# FFI: Converting from \_pointer to byte string
**URL:** https://racket.discourse.group/t/ffi-converting-from-pointer-to-byte-string/1349
**Category:** Questions & Answers
**Tags:** question, ffi
**Created:** [October 4, 2022, 10:07pm UTC](https://racket.discourse.group/t/ffi-converting-from-pointer-to-byte-string/1349 "2022-10-04T22:07:54Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@jA\_cOp](https://racket.discourse.group/u/jA_cOp)
#### Post date: [October 4, 2022, 10:07pm UTC](https://racket.discourse.group/t/ffi-converting-from-pointer-to-byte-string/1349/1 "2022-10-04T22:07:55Z")
I have a C struct like so:
```scheme
(define-cstruct _foo ([data _pointer] [len _int32]))
```
I then get an instance of this struct as an output parameter of a C function. `data` points to a non-null-terminated array of bytes, of length `len`.
How can I convert `foo-data` to a byte string? Or maybe a vector?
What would be the most efficient way to receive this value in Racket FFI?
Thanks!
---
### Author:  [@samth](https://racket.discourse.group/u/samth)
#### Post date: [October 5, 2022, 7:09pm UTC](https://racket.discourse.group/t/ffi-converting-from-pointer-to-byte-string/1349/2 "2022-10-05T19:09:50Z")
Unfortunately, you can't create a byte string without copying. I think `_array` and `array-ref` is probably the simplest option, although I don't know what would be most efficient in your situation.
---
### Author:  [@jA\_cOp](https://racket.discourse.group/u/jA_cOp)
#### Post date: [October 5, 2022, 8:53pm UTC](https://racket.discourse.group/t/ffi-converting-from-pointer-to-byte-string/1349/3 "2022-10-05T20:53:12Z")
Thanks for the suggestion! I'm looking at the docs for `_array`, and it kinda looks like it's for arrays with known length? In my case, the length is only known at runtime, and can vary from call to call - the callee fills in the `len` field of the struct to indicate the length of the array pointed-to by the `data` field. Is this possible to handle with `_array`?
---
### Author:  [@samth](https://racket.discourse.group/u/samth)
#### Post date: [October 5, 2022, 8:57pm UTC](https://racket.discourse.group/t/ffi-converting-from-pointer-to-byte-string/1349/4 "2022-10-05T20:57:48Z")
Yes, I think you want something like this (untested):
```scheme
(define (convert f)
(ptr-ref (foo-data f) (_array _int (foo-len f)) 0))
```
---
### Author:  [@jA\_cOp](https://racket.discourse.group/u/jA_cOp)
#### Post date: [October 5, 2022, 9:29pm UTC](https://racket.discourse.group/t/ffi-converting-from-pointer-to-byte-string/1349/5 "2022-10-05T21:29:03Z")
Hm, OK, that gives me an `_array`! However, there doesn't seem to be much I can do with this array... I'm trying to deserialize a stream of data, and sometimes the array will just contain a string, other times multiple strings, each prefixed with their length (as a 32-bit integer), and so on.
The other day you told me about `integer-bytes->integer`, and I also found `bytes->string/utf-8`. I'd rather not have to reimplement these in terms of `_array`, especially not if I'd have to write them in C to get decent performance.
It seems like a byte string would be a good intermediate representation. How can I convert from `_array` to byte string? I see there's a `sequence->bytes` in an external package, and there's an `in-array` for creating a sequence over an array, but not only do I want to avoid external dependencies, but this seems really inefficient when it should just be a cast or at worst, maybe a memcpy.
---
### Author:  [@samth](https://racket.discourse.group/u/samth)
#### Post date: [October 5, 2022, 10:19pm UTC](https://racket.discourse.group/t/ffi-converting-from-pointer-to-byte-string/1349/6 "2022-10-05T22:19:47Z")
To get a byte string you might just use ptr-ref and a loop that copies into a byte string. Unfortunately there's no way to convert to a byte string without copying because of how they're represented internally. I think there's also not a way to copy the bytes in bulk but that seems like something that could be added.
---
### Author:  [@LiberalArtist](https://racket.discourse.group/u/LiberalArtist)
#### Post date: [October 6, 2022, 12:11am UTC](https://racket.discourse.group/t/ffi-converting-from-pointer-to-byte-string/1349/7 "2022-10-06T00:11:54Z")
> [@samth](#):
>
> I think there's also not a way to copy the bytes in bulk
I would do it like this:
```scheme
#lang racket
(require ffi/unsafe)
(define (convert data len)
(cast data _pointer (_bytes o len)))
(convert #"abcdefghijklmn" 7) ;-> #"abcdefg"
```
(It might be surprising that `cast` _does_ create a copy!)
You can also use Racket functions like `memcpy`.
---
### Author:  [@mflatt](https://racket.discourse.group/u/mflatt)
#### Post date: [October 6, 2022, 3:57am UTC](https://racket.discourse.group/t/ffi-converting-from-pointer-to-byte-string/1349/8 "2022-10-06T03:57:51Z")
For what it's worth, I would stick to working with the pointer instead of converting to a byte string, at least for most cases and where performance is a concern. For example, I'd use `ptr-ref` with `_int` (or another suitable integer type) instead of `integer-bytes->integer`. But there are cases where it's more convenient to work with byte strings.
---
### Author:  [@jA\_cOp](https://racket.discourse.group/u/jA_cOp)
#### Post date: [October 6, 2022, 12:59pm UTC](https://racket.discourse.group/t/ffi-converting-from-pointer-to-byte-string/1349/9 "2022-10-06T12:59:23Z")
Thanks a lot everyone for the helpful responses! I'll play around with `cast`, `ptr-ref` and the likes and see how it goes 🙂
(Looks like I can only mark one answer as the solution, so it's a bit arbitrary, sorry!)