Trait GlobalAlloc

pub unsafe trait GlobalAlloc {
    // Required methods
    unsafe fn alloc(&self, layout: Layout) -> *mut u8;
    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout);

    // Provided methods
    unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 { ... }
    unsafe fn realloc(
        &self,
        ptr: *mut u8,
        layout: Layout,
        new_size: usize,
    ) -> *mut u8 { ... }
}

A memory allocator that can be registered as the standard library’s default through the #[global_allocator] attribute.

Some of the methods require that a memory block be currently allocated via an allocator. This means that:

• the starting address for that memory block was previously returned by a previous call to an allocation method such as alloc, and

• the memory block has not been subsequently deallocated, where blocks are deallocated either by being passed to a deallocation method such as dealloc or by being passed to a reallocation method that returns a non-null pointer.

Example

use std::alloc::{GlobalAlloc, Layout};
use std::cell::UnsafeCell;
use std::ptr::null_mut;
use std::sync::atomic::{AtomicUsize, Ordering::Relaxed};

const ARENA_SIZE: usize = 128 * 1024;
const MAX_SUPPORTED_ALIGN: usize = 4096;
#[repr(C, align(4096))] // 4096 == MAX_SUPPORTED_ALIGN
struct SimpleAllocator {
    arena: UnsafeCell<[u8; ARENA_SIZE]>,
    remaining: AtomicUsize, // we allocate from the top, counting down
}

#[global_allocator]
static ALLOCATOR: SimpleAllocator = SimpleAllocator {
    arena: UnsafeCell::new([0x55; ARENA_SIZE]),
    remaining: AtomicUsize::new(ARENA_SIZE),
};

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