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 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 an OpenJDK install, a copy of the Android SDK command line tools (aapt or aapt2, zipalign, apksigner), and a copy of the Android NDK toolchains. See my article for several ways to acquire these.

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. E.g. using keytool:

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=Doe, S=John, 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=30 ./pull_android_jar.sh
ANDROID_VERSION=30 \
    ANDROID_JAR=./android-30.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 SEGLAPP:I *:S

To uninstall the app you can run:

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