How to Write Custom Allocators in Rust

You write custom allocators 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_ALLOC: MyAlloc = MyAlloc;

struct MyAlloc;

unsafe impl GlobalAlloc for MyAlloc {
    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: MyAlloc = MyAlloc;

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 operating system. You use one when you need to track every memory operation, handle out-of-memory errors differently, or optimize for specific hardware. Think of it as replacing the default bank teller with your own manager who follows your specific rules for handing out cash.