On 10/12/14 18:08, Matt wrote:
Sorry if this has been covered before, and also that I'm a complete
noob, but I'm thinking about trying a first project in Rust, and
trying to learn enough to get started.
My plan is for an on-disk key-value store. I'm going to end up
writing arrays of numbers to disk, and then needing to efficiently
read them out of a mmap-ed file. So I'm wondering how in Rust you
efficiently/zero-copy-ly take some slice of a read-only mmap and
treat it as e.g. a vector of ints?
I see Vec.from_raw_parts() and Vec.from_raw_buf(), but I don't really
understand the difference between them, and also they seem like they
just give you a vector of the pointer's type, so I don't know how you
use them convert the u8s you get from MemoryMap.data() into a vector
of a different type, e.g. 32 bit ints.
It seems like there should be a higher level API for this kind of
thing, where "casting" a slice of a read-only memory buffer into an
immutable vector is not an unsafe operation (I mean, you can do that
in Python ;) Either I don't see it in the docs, or it doesn't exist
yet; just wondering which :)
Vec is probably not what you want, as it owns its memory and frees it
when going out of scope. Use a slice instead. Assuming that’s what you
get from mmap, you can create a slice from a raw pointer and a length:
http://doc.rust-lang.org/std/slice/fn.from_raw_mut_buf.html
http://doc.rust-lang.org/std/slice/fn.from_raw_buf.html
Something like (untested):
use std::slice::from_raw_mut_buf;
use std::mem::size_of;
use libc::c_void;
// Whatever you do for mmap:
let (void_pointer, byte_length): (*mut c_void, uint) = ...;
let i32_pointer: *mut i32 = void_pointer as *mut i32;
let i32_length: uint = byte_length / size_of::<i32>();
let slice: &mut [i32] = from_raw_mut_buf(&i32_pointer, i32_length);
Then you can index into `slice`. You’re responsible for making sure that
the mmap lives at least as long as `slice` does. (Its lifetime it tied
to that of `i32_pointer`.) Most type annotations can probably be
inferred, I added them to show what’s going on.
--
Simon Sapin
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