Implementing a Custom Global Allocator

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Implementing a Custom Global Allocator

Rust allows overriding the default memory allocator for the entire program. By implementing the GlobalAlloc trait and using the #[global_allocator] attribute, you can inject custom logic into every allocation and deallocation, which is useful for memory profiling, telemetry, or specialized embedded requirements.

snippet.rs
rust
use std::alloc::{GlobalAlloc, Layout, System};
 
struct TrackingAllocator;
 
unsafe impl GlobalAlloc for TrackingAllocator {
    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
        println!("Allocating {} bytes", layout.size());
        System.alloc(layout)
    }
 
    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
        println!("Deallocating {} bytes", layout.size());
        System.dealloc(ptr, layout)
    }
}
 
#[global_allocator]
static GLOBAL: TrackingAllocator = TrackingAllocator;

Breakdown

unsafe impl GlobalAlloc

Implementing this trait is unsafe because it must correctly manage raw pointers.

System.alloc(layout)

Delegates the actual memory work to the underlying system allocator (e.g., jemalloc or libc).

#[global_allocator]

A compiler attribute that registers the static variable as the program-wide allocator.

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Implementing a Custom Global Allocator

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