Hello, I'm trying to compile a c++ static lib and ...

# multiplatform

[Kieren Searle]

Hello, I'm trying to compile a c++ static lib and link it to my cinterops compilation. I've managed to compile the c++ lib using cpp-library, but I can't think of how to link the output to the inputs of cinterops. Anyone have any ideas on how to do this, or if I'm doing something completely wrong?

[Kieren Searle]

For my cpp-library, here it the build.gradle file:

plugins {
    id "cpp-library"
}

library {
    linkage = Linkage.STATIC
}

This builds with and outputs libmylibrary.a

[Kieren Searle]

I can include this library in my def file by setting staticLibraries and libraryPaths accordingly

[Kieren Searle]

But, how do I set up my multiplatform gradle to compile my cpp-library before running the cinterops?

[Kieren Searle]

Or.. is there a better way to do this?

[Big Chungus]

Kotlin/Native does not support cpp interop yet. There's an open issue for it, but it's postponed in favour of KMM

[Big Chungus]

Wait, did you actually get it to work?

[Big Chungus]

For gradle stuff you can add cppCompile task as a dependency to cinterops task

[Kieren Searle]

No, I wish I got cpp interop working 😛 I'm using cinterop with c headers

[Big Chungus]

Makes sense. Thought I've missed cpp interop announcement 😀

[Kieren Searle]

Haha, sorry

[Big Chungus]

Here's another gradle setup example for jni case petuska.dev/fake-kamera
Native binary is compiled via kotlin native (so an equivalent to your cpp module) and added to jvm consumer (so your kotlin native module)

Ignore the source code.

[Kieren Searle]

Thanks, I'll take a look and let you know how I get on ..

[Big Chungus]

Btw, all of this is just to help with your gradle setup. This will not magically make cpp interop work with kotlin 😀

[Kieren Searle]

Yeah, I appreciate that. Nothing about any of this project has been simple so far. One step at a time!

[Landry Norris]

Depending on how often your cpp code changes, you could write a separate gradle task that compiles it and moves the binary somewhere it can be shared. I do this on a project at work to avoid the cost of compiling the native code.

[Kieren Searle]

I've been considering that as this code is due to be replaced with Kotlin later anyway. How are you compiling and copying? Do you have a sharable example of that?

[Kieren Searle]

Ah, you said you're using a gradle task. I don't suppose you could share that?

[Landry Norris]

tasks {
    //We want to create a task that can copy our resulting Android library into the analyzer-android module.
    val prepareAndroidNdkSo by creating {
        dependsOn(build)
        val debugArm32SoFolder = File(buildDir, "bin/androidNativeArm32/debugShared")
        val jniArm32Folder = File(projectDir, "../a-folder/src/androidMain/jniLibs/armeabi-v7a")
        val debugArm64SoFolder = File(buildDir, "bin/androidNativeArm64/debugShared")
        val jniArm64Folder = File(projectDir, "../a-folder/src/androidMain/jniLibs/arm64-v8a")

        val debugX86SoFolder = File(buildDir, "bin/androidNativeX86/debugShared")
        val jniX86Folder = File(projectDir, "../a-folder/src/androidMain/jniLibs/x86")
        val debugX64SoFolder = File(buildDir, "bin/androidNativeX64/debugShared")
        val jniX64Folder = File(projectDir, "../a-folder/src/androidMain/jniLibs/x86_64")

        doLast {
            copy {
                from(debugArm32SoFolder)
                into(jniArm32Folder)
                include("*.so")
            }
            copy {
                from(debugArm64SoFolder)
                into(jniArm64Folder)
                include("*.so")
            }
            copy {
                from(debugX86SoFolder)
                into(jniX86Folder)
                include("*.so")
            }
            copy {
                from(debugX64SoFolder)
                into(jniX64Folder)
                include("*.so")
            }
        }
    }
}

[Kieren Searle]

Amazing, thank you 😄

[Landry Norris]

I just realized I’ve been copying debug builds over.

[Landry Norris]

There are other improvements you could make, like setting dependsOn to just the tasks you want, not a full build

[Kieren Searle]

Nice. Good to review old code every now and again

[Kieren Searle]

Yeah, I was just thinking about the dependsOn part. Looks quite useful

[Landry Norris]

You can do some more gradle magic to make this happen at various parts of the build process, but I have it set up where I would have to run this manually.

[Kieren Searle]

That was my thought. I'm not sure how practical it is though. It's a tiny lib so there might not be much overhead

[Kieren Searle]

Hello again. Just to close this one out I got it to work with the following:

• Stopped using cpp-library
• Created a CMakeLists.txt to compile the project instead
• Created a task in my gradle that runs 2 exec steps
a. The first uses cmake to generate an Xcode project: cmake -GXcode . -bbuild
b. The second then uses xcodebuild to build it
• I call this task from my script in Xcode which builds my KMM module
• I can then use the build lib in cinterops
It's not the most efficient, but it works and I can improve on it in the future.

[Kieren Searle]

Thanks for all the pointers!