Struct std::alloc::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 and Rust programs by default 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

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impl Allocator for System

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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. Read more

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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. Read more

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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. Read more

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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. Read more

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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. Read more

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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. Read more

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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. Read more

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impl Clone for System

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fn clone(&self) -> System

Returns a copy of the value. Read more

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

Performs copy-assignment from source. Read more

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impl Debug for System

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

Formats the value using the given formatter. Read more

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impl Default for System

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fn default() -> System

Returns the “default value” for a type. Read more

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impl GlobalAlloc for System

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

Allocate memory as described by the given layout. Read more

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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. Read more

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

Deallocate the block of memory at the given ptr pointer with the given layout. Read more

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

Shrink or grow a block of memory to the given new_size in bytes. The block is described by the given ptr pointer and layout. Read more

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impl Copy for System

Auto Trait Implementations

impl RefUnwindSafe for System

impl Send for System

impl Sync for System

impl Unpin for System

impl UnwindSafe for System

Blanket Implementations

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impl<T> Any for T
where
    T: 'static + ?Sized,

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fn type_id(&self) -> TypeId

Gets the TypeId of self. Read more

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impl<T> Borrow<T> for T
where
    T: ?Sized,

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fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more

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impl<T> BorrowMut<T> for T
where
    T: ?Sized,

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fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more

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impl<T> From<T> for T

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fn from(t: T) -> T

Returns the argument unchanged.

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impl<T, U> Into<U> for T
where
    U: From<T>,

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fn into(self) -> U

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

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impl<T> ToOwned for T
where
    T: Clone,

type Owned = T

The resulting type after obtaining ownership.

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fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more

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fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more

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impl<T, U> TryFrom<U> for T
where
    U: Into<T>,

type Error = Infallible

The type returned in the event of a conversion error.

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fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.

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impl<T, U> TryInto<U> for T
where
    U: TryFrom<T>,

type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.

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fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.

© 2010 The Rust Project Developers
Licensed under the Apache License, Version 2.0 or the MIT license, at your option.
https://doc.rust-lang.org/std/alloc/struct.System.html