Hi. I'm having some trouble getting c interop to work with my KMM library.

# multiplatform

[Kieren Searle]

Hi. I'm having some trouble getting c interop to work with my KMM library.

[Kieren Searle]

I have the following c header:

#ifndef __MYWRAPPER_H
#define __MYWRAPPER_H

#ifdef __cplusplus
extern "C" {
#endif

typedef struct MyClass MyClass;

MyClass* newMyClass();

void MyClass_int_set(MyClass* v, int i);

int MyClass_int_get(MyClass* v);

void deleteMyClass(MyClass* v);

#ifdef __cplusplus
}
#endif
#endif

[Kieren Searle]

And the following def file

package = testlib

headers = MyWrapper.h
headerFilter = MyWrapper.h

staticLibraries = libtest.a
libraryPaths = src/nativeInterop/cinterop/libtest/
compilerOpts = -Isrc/nativeInterop/cinterop/libtest

[Kieren Searle]

And here is my gradle

kotlin {
    android()

    listOf(
        iosX64(),
        iosArm64(),
        iosSimulatorArm64()
    ).forEach {
        it.binaries.framework {
            baseName = "shared"
        }

        it.apply {
            compilations.getByName("main") {
                val libtest by cinterops.creating {
                    defFile(project.file("src/nativeInterop/cinterop/libtest.def"))
                    packageName("testlib")
                }
            }
        }
    }

    sourceSets {
        val commonMain by getting
        val commonTest by getting {
            dependencies {
                implementation(kotlin("test"))
            }
        }
        val androidMain by getting
        val androidTest by getting
        val iosX64Main by getting
        val iosArm64Main by getting
        val iosSimulatorArm64Main by getting
        val iosMain by creating {
            dependsOn(commonMain)
            iosX64Main.dependsOn(this)
            iosArm64Main.dependsOn(this)
            iosSimulatorArm64Main.dependsOn(this)
        }
        val iosX64Test by getting
        val iosArm64Test by getting
        val iosSimulatorArm64Test by getting
        val iosTest by creating {
            dependsOn(commonTest)
            iosX64Test.dependsOn(this)
            iosArm64Test.dependsOn(this)
            iosSimulatorArm64Test.dependsOn(this)
        }
    }
}

[Kieren Searle]

Everything seems to build fine, but I can't access testlib from Kotlin (Android, iOS, or common)

[Landry Norris]

I think the methods need to be static.

[Landry Norris]

Is src in the root of your project, or in a module’s folder?

[Kieren Searle]

It's the module folder

[Landry Norris]

You should be able to remove the defFile from your gradle and just use val libtest by cinterops.creating . It will automatically check src/nativeInterop/cinterop/libtest.def

[Kieren Searle]

I have tried that too, I just added it while I was testing the packageName. It's the same behavior with this too

it.apply {
    compilations.getByName("main") {
        val libtest by cinterops.creating
    }
}

[Landry Norris]

Are there any warnings in the console?

[Kieren Searle]

Oh! Yes there is!

[WARNING] The project is using Kotlin Multiplatform with hierarchical structure and disabled 'cinterop commonization'
    See: <https://kotlinlang.org/docs/mpp-share-on-platforms.html#use-native-libraries-in-the-hierarchical-structure>

    'cinterop commonization' can be enabled in your 'gradle.properties'
    kotlin.mpp.enableCInteropCommonization=true
    
    To hide this message, add to your 'gradle.properties'
    kotlin.mpp.enableCInteropCommonization.nowarn=true 

    The following source sets are affected: 
    [iosMain]
    
    The following cinterops are affected: 
    [cinterop:compilation/compileKotlinIosArm64/libtest, cinterop:compilation/compileKotlinIosSimulatorArm64/libtest, cinterop:compilation/compileKotlinIosX64/libtest]

[Kieren Searle]

Setting that doesn't seem to help though 😞

[Landry Norris]

Any new warnings?

[Kieren Searle]

No

[Kieren Searle]

> Configure project :shared
Kotlin Multiplatform Projects are an Alpha feature. See: <https://kotlinlang.org/docs/reference/evolution/components-stability.html>. To hide this message, add 'kotlin.mpp.stability.nowarn=true' to the Gradle properties.


The following Kotlin source sets were configured but not added to any Kotlin compilation:
 * androidAndroidTestRelease
 * androidTestFixtures
 * androidTestFixturesDebug
 * androidTestFixturesRelease
You can add a source set to a target's compilation by connecting it with the compilation's default source set using 'dependsOn'.
See <https://kotlinlang.org/docs/reference/building-mpp-with-gradle.html#connecting-source-sets>

> Task :commonizeNativeDistribution
> Task :commonize
> Task :runCommonizer
> Task :shared:cinteropLibtestIosArm64 UP-TO-DATE
> Task :shared:cinteropLibtestIosSimulatorArm64 UP-TO-DATE
> Task :shared:cinteropLibtestIosX64 UP-TO-DATE
> Task :shared:commonizeCInterop UP-TO-DATE
> Task :shared:copyCommonizeCInteropForIde UP-TO-DATE
> Task :shared:commonize
> Task :shared:transformIosMainCInteropDependenciesMetadataForIde UP-TO-DATE
> Task :shared:transformIosTestCInteropDependenciesMetadataForIde UP-TO-DATE
> Task :shared:runCommonizer
> Task :prepareKotlinBuildScriptModel UP-TO-DATE

BUILD SUCCESSFUL in 1s
8 actionable tasks: 1 executed, 7 up-to-date

[Landry Norris]

Looks like it’s trying to cinterop. You don’t see MyClass in the module?

[Kieren Searle]

Inside the aar?

[Landry Norris]

In the Kotlin/Native source set you want to use it in. Android uses JVM, so cinterop won’t be available without a JNI wrapper.

[Kieren Searle]

Is there anywhere I should look sorry? I can't use testlib or MyClass inside commonMain and I can't see anything under External Libraries in the project window

[Landry Norris]

What are your targets? Is it Android and iOS?

[Kieren Searle]

Yeah

[Landry Norris]

Android uses JVM, so you can’t directly use anything from C. For now, you’ll have to stick with your C library in the iOS source set. On Android, you’ll have to have a JVM implementation.

[Kieren Searle]

Okay. That's a shame about Android. But it makes sense as to why I can't add cinterops to the android() section of my gradle

[Landry Norris]

For Android, this means you have 2 options:
1. Write an equivalent of testlib in Kotlin
2. If you can’t do that, use the androidNative* source sets to create a JNI wrapper. You can look at https://github.com/LandryNorris/JniUtils/sample for an example of how to do this.

[Kieren Searle]

Oh! It's available in iOS module now! I must have been that warning! Thanks 🙂

[Landry Norris]

Is testlib something you control or something distributed by some company? If a company distributes an iOS binary, they almost certainly have an Android version using JVM.

[Kieren Searle]

The library we want to use is our internal lib. It's currently a c++ library with a jni wrapper for Android. We're looking to share it with iOS so was hoping KMM would allow us to do this easily

[Kieren Searle]

But it sounds like it's going to have to be something like the following?
iOS: C++ with a C wrapper and cinterop
Android: C++ with jni

[Landry Norris]

In this case, you can still use your JNI wrapper on Android. You can create a wrapper in commonMain with expect, then add actual implementations in your androidMain and iosMain

[Landry Norris]

If you already have C++ code for Android, you can use the JNI normally, then provide the JVM class as an actual for your common expect.

[Kieren Searle]

Great. I've already hooked that up for the Android side. Glad to know my instincts were correct there then

[Kieren Searle]

Thanks for all the help, I really appriciate it. It's been driving me crazy for the last few hours!

[Kieren Searle]

🍺