Could anyone please provide an example of how to c...

# kotlin-native

[andreasmattsson]

Could anyone please provide an example of how to call a c-function that takes a va_list argument from Kotlin Native? e.g. vsnprintf

[Big Chungus]

You mean an array?

[andreasmattsson]

yeah, calling vsnprintf with an array of args

[Big Chungus]

Something like this:

fun run(args: Array<String>? = null) = memScoped {
    vsnprintf(args?.size ?: 0, args?.toCStringArray(this))
  }

[andreasmattsson]

Trying to pass the result of toCStringArray() where va_list is expected gives me this:

e: /Users/am/projects/sdg12-deeds-ios/deedster-common/lib-common/src/iOSMain/kotlin/se/sdg12/deedster/util/sprintf.kt: (21, 25): Type mismatch: inferred type is CPointer<CPointerVar<ByteVar /* = ByteVarOf<Byte> */> /* = CPointerVarOf<CPointer<ByteVarOf<Byte>>> */> but va_list? /* = CPointer<__va_list_tag>? */ was expected

[Big Chungus]

Hmm, try adding .reinterpret() after it

[andreasmattsson]

hmmm, some sort of crash unfortunately.
Thanks for your help, gotta run unfortunately will revisit this later

[Dominaezzz]

I was researching this recently and I don't think it's possible. The only reason the va_list function is exposed at all, is so you can pass null as the argument.

[Dominaezzz]

If you have no other alternative, you can use cinterop to make a ... wrapper for it. Then you can call it through that.

[Dominaezzz]

https://github.com/JetBrains/kotlin-native/issues/740

https://github.com/JetBrains/kotlin-native/issues/1834

[andreasmattsson]

Yeah I found those. Don't really think that solves my problem though. As I understand it I'm only allowed to call the vararg version of c methods with *àrrayOf(...), which means I need to know the amount of args ahead of time. If I try to call with *runtimeConstructedAssignedArrayVariable it failes to compile.

[Dominaezzz]

Yeah, I have the same issue in a project of mine. Variadic functions are also quite limited in C it seems.

[Dominaezzz]

You also need to know the type of the arguments ahead of time.

[andreasmattsson]

Okay, so maybe this is a dead end... Did you find some workaround?

Basically I'm just trying to get a cross-platform equivalent of String.format() / snprintf where I don't need to know types and amount of args beforehand.

[Dominaezzz]

Same issue here. Best to just implement sprintf in pure Kotlin. I'm surprised no one has done it yet (AFAIA).

[andreasmattsson]

okay, thanks

[msink]

Weird. I see in `platform.posix`:

@CCall("knifunptr_platform_posix672_sprintf")
external fun sprintf(__s: CValuesRef<ByteVar>?, @CCall.CString __format: String?, vararg variadicArguments: Any?): Int

@CCall("knifunptr_platform_posix675_vsprintf")
external fun vsprintf(__s: CValuesRef<ByteVar>?, @CCall.CString __format: String?, __arg: __gnuc_va_list?): Int

[Dominaezzz]

You can't call it without knowing the type and count of the args at compile time. And there's no way to provide a compile time wrapper, with an inline function of something.

[msink]

import platform.posix.printf
fun main() {
    printf("Hello %s%c", "World", '!'.toInt())
}

Hello World!

[andreasmattsson]

Yeah that works @msink, but now try it where the args ("World" and "!") are from an array.

[Dominaezzz]

Like this. https://github.com/Dominaezzz/kotlin-imgui/issues/1

[napperley]

Variadic arguments are definitely, a "*pain* in the void" 😆 . Using the GTK libraries, and found in a few cases if there is a corresponding C function that takes a single item (instead of variadic arguments) then it is easier to use that function instead.

[msink]

Hm, I think in K/N stdlib should be (currently missed) variant of memScoped function with vararg parameter, something like

fun memScoped(va: Array<>, block: (va_list) -> Unit)