Your code is passing bytes by value, but bytes are themselves
pointers, so you are passing copies of the pointer, not copies of the
bytes. When you modify it, with `bytes-set!` you are modifying the
underlying structure. When you copy it with `subbytes` or
`bytes-copy`, you are making a new object with a new pointer.

--
Jay McCarthy
Associate Professor @ CS @ UMass Lowell
http://jeapostrophe.github.io
Vincit qui se vincit.

On Thu, Jun 6, 2019 at 12:00 PM David Storrs <david.sto...@gmail.com> wrote:
>
> My understanding is that Racket is call by value, not call by reference.  My 
> application will often be passing around large-ish byte strings; will they be 
> copied every time I pass them, or will the interpreter use copy-on-write?
>
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