Hey all. So I'm a little confused about c strings ...

# kotlin-native

[Nico Buescher]

Hey all. So I'm a little confused about c strings in relation to struct members. If I have a c struct defined as such:

typedef struct {
    const char *str;
} Example;

What is the intended way to store a string literal in str from Kotlin?
Using "String".cstr doesn't work since str is a CPointer<ByteVar>, and String.cstr is a CValues<ByteVar>, and when I use memScoped { ... } with getPointer(memScope), the CString is obviously freed as soon as the scope is finished, leaving me with an empty string in the struct member.

[Dico]

You need to copy the characters over in the memScoped block.

[olonho]

Use longer living placements, according to your string lifetime, like foo.cstr.getPointer(nativeHeap), as documented in https://github.com/JetBrains/kotlin-native/blob/master/INTEROP.md#working-with-the-strings

[Nico Buescher]

@Dico Copying the characters over doesn't work since the struct only has room for a pointer to the char array. In C, this isn't a problem since string literals are allocated in shared read-only memory, but this doesn't work in kotlin.

[Nico Buescher]

@olonho foo.cstr.getPointer(nativeHeap) doesn't work since getPointer(...) expects an AutoFreeScope, and nativeHeap is only a NativeFreeablePlacement. Same with placeTo. This is with kotlin 1.3.21. The docs seem to be a bit out of date.

[Dico]

Allocate memory in nativeHeap, copy the characters, set the ptr?

[Nico Buescher]

Oh, yeah, that works. But as per the docs, as olonho mentioned, that didn't seem like the intended solution. But it looks like for now it's the only one?

[olonho]

Indeed docs somewhat outdated, better use smth like

val a = Arena()
val ptr = "Hello".cstr.getPointer(a)
//  a.clear() when pointer no longer needed

[Nico Buescher]

That works perfectly, thanks for the help!