CODEWITHSUNDEEP
rust
nlta
nlta

Reputation: 1914

How to read file into a pointer / raw vec?

I'm using a copy of the stdlib's raw vec to build my own data structure. I'd like to reaad a chunk of a file directly into my data structure (no extra copies). RawVec has a *const u8 as it's underlying storage and I'd like to read the file directly into that.

// Goal:
// Takes a file, a pointer to read bytes into, a number of free bytes @ that pointer
// and returns the number of bytes read
fn read_into_ptr(file: &mut File, ptr: *mut u8, free_space: usize) -> usize {
    // read up to free_space bytes into ptr
    todo!()
}

// What I have now requires an extra copy. First I read into my read buffer 
// Then move copy into my destination where I actually want to data.
// How can I remove this extra copy?
fn read_into_ptr(file: &mut File, ptr: *mut u8, read_buf: &mut[u8; 4096]) -> usize {
    let num_bytes = file.read(read_buf).unwrap();
    unsafe { 
      ptr::copy_nonoverlapping(...)
    }
    num_bytes
}
``

Upvotes: 1

Views: 545

Answers (1)

Chayim Friedman
Chayim Friedman

Reputation: 70970

Create a slice from the pointer, and read into it:

let slice = unsafe { std::slice::from_raw_parts_mut(ptr, free_space) };
file.read(slice).unwrap()

Upvotes: 2

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