Can anyone confirm if `char ` in C is represented as `CPoint

Question

Can anyone confirm if `char ` in C is represented as `CPointerVar lt Char gt ` in kotlin

vbsteven · Answer

On Linux I ve seen it mapped mostly as `CPointerVar lt ByteVar gt `

vbsteven · Answer

Assuming you are working on GTK I ve used the following for most of the `char ` related conversions ``` Convert a Kotlin List lt String gt into its native equivalent public fun List lt String gt toCStringList scope MemScope CArrayPointer lt CPointerVarOf lt CPointer lt ByteVar gt gt gt = with scope return allocArrayOf this toCStringList map it cstr getPointer this + null Convert a native char into a Kotlin List lt String gt public fun CArrayPointer lt CPointerVarOf lt CPointer lt ByteVar gt gt gt toKStringList List lt String gt = buildList var i = 0 while this toKStringList i = null add this toKStringList i toKString i++ ``` PoC code this won t work for all methods As sometimes the `char ` is null terminated and sometimes it has a fixed size

Big Chungus · Answer

Thanks

Big Chungus · Answer

Do type in c always mean an array or are there other ways to represent an array in C

vbsteven · Answer

not always an array could also be a pointer to pointer

vbsteven · Answer

in the GTK case you ll see ` ` for GError out pointers where you want to pass in a pointer to a `GError` pointer so GTK can fill it with a pointer to the error when it encounters an error

Big Chungus · Answer

Are you saying there is no standard way to declare an array in C but rather just type convention

vbsteven · Answer

I m not super fluent in C so there might be other ways In the case of string arrays here the `char ` means a pointer to the first element in an array of `char `

vbsteven · Answer

and c strings if I m not mistaken are also arrays and `char ` means a pointer to the first element in an array of `char` And C strings are an array of `char` terminated by ` 0`

Big Chungus · Answer

Odd why not null terminated

vbsteven · Answer

so for the `char ` case for GTK null terminated string arrays in memory it will look like ` ptr ptr ptr null ` where each `ptr` points to ` char char char 0 ` something like that

Big Chungus · Answer

Thanks that s super clear now I remember having to do some pointer voodoo like this back at the university and not liking it very much grinning

vbsteven · Answer

in the case of C strings the ` 0` is a null byte because char is typically byte In the case of `char ` since it s an array of pointers the final element is a `null` pointer

Big Chungus · Answer

Ah makes sense now

vbsteven · Answer

also cinterop handles the C strings for you mostly and you can use `toKstring ` to convert it to a Kotlin String and you can do `myKotlinString cstr` to get a `char ` pointer to the c String for your kotlin string

Big Chungus · Answer

Yeah I m aware of those Just trying to understand how it all ties together under the hood

Landry Norris · Answer

C does let you declare `char strings size ` which is explicitly an array I believe Kotlin will read this as a ~CArrayPointer~ turns out it s CValuesRef CArrayPointer comes from allocArray

Landry Norris · Answer

For a real world example the FFmpeg sws scale sees ` uint8 t const srcSlice ` as `srcSlice CValuesRef lt CPointerVar lt uint8 tVar gt gt ` in Kotlin

ephemient · Answer

C arrays decay to pointers across most boundaries

Landry Norris · Answer

Also small pedantic note on 0 vs null NULL in C Both are actually the same 0 is just a nice way of saying 0 and so is NULL C doesn t really have a strong concept of runtime types like Kotlin or Java You could set `char c = char NULL` just as easily In the same way `char words` is essentially equivalent to `char words ` C also lets you do interesting things like `2 words = This works ` or ` words + 3 = This works too ` C has been around for a long time so there s a lot of variations on almost anything Some lists like strings use a null terminator and others like the srcSlice I mentioned earlier will require you to calculate the size the calculation for srcSlice length is not fun or know it

ephemient · Answer

in theory standard C doesn t mandate that NULL is represented by 0 only that it is integer convertible to from 0 but it does mandate that a byte with all bits set to 0 is interpreted as a null character

ephemient · Answer

<https c faq com null machexamp html> has some specific examples of historical architectures where NULL was not physically zero

Landry Norris · Answer

C is just one of those languages that gets more terrifying as you look further into it No wonder one of my professors in college refused to acknowledge that anything past ANSI C exists