Where can I find an example of allocating an array of structs in order to pass a `CValuesRef` to the...

# kotlin-native

[jw]

Where can I find an example of allocating an array of structs in order to pass a CValuesRef to the leading item + item count to a C function? I've done with things like ByteArray where I can call refTo(0) but I don't see an explanation or example of how to do this with a struct type.

[mbonnin]

I've done this: https://github.com/apollographql/apollo-android/blob/aa5137599b957a9dcf2702eefbafa[…]appleMain/kotlin/com/apollographql/apollo3/mockserver/Socket.kt

[jw]

thanks. this will unblock me, i think, but it's far from ideal...

[jw]

i'm also getting structs back from a call to a C function so i have to copy their contents into each index which is not pretty

[jw]

val args = allocArray<Thing>(4)
    arg0.useContents {
      args[0].tag = tag
      args[0].u.int32 = u.int32
      args[0].u.float64 = u.float64
    }
    // etc.

[jw]

and sure I can put it behind a helper function, but it's still very awkward

[mbonnin]

But C cannot return array of structs by values, right? So it's all pointer based ?

[mbonnin]

void myfunction(struct thing * input_output_param, int *size)

[mbonnin]

Then you can use the same allocated array for both input + output maybe?

[Gavin Ray]

Maybe this can be helpful?
• https://github.com/Durun/vst3kotlin/blob/f1082278b6c763fbebb4318c83f7aecdf501e233/[…]n/kotlin/durun/vst3kotlin/pluginterface/vst/ParameterChanges.kt
• https://github.com/Durun/vst3kotlin/blob/f1082278b6c763fbebb4318c83f7aecdf501e233/vst3cwrapper/src/main/headers/cppinterface/memory.h#L16-L24
I have been using this as a reference for some of the lower-level KN stuff.
This person defined:

template<typename T>
T *allocArray(int length) {
    auto ptr = reinterpret_cast<T *>(malloc(sizeof(T) * length));
    if (ptr == nullptr) {
        fprintf(stderr, "Failed to allocate array of T (length=%d).\n", length);
        return nullptr;
    }
    return ptr;
}

And then uses it like you see in ParameterChanges.kt (or other places if you search for allocArray)

[Gavin Ray]

Disclaimer: I genuinely don't understand how some of the things in this repo work, but the person appears to know what they were doing

[jw]

I think I cracked it

internal inline operator fun <reified T : CVariable> CPointer<T>.set(index: Int, item: CValues<T>) {
  val offset = index * sizeOf<T>()
  item.place(interpretCPointer(rawValue + offset)!!)
}

internal inline fun <reified T : CVariable> NativePlacement.allocArrayOf(
  vararg items: CValues<T>,
): CArrayPointer<T> {
  val array = allocArray<T>(items.size)
  items.forEachIndexed { index, item ->
    array[index] = item
  }
  return array
}

[jw]

it's really the first part. the second is just convenience.

[Gavin Ray]

This is super useful -- thank you!