std::alloc::System - Rust
Description: API documentation for the Rust `System` struct in crate `std`.
Keywords: rust, rustlang, rust-lang, System

# 1.28.0[−][src]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.

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);
    }

Run

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::SeqCst};

    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(), SeqCst);
            }
            return ret
        }

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

    #[global_allocator]
    static A: Counter = Counter;

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

Run

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 AllocRef for System`[src]

#### `fn alloc(
&mut self,
layout: Layout,
init: AllocInit
) -> Result<MemoryBlock, AllocErr>`[src]

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

Attempts to allocate a block of memory. Read more

#### `unsafe fn dealloc(&mut self, ptr: NonNull<u8>, layout: Layout)`[src]

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

Deallocates the memory referenced by `ptr`. Read more

#### `unsafe fn grow(
&mut self,
ptr: NonNull<u8>,
layout: Layout,
new_size: usize,
placement: ReallocPlacement,
init: AllocInit
) -> Result<MemoryBlock, AllocErr>`[src]

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

Attempts to extend the memory block. Read more

#### `unsafe fn shrink(
&mut self,
ptr: NonNull<u8>,
layout: Layout,
new_size: usize,
placement: ReallocPlacement
) -> Result<MemoryBlock, AllocErr>`[src]

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

Attempts to shrink the memory block. Read more

#### `fn by_ref(&mut self) -> &mut Self`[src]

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

Creates a "by reference" adaptor for this instance of `AllocRef`. Read more

### `impl Clone for System`[src]

#### `fn clone(&self) -> System`[src]

Returns a copy of the value. Read more

#### `fn clone_from(&mut self, source: &Self)`1.0.0[src]

Performs copy-assignment from `source`. Read more

### `impl Copy for System`[src]

### `impl Debug for System`[src]

#### `fn fmt(&self, f: &mut Formatter) -> Result`[src]

Formats the value using the given formatter. Read more

### `impl Default for System`[src]

#### `fn default() -> System`[src]

Returns the "default value" for a type. Read more

### `impl GlobalAlloc for System`[src]

#### `unsafe fn alloc(&self, layout: Layout) -> *mut u8`[src]

Allocate memory as described by the given `layout`. Read more

#### `unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8`[src]

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

#### `unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout)`[src]

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

#### `unsafe fn realloc(
&self,
ptr: *mut u8,
layout: Layout,
new_size: usize
) -> *mut u8`[src]

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

## 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

### `impl<T> Any for T where
T: 'static + ?Sized,` [src]

#### `fn type_id(&self) -> TypeId`[src]

Gets the `TypeId` of `self`. Read more

### `impl<T> Borrow<T> for T where
T: ?Sized,` [src]

#### `fn borrow(&self) -> &T`[src]

Immutably borrows from an owned value. Read more

### `impl<T> BorrowMut<T> for T where
T: ?Sized,` [src]

#### `fn borrow_mut(&mut self) -> &mut T`[src]

Mutably borrows from an owned value. Read more

### `impl<T> From<T> for T`[src]

#### `fn from(t: T) -> T`[src]

Performs the conversion.

### `impl<T, U> Into<U> for T where
U: From<T>,` [src]

#### `fn into(self) -> U`[src]

Performs the conversion.

### `impl<T> ToOwned for T where
T: Clone,` [src]

#### `type Owned = T`

The resulting type after obtaining ownership.

#### `fn to_owned(&self) -> T`[src]

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

#### `fn clone_into(&self, target: &mut T)`[src]

🔬 This is a nightly-only experimental API. (`toowned_clone_into` #41263)

recently added

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

### `impl<T, U> TryFrom<U> for T where
U: Into<T>,` [src]

#### `type Error = Infallible`

The type returned in the event of a conversion error.

#### `fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>`[src]

Performs the conversion.

### `impl<T, U> TryInto<U> for T where
U: TryFrom<T>,` [src]

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

The type returned in the event of a conversion error.

#### `fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>`[src]

Performs the conversion.