Android NDK: How to use CMake and ndk-build together in one project

Android NDK currently provides us with at least two ways to build native libraries: CMake and ndk-build.

You may need to use both build systems at the same time to compile some existing libraries. But if you specify two paths to the build scripts CMakeLists.txt and Android.mk in the build.gradle file, then Gradle will show an error “More than one externalNativeBuild path specified” and will not compile anything. I will show one of the possible ways to solve this problem.

The most obvious solution would be to use CMake by default and run ndk-build separately. But I don’t want to run it manually every time, so in this article, we will use Gradle’s ability to invoke external commands and configure ndk-build to run automatically during application build.

I created a simple project on GitHub with two C libraries: one with a build script CMakeLists.txt and the other built via Android.mk. You can view the files in the app/src/main/jni/ directory.

In the app/build.gradle file we configure the library build via CMake in the usual way:

android {
    defaultConfig {
        ndk {
            abiFilters 'armeabi-v7a', 'arm64-v8a', 'x86', 'x86_64'
        }
    }

    externalNativeBuild {
        cmake {
            path "src/main/jni/cmake/CMakeLists.txt"
        }
    }
}

This will allow us to compile the first library via CMake by simply starting the build of the application. To build the second library using ndk-build, we have the following task at the end of the app/build.gradle file:

task runNdkBuild(type: Exec) {
    def ndkDir = android.ndkDirectory
    executable = "$ndkDir/ndk-build"
    args = ['NDK_PROJECT_PATH=build/intermediates/ndk',
            'NDK_LIBS_OUT=src/main/jniLibs',
            'APP_BUILD_SCRIPT=src/main/jni/ndkbuild/Android.mk',
            'NDK_APPLICATION_MK=src/main/jni/ndkbuild/Application.mk']
}

Tasks of Exec type are used to run external commands. We must assign the path to the command to the executable property. To do this, we get the path to the installed NDK in the first line and add the name "/ndk-build" to it. We also pass an array of arguments for the command in the args property:

• NDK_LIBS_OUT - the directory in which the libraries will be located after compilation. Here we specify the path src/main/jniLibs because the libraries from this directory are automatically included in the APK after compiling the project. You will also have to override the property android.sourceSets.main.jniLibs.srcDirs if you need to specify a different directory here.

• APP_BUILD_SCRIPT - specifies the path to our build script Android.mk

• NDK_APPLICATION_MK - specifies the path to the Application.mk file, where we put the compilation settings

We can run this task now with gradle runNdkBuild command. You will see the freshly compiled *.so files in the app/src/main/jniLibs directory if the compilation was successful.

It’s time to add automatic execution of this task during the project build. Add a line to the end of the app/build.gradle file to do this:

preBuild.dependsOn runNdkBuild

Here we have indicated that starting the build of the project depends on our task, so ndk-build will run just before the build. You can run gradle clean and gradle build to make sure everything builds as it should.