I'm writing a Kotlin/Native application targeted for Windows, Linux and Mac and I need to call platform.posix._popen on Windows and platform.posix.popen on the other.
If I use a simple conditional (if (Platform.osFamily == OsFamily.WINDOWS)), I get a compile error.
There is no such thing as #ifdef in Kotlin, but you can use the expect/actual mechanism to achieve what you need.
In common code, define an expect fun without a body, and then define an actual fun implementation separately in linux/windows/macOS source sets.
You can find more details about how to do this in the Kotlin multiplatform documentation.