request2size.rs

use std::env;
use std::mem::size_of;

const SIZE_SZ: usize = size_of::<usize>();

#[repr(C)]
struct malloc_chunk {
    mchunk_prev_size: usize,      /* Size of previous chunk (if free).  */
    mchunk_size: usize,           /* Size in bytes, including overhead. */
    fd: *mut malloc_chunk,                  /* double links -- used only if free. */
    bk: *mut malloc_chunk,
}

/* The smallest possible chunk */
const MIN_CHUNK_SIZE: usize = size_of::<malloc_chunk>();

const MALLOC_ALIGNMENT: usize = 2 * SIZE_SZ;
const MALLOC_ALIGN_MASK: usize = MALLOC_ALIGNMENT - 1;

const MINSIZE: usize = (MIN_CHUNK_SIZE + MALLOC_ALIGN_MASK) & MALLOC_ALIGN_MASK;

fn request2size(req: usize) -> usize {
    let size = req + SIZE_SZ + MALLOC_ALIGN_MASK;
    if size < MINSIZE {
        MINSIZE
    } else {
        size & !MALLOC_ALIGN_MASK
    }
}

fn main() {
    let args: Vec<_> = env::args().collect();
    if args.len() != 2 {
        println!("usage: {} <size>", args[0]);
        return;
    }
    let req = args[1].parse::<usize>().unwrap();
    let size = request2size(req);
    println!(
        "request:   {}\n\
         size:      {}",
        req, size,
    );
}