crosvm/data_model/src/flexible_array.rs

// Copyright 2019 The Chromium OS Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

//! A wrapper for structures that contain flexible arrays.

use std::mem::size_of;

// Returns a `Vec<T>` with a size in bytes at least as large as `size_in_bytes`.
fn vec_with_size_in_bytes<T: Default>(size_in_bytes: usize) -> Vec<T> {
    let rounded_size = (size_in_bytes + size_of::<T>() - 1) / size_of::<T>();
    let mut v = Vec::with_capacity(rounded_size);
    for _ in 0..rounded_size {
        v.push(T::default())
    }
    v
}

/// The kernel API has many structs that resemble the following `Foo` structure:
///
/// ```ignore
/// #[repr(C)]
/// struct Foo {
///    some_data: u32,
///    entries: __IncompleteArrayField<__u32>,
/// }
/// ```
///
/// In order to allocate such a structure, `size_of::<Foo>()` would be too small because it would
/// not include any space for `entries`. To make the allocation large enough while still being
/// aligned for `Foo`, a `Vec<Foo>` is created. Only the first element of `Vec<Foo>` would actually
/// be used as a `Foo`. The remaining memory in the `Vec<Foo>` is for `entries`, which must be
/// contiguous with `Foo`. This function is used to make the `Vec<Foo>` with enough space for
/// `count` entries.
pub fn vec_with_array_field<T: Default, F>(count: usize) -> Vec<T> {
    let element_space = count * size_of::<F>();
    let vec_size_bytes = size_of::<T>() + element_space;
    vec_with_size_in_bytes(vec_size_bytes)
}