# C integration: Accessing lists (primitive types) via C API
**URL:** https://forum.kx.com/t/c-integration-accessing-lists-primitive-types-via-c-api/11035
**Category:** Community Support
**Tags:** kdb-and-q
**Created:** [May 29, 2016, 12:41pm UTC](https://forum.kx.com/t/c-integration-accessing-lists-primitive-types-via-c-api/11035 "2016-05-29T12:41:00Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Noob](https://forum.kx.com/u/Noob)
#### Post date: [May 29, 2016, 12:41pm UTC](https://forum.kx.com/t/c-integration-accessing-lists-primitive-types-via-c-api/11035/1 "2016-05-29T12:41:00Z")
Apologies in advance, I’m sure this was answered in [http://code.kx.com/wiki/Cookbook/InterfacingWithC](http://code.kx.com/wiki/Cookbook/InterfacingWithC) but I just didn’t get it.
I’m trying to figure out how to pass primitive arrays (most commonly, byte arrays) back and forth across c/q. Specifically, I will to call an external library that wants to operate on lists as C-arrays.
To that end, a few obvious things I haven’t figured out:
How do I query the length of a list?
How are these lists stored in kdb? I’d thought someone told me that kdb optimizes access by internally representing primitive lists as C-style arrays and accessing via pointer arithmetic.
Thanks!
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### Author:  [@effbiae](https://forum.kx.com/u/effbiae)
#### Post date: [May 30, 2016, 1:33am UTC](https://forum.kx.com/t/c-integration-accessing-lists-primitive-types-via-c-api/11035/2 "2016-05-30T01:33:00Z")
Dear Mr Noob,
\> I’m trying to figure out how to pass primitive arrays (most commonly, byte arrays) back and forth across c/q. Specifically, I will to call an external library that wants to operate on lists as C-arrays.
How about we define a function that returns a reverse of a byte array:
K reverse(K x)
{K r; //the return value
if(x-\>t!=KG)return krr(“type”); //check type is array of bytes
r=ktn(KG,x-\>n); //create a new array of bytes with length the same as x
DO(x-\>n,kG(r)[i]=kG(x)[(n-1)-i]) //DO defined in k.h, kG is accessor for byte array
return r;
}
\> How do I query the length of a list?
list-\>n
\> How are these lists stored in kdb? I’d thought someone told me that kdb optimizes access by internally representing primitive lists as C-style arrays and accessing via pointer arithmetic.
yes - see the struct defined in k.h. the n field is the number of elements. the G0 element is the start of the array.
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### Author:  [@Noob](https://forum.kx.com/u/Noob)
#### Post date: [May 30, 2016, 1:21pm UTC](https://forum.kx.com/t/c-integration-accessing-lists-primitive-types-via-c-api/11035/3 "2016-05-30T13:21:00Z")
On Sunday, May 29, 2016 at 8:34:13 PM UTC-4, effbiae wrote:
> How about we define a function that returns a reverse of a byte array:
>
> K reverse(K x)
>
> {K r; //the return value
> if(x-\>t!=KG)return krr(“type”); //check type is array of bytes
>
> r=ktn(KG,x-\>n); //create a new array of bytes with length the same as x
>
> DO(x-\>n,kG(r)[i]=kG(x)[(n-1)-i]) //DO defined in k.h, kG is accessor for byte array
>
> return r;
>
> }
>
> \> How are these lists stored in kdb? I’d thought someone told me that kdb optimizes access by internally representing primitive lists as C-style arrays and accessing via pointer arithmetic.
>
> yes - see the struct defined in k.h. the n field is the number of elements. the G0 element is the start of the array.
Thanks, that example helps quite a bit and I like the DO definition. The internal struct wrapping fields n and G0 threw me off as I didn’t notice it was anonymous.
So it looks like kG(x) returns the pointer x-\>G0. If I have to pass that buffer of data to a C function as a void\*, can I safely pass kG(x) as long as the function will not modify the buffer?
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### Author:  [@effbiae](https://forum.kx.com/u/effbiae)
#### Post date: [May 31, 2016, 1:58am UTC](https://forum.kx.com/t/c-integration-accessing-lists-primitive-types-via-c-api/11035/4 "2016-05-31T01:58:00Z")
\> So it looks like kG(x) returns the pointer x-\>G0. If I have to pass that buffer of data to a C function as a void\*, can I safely pass kG(x) as long as the function will not modify the buffer?
Yes. It’s convenient that any simple list (byte, char, int, ..) in kdb+ is implemented as a simple array in x-\>G0. You can use or pass kG(x) as you like. You can even reduce x-\>n with expected results (and memory management will still work).
As you can see from the example, you can modify a K (after ktn), so you can pass a K object to a function that modifies it’s contents, but be careful when modifying any arguments passed to your K function - K’s policy is “pass by value” (copy on write).