libprotobuf.a: X compile with Android NDK using Bazel

To build libprotobuf.a using Bazel and the Android NDK, follow these steps:

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 Bazel:

• Follow the instructions from Bazel’s official site to install Bazel for our Linux distribution. We can also install it using:

sudo apt install apt-transport-https curl gnupg -y

curl -fsSL https://bazel.build/bazel-release.pub.gpg | gpg --dearmor >bazel-archive-keyring.gpg

sudo mv bazel-archive-keyring.gpg /usr/share/keyrings

echo "deb [arch=amd64 signed-by=/usr/share/keyrings/bazel-archive-keyring.gpg] https://storage.googleapis.com/bazel-apt stable jdk1.8" | sudo tee /etc/apt/sources.list.d/bazel.list

sudo apt update && sudo apt-get install bazel

4. Download Protobuf Source:

• Clone the protobuf repository and check out version 5.27.1:

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

cd protobuf

git checkout v5.27.1

5. Install Python (if needed):

• Bazel will need Python to configure the protobuf build.

Step-by-Step Process

1. Setup Bazel Configuration for Android NDK

We need to configure Bazel to use the Android NDK. To do that, we need to set up an ndk in our Bazel WORKSPACE file.

1. Open or create the WORKSPACE file in the root of our protobuf directory:

vim WORKSPACE

1. Add the Android NDK and SDK configurations to our WORKSPACE file:

android_ndk_repository(     
     name = "androidndk",
     path = "/path/to/our/android-ndk",
     api_level = 21,  # Set to the desired Android API level.
)

• Replace /path/to/our/android-ndk with the actual paths to your Android NDK installations.

2. Configure Android NDK in .bazelrc

Bazel uses .bazelrc to manage the build configuration. We need to specify the Android NDK and the desired architecture in this file.

1. Create or open the .bazelrc file:

vim .bazelrc

1. Add the following configuration to build for Android:

build --config=android 
build:android --cpu=arm64-v8a 
build:android --fat_apk_cpu=arm64-v8a  

• We can change the CPU architecture (arm64-v8a, armeabi-v7a, x86_64) according to the architecture we need.

3. Build libprotobuf.a using Bazel

Bazel allows we to directly target and build specific libraries. In this case, we’ll be building the static library (libprotobuf.a).

1. Run the following Bazel command to build libprotobuf.a:

bazel build //src/google/protobuf:protobuf --config=android --cpu=arm64-v8a

4. Locate the Output

After the build is successful, Bazel will output the compiled static library (libprotobuf.a). We can find it under the bazel-bin directory inside our project:

bazel-bin/src/google/protobuf/libprotobuf.a
bazel-bin/src/google/protobuf/libprotobuf.so

• Multiple Architectures: To build for multiple architectures, we can pass different --cpu values like arm64-v8a, armeabi-v7a, x86_64, etc. We can build for multiple architectures in one go by passing --fat_apk_cpu=arm64-v8a,armeabi-v7a,x86_64.
• Building Other Protobuf Components: We can replace //:protobuf with other targets, such as //:protoc_lib or //:protobuf_lite, depending on our needs.

This process cross-compiles Protobuf for Android and outputs the libprotobuf.a static library using Bazel and the NDK.