you ls the path, it must include platforms, e.g.:

$ ls /Users/cuongvng/Library/Android/sdk/ndk/21.1.6352462
CHANGELOG.md      build             ndk-stack         prebuilt          source.properties wrap.sh
NOTICE            meta              ndk-which         python-packages   sources
NOTICE.toolchain  ndk-build         package.xml       shader-tools      sysroot
README.md         ndk-gdb           platforms         simpleperf        toolchains

For now I ignore the resulting WARNING, then choose the min NDK API level

WARNING: The NDK version in /Users/cuongvng/Library/Android/sdk/ndk/21.1.6352462 is 21, which is not supported by Bazel (officially supported versions: [10, 11, 12, 13, 14, 15, 16, 17, 18]). Please use another version. Compiling Android targets may result in confusing errors.

Please specify the (min) Android NDK API level to use. [Available levels: ['16', '17', '18', '19', '21', '22', '23', '24', '26', '27', '28', '29']] [Default is 21]: 29

Next

Please specify the home path of the Android SDK to use. [Default is /Users/cuongvng/library/Android/Sdk]: /Users/cuongvng/Library/Android/sdk

Please specify the Android SDK API level to use. [Available levels: ['28', '29', '30']] [Default is 30]: 30

Please specify an Android build tools version to use. [Available versions: ['29.0.2', '29.0.3', '30.0.3', '31.0.0-rc1']] [Default is 31.0.0-rc1]: 30.0.3

That is all for Android build configs. Choose N for all questions appearing later:

• Step 4: Build the shared library (.so) Now you can run the bazel build command to generate libraries for your target architecture:

bazel build -c opt --config=android_arm //tensorflow/lite:libtensorflowlite.so
# or
bazel build -c opt --config=android_arm64 //tensorflow/lite:libtensorflowlite.so

It should work without errors. The generated library would be saved at ./bazel-bin/tensorflow/lite/libtensorflowlite.so.

How to integrate the libraries in Android apps (using Android Studio)

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This repo consists of:

• Pre-built TF Lite C++ libraries for armeabi-v7a and arm64-v8a ABIs in the ./generated-libs directory.
• All header files needed in the ./include directory, as stated in the guide

Currently, there is no straightforward way to extract all header files needed, so you must include all header files in tensorflow/lite/ from the TensorFlow repository. Additionally, you will need header files from FlatBuffers and Abseil. The tensorflow headers are at commit @8f46088df45cc9824b2901378106572aa0a89406.

Guide to import the libraries:

• Create a new android studio project with native C++
• At the project root, initialize it as a git repo by

$ git init

• Then cd to `app/src/main/cpp/
• Add this repo as a git submodule (recursively, as this repo has other submodules), by

$ git submodule add  https://github.com/cuongvng/TF-Lite-Cpp-API-for-Android tf-lite-api
$ git submodule update --init --recursive

• Set ABI filters in build.gradle (app), inside android.defaultConfig.externalNativeBuild.cmake

android {
    ...
    defaultConfig {
        ...
        externalNativeBuild {
            cmake {
                cppFlags "-frtti -fexceptions"
                abiFilters 'armeabi-v7a', 'arm64-v8a'
            }
        }
        ...
    }

• Modify CMakeLists.txt to link your app with the prebuilt TF Lite libs

# For more information about using CMake with Android Studio, read the
# documentation: https://d.android.com/studio/projects/add-native-code.html

# Sets the minimum version of CMake required to build the native library.

cmake_minimum_required(VERSION 3.10.2)

project("tflitecxx")

# Specify where to find the header files for TF Lite C++
set( INCLUDE_DIRS
        ${CMAKE_CURRENT_LIST_DIR}/tf-lite-api/include
        ${CMAKE_CURRENT_LIST_DIR}/tf-lite-api/include/flatbuffers/include)
include_directories(${INCLUDE_DIRS})

add_library( tflite SHARED IMPORTED )
set_target_properties( tflite PROPERTIES IMPORTED_LOCATION
        ${CMAKE_CURRENT_LIST_DIR}/tf-lite-api/generated-libs/${ANDROID_ABI}/libtensorflowlite.so )

# Build the main target `native-lib` that will use TF Lite
add_library( native-lib SHARED native-lib.cpp )

find_library( log-lib log ) # Library required by NDK.
find_library(android-lib android) # for AssetManager functionality

# Link the main target with two required libs: `log` and `libtensorflowlite.so`
target_link_libraries( native-lib ${log-lib} ${android-lib} tflite)

I have created a example app that shows how to integrate the TF Lite C++ APIs in an Android app, to load a .tflite model. Check it out!

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