Simple Native Android EGL App

A minimal example of building a C Android NDK app from the command line without writing java code. I also wrote an accompanying article.

Motivation

I currently use a musl libc based distro (Chimera Linux) and Google does not distribute Android Studio for such a system. Luckily, a good samaritan has gone through the work of compiling the Android SDK tools and Android NDK into static binaries that will work on any Linux system.

The rawdraw Android project provides a fantastic example of how to build a native Android applicaiton from the command line, but it is still somewhat complicated. After getting my GLFW applications to compile for Android, I distilled the process down to this tiny OpenGL ES 2.0 example.

Dependencies

You'll need to install OpenJDK, and acquire a copy of the Android SDK command line tools (aapt or aapt2, zipalign, apksigner), and the Android NDK clang toolchain. See my article for a walkthrough setting up a build environment.

You will also need a copy of android.jar for the target android SDK version. A copy can be pulled from the Sable collection by running ANDROID_VERSION=XX ./pull_android_jar.sh.

Finally, you will need a keystore file to sign your application. A dummy debug keystore can be generated using keytool from OpenJDK:

keytool -genkey -v -keystore mykey.keystore -alias mykey -keyalg RSA -keysize 2048 \
    -validity 10000 -storepass mypassword -keypass mypassword \
    -dname "CN=example.com, OU=ID, O=Example, L=Callar, S=Morvern, C=GB"

Building

You will need to export the following environment variables in order to run ./build.sh.

#     ANDROID_VERSION: the target android version number (must be >=22)
#     ANDROID_JAR: a path to the android.jar file (see pull_android_jar.sh)
#     ANDROID_AAPT: path to android sdk aapt
#     ANDROID_ZIPALIGN: path to android sdk zipalign
#     ANDROID_APKSIGNER: path to android sdk apksizinger
#     ANDROID_CLANG: path to android ndk clang
#     APP_NAME: the name of your app
#     ORG_NAME: the name of your organization
#     KEYSTORE_FILE: path to keystore file
#     STORE_PASS: keystore password
#     KEY_PASS: key password
#     CFLAGS: common flags to be passed to C compiler
#     LDFLAGS: common flags to be passed to the linker

An example build command:

ANDROID_VERSION=35 ./pull_android_jar.sh
ANDROID_VERSION=35 \
    ANDROID_JAR=./android-35.jar \
    ANDROID_AAPT=/home/user/android_sdk/build-tools/35.0.1/aapt \
    ANDROID_ZIPALIGN=/home/user/android_sdk/build-tools/35.0.1/zipalign \
    ANDROID_APKSIGNER=/home/user/android_sdk/build-tools/35.0.1/apksigner \
    ANDROID_CLANG=/home/user/android_sdk/ndk/29.0.13599879/toolchains/llvm/prebuilt/linux-x86_64/bin/clang \
    APP_NAME=seglapp \
    ORG_NAME=avensegl \
    KEYSTORE_FILE=./mykey.keystore \
    STORE_PASS=mypassword \
    KEY_PASS=mypassword \
    CFLAGS="-Wall -Wextra -Wno-unused-parameter -std=c11 -O3 -g0 -s" \
    ./build.sh

Installing and testing

You will need to enable USB Debugging on the test device (or use an emulator) and then install the APK with adb:

adb install ./build_android/seglapp.apk # replace seglapp.apk with your app name

To view debug logs you can run:

adb shell logcat SEGL:I *:S

To uninstall the app you can run:

adb uninstall org.avensegl.seglapp # replace avensegl and seglapp with your org and app names