How to Write a Custom Allocator in Rust

You write a custom allocator in Rust by implementing the GlobalAlloc trait and using the #[global_allocator] attribute on a static instance. This replaces the default system allocator for your entire binary.

use std::alloc::{GlobalAlloc, Layout, System};
use std::sync::atomic::{AtomicBool, Ordering};

static MY_ALLOCATOR: MyAllocator = MyAllocator;

struct MyAllocator;

unsafe impl GlobalAlloc for MyAllocator {
    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
        System.alloc(layout)
    }

    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
        System.dealloc(ptr, layout)
    }
}

#[global_allocator]
static GLOBAL: MyAllocator = MyAllocator;

fn main() {
    println!("Custom allocator active");
}

A custom allocator is a piece of code that decides how your program requests and returns memory from the computer. You use it when you need to track every memory operation, prevent memory fragmentation, or integrate with a specific operating system environment. Think of it as replacing the default librarian with your own who follows a specific set of rules for shelving books.