Hey, can anyone point me to some info on how to pass variadic args to a c function?

# kotlin-native

[adk]

Hey, can anyone point me to some info on how to pass variadic args to a c function?

[Kris Wong]

look at the generated kotlin binding. what type is the parameter?

[adk]

the binding says:

vararg variadicArguments: kotlin.Any?

[adk]

I'm trying to wrap calls to snprintf in a kotlin actual function

[Kris Wong]

then you should be able to call it like any variadic function

[adk]

I can call it with hard-coded values, but I'm having difficulty figuring out how to wrap the kotlin

vararg args: Any?

[adk]

that comes from the wrapping function

[adk]

alternatively, any suggestions on how to emulate java's String.format() in K/N would be most welcome! :-)

[adk]

Hmmm, I appear to have found the answer to my question - you can't do that: http://c-faq.com/varargs/handoff.html

[napperley]

Have you tried using the printf function ( https://pubs.opengroup.org/onlinepubs/9699919799/functions/fprintf.html ) from the POSIX library?

[adk]

Yeah, so the problem is that I'm trying to forward from a vararg kotlin function (my expect function) to a C function that takes variadic args. Turns out that this is impossible.

[adk]

My google searches have turned up a fair bit of into on how impossible this is: http://c-faq.com/varargs/wacky.html

[adk]

So it appears that a String.format function for kotlin common is not as simple as just calling printf and living with the differences.

[Kris Wong]

so I just added a random call to printf in my iosMain source set, and the IDE doesn't seem to have any problem with it

[adk]

It is the forwarding of the variadic args in C that is the hard bit

[adk]

calling printf with a fixed number of args is straightforward

[adk]

also, in IOS, you're using objective-c, aren't you?

[adk]

(I know nothing about IOS)

[Kris Wong]

fun somefun(vararg args: Any) {
    printf("", args)
}

[adk]

I believe that passes the array as a single arg

[adk]

When you execute the code, it will come back with this:

type kotlin.Array<out kotlin.Any?>  is not supported here: doesn't correspond to any C type

[adk]

My code it running on linux, btw - although the snprintf function that I'm using is posix, so exists most places

[Kris Wong]

i see what you mean. I am certain there's a way to do this, but i don't have time to dig through the cinterop types ATM.

[adk]

Don't worry - I'm pretty sure there isn't actually a way; it's a limitation of C. Thanks for your help! 🙂

[adk]

Others have solve this in the C world with things like dyncall and libffi (which used to be part of K/N) but these are all non-portable.

[adk]

Who new something so simple could be so difficult?