can someone help me out with the `CValue` concept and passing structs by reference to C functions?

# kotlin-native

[czuckie]

can someone help me out with the CValue concept and passing structs by reference to C functions?

[czuckie]

when I have a C function like

void do_something(struct_t *boop)

I can pass in the result of cValue<struct_t>() into the function, but any modifications done to the struct are not carried over

If I use memScoped and pass the ptr value in, the same happens.

If I dynamically allocated with alloc<struct_t>() THEN it works

[czuckie]

I guess what I want is to figure out what the &structVar equivalent would be in kotlin

[czuckie]

https://kotlinlang.org/docs/tutorials/native/mapping-struct-union-types-from-c.html has been my bible, but it doesn't explain how to do it without dynamic allocation

[Dominaezzz]

If I use memScoped and pass the ptr value in, the same happens.

Should have worked.

[Dominaezzz]

That's the de facto way.

[Dominaezzz]

You have to call alloc<...> in a memScoped block.

[czuckie]

so! I've gone with something I hope isn't filthy

[czuckie]

class MyStructWrapper {
   private val arena = Arena()
   private val myStruct = arena.alloc<my_struct_t>().ptr

   fun doSomething() = c_api_do_something(myStruct)
}

[czuckie]

it works but not sure if I'm just pouring memory out everywhere

[czuckie]

my understanding is the arena will deallocate when the containing object is deallocted

[Dominaezzz]

You have to manually free the arena.

[Dominaezzz]

memScoped {
   val structVar = alloc<struct_t>()
   doSomething(structVar.ptr)
   // Do stuff with structVar
}
// structVar is freed.

[czuckie]

hmm, so what would be the recommended way to have a dynamically allocated object be a field member of something that wasn't

[czuckie]

or, 😅 if you were going to do something similar to the code snippet I posted, how would you go about it?

[Dominaezzz]

Ideally, you would just copy it into a Kotlin object.

[Dominaezzz]

Are you going to be rewriting the value or just write once and use many?

[czuckie]

rewriting unfortunately, I think that's where the problem is

[Dominaezzz]

Hmm, I guess you can use nativeHeap.

[Dominaezzz]

You only need one struct right?

[czuckie]

yeah

[czuckie]

how would I go about using nativeHeap? (cheers for your help dude, I can't imagine what kind of lemon soaked razor blades you've crawled over to get to this point!)

[Dominaezzz]

Haha, yeah it's been fun.

[Dominaezzz]

Then nativeHeap is the way to go. Just don't forget to call free when you're done with the class. Have a Closeable interface or something.

[Dominaezzz]

val struct = nativeHeap.alloc<struct_t>

[Dominaezzz]

nativeHeap.free(struct) when you're done.

[czuckie]

oki doki 💪 I'll see if I can't work that in some how

[czuckie]

as it happens, I'm having a little play about with wrapping Cairo - https://cairographics.org/samples the object I'm messing about with is a cairo_matrix_t. All my unit tests were generating the correct output per-each sample, except the image pattern one

[czuckie]

I've wrapped the cairo_matrix_t variable in a CairoMatrix class, but I couldn't figure out how get things working without dynamic allocation which is doing my nut in! But it's only a play thing for me right now, so I'll persevere... I haven't even gotten to the lemon soaked razors yet 😂

[Dominaezzz]

Ah I see. Representing stuff like that in Kotlin can be a bit awkward. Especially when there's stack allocation involved.

[Dominaezzz]

I'm guessing you have a CairoSurface class?

[czuckie]

I do, but fortunately all the allocation is handled by the C API (but I do have a destroy method)

[Dominaezzz]

I guess you'll be giving all your objects destroy methods.

[czuckie]

I miss garbage collection all of a sudden!

[czuckie]

so stack allocation isn't directly mappable to kotlin?

[Dominaezzz]

Not really. You can always make your own stack.

[Dominaezzz]

Allocate a big blob and the sub allocate from it when you need to.

[czuckie]

cheers dude, this has been really useful- I'm kind of feeling my way through things at the minute, not everything has been super intuitive, but I am seeing results and making progress. Understanding the mechanics more and things to look out for is going to be massively helpful

[Dominaezzz]

No problem

[svyatoslav.scherbina]

Passing CValue for pointer parameter creates a temporary copy of it, because CValue is immutable.
Taking .ptr on CValue does the same for the same reason.

I’ve wrapped the cairo_matrix_t variable in a CairoMatrix class, but I couldn’t figure out how get things working without dynamic allocation

Does anything prevent you from having val value: CValue<cairo_matrix_t> in your CairoMatrix class?

[Dominaezzz]

He needs to be able to rewrite the value and CValue<... > is immutable.

[svyatoslav.scherbina]

Then mutable property (var) could be used.

[Dominaezzz]

How are `CValue`s stored? Are they like value types? Or is does it use nativeHeap underneath and free the memory when garbage collected?

[svyatoslav.scherbina]

Does it matter for this purpose?

[Dominaezzz]

Kind of, it would be a shame to have to call nativeHeap.alloc<...> every time the value needs changing (or reading) when a single allocation would do.

[svyatoslav.scherbina]

CValue is stored in the heap. You can’t reuse the same memory for a new value.