How to use Cmake in android for adding c/c++ function

 cmake 활용하여 c/c++ 함수 사용하기

최근들어 안드로이드 프로젝트에서 c 나 c++ 언어를 사용해야할 일이 생긴다.

이전에는 ndk빌드를 통해서 c 나 c++ 함수를 사용했는데 이번에는 cmake를 사용해보았다.

안드로이드 공식문서에서도 ndk 빌드보다는 cmake 스크립트를 통해 빌드하는 것을 권장하고있다.

Tool - SDK Manager 들어가기

Select Cmake version you want to install

On left Project menu, make foler named by cpp

On cpp folder, we put several files about c++ code

Make CMakeLists.txt in cpp folder

Make cpp file on cpp folder

.cpp file is named "native-lib"

It's okay if you want different file name.

cmake_minimum_required(VERSION 3.4.1)

project("sampleproject")

add_library(
        native-lib
        SHARED
        native-lib.cpp
)

find_library(
        native-lib
        log
)

target_link_libraries(
        native-lib
        ${logd-lib}
)

Fill the code in CMakeLists.txt like above codes.

plugins {
    id("com.android.application")
    id("org.jetbrains.kotlin.android")
}

android {
    ...

    defaultConfig {
    ...
        externalNativeBuild {
            cmake {
               arguments += listOf("-DANDROID_STL=c++_shared")
            }
        }
    }

    buildTypes {
        release {
            isMinifyEnabled = false
            proguardFiles(
                getDefaultProguardFile("proguard-android-optimize.txt"),
                "proguard-rules.pro"
            )
        }
    }
    compileOptions {
        sourceCompatibility = JavaVersion.VERSION_1_8
        targetCompatibility = JavaVersion.VERSION_1_8
    }
    kotlinOptions {
        jvmTarget = "1.8"
    }

    externalNativeBuild {
        cmake {
            path = File("src/main/cpp/CMakeLists.txt")
            version = "3.22.1"
        }
    }
}

Above code is build.gradle(levle: app)

There is code to build CMakeLists.txt

extern "C"
JNIEXPORT jstring JNICALL
Java_com_example_sampleproject_MainActivity_stringFromJni(JNIEnv *env, jobject thiz) {
    std::string hello = "Hello from C++";
    return env->NewStringUTF(hello.c_str());
}

Define the function we use in MainActivity

class MainActivity : AppCompatActivity() {
    val TAG = MainActivity::class.java.simpleName

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        Log.d(TAG, "stringFromJni() : ${stringFromJni()}")

    }

    external fun stringFromJni() : String
}

We can use stringFromJni() function which is defined by c++ code

Verify the result on the Logcat

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