Skip to content

Unlocking Complex Layouts with Rust's Alloc Layout Extra Feature

Core Problem

The alloc_layout_extra feature in Rust provides a powerful way to construct complex allocation layouts for use with the stable global allocator API. However, its usage and limitations are not yet fully understood.

Solution & Analysis

Public API

pub const fn padding_needed_for(&self, align: usize) -> usize;
pub const fn repeat(&self, n: usize) -> Result<(Layout, usize), LayoutErr>;
pub const fn repeat_packed(&self, n: usize) -> Result<Layout, LayoutErr>;
pub const fn extend_packed(&self, next: Layout) -> Result<Layout, LayoutErr>;
pub const fn dangling(&self) -> NonNull<u8>;

To construct complex allocation layouts, these methods can be used together. For example:

  • std::collections::HashMap uses repeat_packed to create a packed layout.
  • hashbrown uses extend_packed to extend a packed layout.

Code Example

use std::alloc::{Layout, LayoutErr};
use std::ptr;

// Create a new layout with some padding needed for alignment
let layout = Layout::new::<u8>().align_to(16).unwrap();

// Calculate the remaining space in the layout
let remaining_space = padding_needed_for(&layout, 16);

// Repeat the layout n times
let repeated_layout = repeat_packed(&layout, 5).unwrap();

// Extend the packed layout with some extra space
let extended_layout = extend_packed(&repeated_layout, Layout::new::<u8>().align_to(32).unwrap());

fn main() {
    // Allocate memory using the extended layout
    let ptr = unsafe { std::alloc::alloc(extended_layout) };

    // Store a pointer to the allocated memory
    *ptr as *const u8 = ptr;

    // Free the allocated memory
    unsafe { std::alloc::dealloc(ptr, extended_layout) };
}

Conclusion

The alloc_layout_extra feature provides a powerful way to construct complex allocation layouts in Rust. By using methods like repeat_packed, extend_packed, and padding_needed_for, developers can create custom allocation strategies for their use cases. However, more examples and doctests are needed to fully understand the limitations of this feature.

Reference