Anyone know how I might check if an API is availab...

# kotlin-native

[Matt Nelson]

Anyone know how I might check if an API is available, and then call that function?

Linux glibc 2.29+ supports the following function posix_spawn_file_actions_addchdir_fp, but because kotlin compiles against glibc < 2.29, it is not available from Kotlin.

I would like to call it if the gnu_get_libc_version returns 2.29 or greater, instead of having to fall back to doing fork/exec.

As far as I am aware, syscall cannot be used to call this function, so.

Any thoughts?

[Matt Nelson]

If I add it as an external fun in linuxMain source set, would that cause issues for people running this whereby the function is not available?

What if it is never called (because I check if glibc is > 2.28 before hand), would the external fun cause issue?

[Matt Nelson]

Is there any way to express a weak reference, like in rust?

This stackoverflow answer suggests to use a weak reference to it, and in fact that is what I am seeing in the Rust lib that I am looking at.

https://stackoverflow.com/questions/8814705/how-to-check-if-a-function-exists-in-c-c/43854514#43854514

[Matt Nelson]

https://doc.rust-lang.org/src/std/sys/unix/process/process_unix.rs.html#529

[ephemient]

dlsym should work in Kotlin/Native

[Matt Nelson]

Could I hold onto that OpaquePointer statically, or do I have to release it and perform dlsym every time the function is invoked?

e.g.

// Example.kt

package io.matthewnelson.kmp.process.internal

internal val P_SPAWN_ADDCHDIR_NP by lazy {
    val ptr = dlsym(null, "posix_spawn_file_actions_addchdir_np")
    if (ptr == null) return@lazy null
    CFunction<(actions: CValuesRef<posix_spawn_file_actions_t>, path: String) -> Int>(ptr.rawValue)
}

[ephemient]

you can hold onto it until the library is unloaded (and since it's libc it won't be unloaded)

[Matt Nelson]

We're close, but any ideas on how to pass a kotlin.String as a function parameter? Keep getting the following error

Error is type kotlin.string? is not supported here: doesn't correspond to any C type

[Matt Nelson]

Figured it out... a little unsafe cast and we good

private val ADDCHDIR_NP by lazy {
    val ptr = dlsym(null, "posix_spawn_file_actions_addchdir_np")
        ?: return@lazy null

    @Suppress("UNCHECKED_CAST")
    ptr as CPointer<CFunction<(CValuesRef<posix_spawn_file_actions_t>, CPointer<ByteVarOf<Byte>>) -> Int>>
}

@Throws(UnsupportedOperationException::class)
internal fun FileActions.try_spawn_file_actions_addchdir(directory: File, scope: MemScope): Int = with(scope) {
    ADDCHDIR_NP?.invoke(ref, chdir.path.cstr.ptr)
        ?: throw UnsupportedOperationException()
}