libprotobuf.a: X compile with Android NDK

Useful while working on Native library.

To cross-compile Protobuf for Android using the NDK toolchain and generate a static library (libprotobuf.a), follow the detailed steps below.

Prerequisites

These prerequisites are what i tried with while building.

1. Ubuntu: 20.4
2. Install Android NDK: Download and install the Android NDK from the NDK official site. I tried with NDK versionv21e
3. Install build tools: We will need autoconf, automake, libtool, cmake, and ninja. We can install them via our package manager:

sudo apt-get install autoconf automake libtool cmake ninja-build

Step-by-Step Guide

Step 1: Download and Extract Protobuf

1. Clone the latest release of Protobuf from GitHub, but fetch v.3.5.1 because post this protobuf user bazel for building. (This was a surprise for me, and took quite some to figure out)

git clone https://github.com/protocolbuffers/protobuf.git

cd protobuf && git checkout v3.5.1

Step 2: Set Up the Android NDK Toolchain

1. Set up environment variables for the Android target (replace arm64-v8a with our target architecture):

export ANDROID_NDK=<path-to-android-ndk>
export TOOLCHAIN=$ANDROID_NDK/toolchains/llvm/prebuilt/linux-x86_64
export TARGET=aarch64-linux-android
export API=21  # We can change this depending on our minimum supported Android version

2. Set the cross-compiling environment variables for aarch64:

export AR=$TOOLCHAIN/bin/$TARGET-ar
export AS=$TOOLCHAIN/bin/$TARGET-as
export CC=$TOOLCHAIN/bin/$TARGET$API-clang
export CXX=$TOOLCHAIN/bin/$TARGET$API-clang++
export LD=$TOOLCHAIN/bin/$TARGET-ld
export RANLIB=$TOOLCHAIN/bin/$TARGET-ranlib
export STRIP=$TOOLCHAIN/bin/$TARGET-strip

Step 3: Configure Protobuf for Cross-Compilation

1. Install protobuf build dependencies: We need to install libtool, autoconf, and automake:

sudo apt-get install libtool autoconf automake

2. Generate the configuration files for Protobuf:

./autogen.sh

mkdir build && cd build

3. Configure Protobuf with Android toolchain: Now configure the Protobuf library to use the Android NDK toolchain:

../configure --host=$TARGET --with-sysroot=$TOOLCHAIN/sysroot --enable-static --disable-shared --disable-tests CXXFLAGS="-fPIC" CC=$CC CXX=$CXX LDFLAGS="-llog -lz"

NOTE: We are disabling test, else you will see build error because the tests the target and build envornments are different.

Step 4: Compile Protobuf

1. Build the static library:

make -j 2

This will build libprotobuf.a for Android for above specified target type. Target different architectures by changing the TARGET and API values (e.g., armv7a-linux-androideabi for ARM 32-bit, x86_64-linux-android for x86_64).

NOTE: Make sure all the build tools have proper path, example: for armv7a-linux-androideabi the AR/AS/LD/… tool has following path:

export AR=$TOOLCHAIN/bin/arm-linux-androideabi-ar

Step 5: Verify the Output

1. After the build completes, check for the generated libprotobuf.a file in the src/.libs/ directory.

ls src/.libs/libprotobuf.a

Final words, if configure fails, mostly it is something to do with the export path, double check them all.