Struct System

pub struct System;

The default memory allocator provided by the operating system.

This is based on malloc on Unix platforms and HeapAlloc on Windows, plus related functions. However, it is not valid to mix use of the backing system allocator with System, as this implementation may include extra work, such as to serve alignment requests greater than the alignment provided directly by the backing system allocator.

This type implements the GlobalAlloc trait. Currently the default global allocator is unspecified. Libraries, however, like cdylibs and staticlibs are guaranteed to use the System by default and as such work as if they had this definition:

use std::alloc::System;

#[global_allocator]
static A: System = System;

fn main() {
    let a = Box::new(4); // Allocates from the system allocator.
    println!("{a}");
}

You can also define your own wrapper around System if you’d like, such as keeping track of the number of all bytes allocated:

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

struct Counter;

static ALLOCATED: AtomicUsize = AtomicUsize::new(0);

unsafe impl GlobalAlloc for Counter {
    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
        let ret = System.alloc(layout);
        if !ret.is_null() {
            ALLOCATED.fetch_add(layout.size(), Relaxed);
        }
        ret
    }

    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
        System.dealloc(ptr, layout);
        ALLOCATED.fetch_sub(layout.size(), Relaxed);
    }
}

#[global_allocator]
static A: Counter = Counter;

fn main() {
    println!("allocated bytes before main: {}", ALLOCATED.load(Relaxed));
}

It can also be used directly to allocate memory independently of whatever global allocator has been selected for a Rust program. For example if a Rust program opts in to using jemalloc as the global allocator, System will still allocate memory using malloc and HeapAlloc.

Trait Implementations

impl Allocator for System

fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Attempts to allocate a block of memory.

fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Behaves like allocate, but also ensures that the returned memory is zero-initialized.

unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout)

🔬This is a nightly-only experimental API. (allocator_api #32838)

Deallocates the memory referenced by ptr.

unsafe fn grow( &self, ptr: NonNull<u8>, old_layout: Layout, new_layout: Layout, ) -> Result<NonNull<[u8]>, AllocError>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Attempts to extend the memory block.

unsafe fn grow_zeroed( &self, ptr: NonNull<u8>, old_layout: Layout, new_layout: Layout, ) -> Result<NonNull<[u8]>, AllocError>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Behaves like grow, but also ensures that the new contents are set to zero before being returned.

unsafe fn shrink( &self, ptr: NonNull<u8>, old_layout: Layout, new_layout: Layout, ) -> Result<NonNull<[u8]>, AllocError>

🔬This is a nightly-only experimental API. (allocator_api #32838)

Attempts to shrink the memory block.

fn by_ref(&self) -> &Self

where Self: Sized,

🔬This is a nightly-only experimental API. (allocator_api #32838)

Creates a “by reference” adapter for this instance of Allocator.

impl Clone for System

fn clone(&self) -> System

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for System

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for System

fn default() -> System

Returns the “default value” for a type.

impl GlobalAlloc for System

unsafe fn alloc(&self, layout: Layout) -> *mut u8

Allocates memory as described by the given layout.

unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8

Behaves like alloc, but also ensures that the contents are set to zero before being returned.

unsafe fn dealloc(&self, ptr: *mut u8, _layout: Layout)

Deallocates the block of memory at the given ptr pointer with the given layout.

unsafe fn realloc( &self, ptr: *mut u8, layout: Layout, new_size: usize, ) -> *mut u8

Shrinks or grows a block of memory to the given new_size in bytes. The block is described by the given ptr pointer and layout.

impl Copy for System