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 Debug for System`[src]

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

Formats the value using the given formatter. Read more

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

### `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 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

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

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

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

Returns a pointer meeting the size and alignment guarantees of `layout`. Read more

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

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

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

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

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

Deallocate the memory referenced by `ptr`. Read more

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

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

Returns a pointer suitable for holding data described by a new layout with `layout`’s alignment and a size given by `new_size`. To accomplish this, this may extend or shrink the allocation referenced by `ptr` to fit the new layout. Read more

#### `fn usable_size(&self, layout: &Layout) -> (usize, usize)`[src]

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

Returns bounds on the guaranteed usable size of a successful allocation created with the specified `layout`. Read more

#### `unsafe fn alloc_excess(&mut self, layout: Layout) -> Result<Excess, AllocErr>`[src]

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

Behaves like `alloc`, but also returns the whole size of the returned block. For some `layout` inputs, like arrays, this may include extra storage usable for additional data. Read more

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

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

Behaves like `realloc`, but also returns the whole size of the returned block. For some `layout` inputs, like arrays, this may include extra storage usable for additional data. Read more

#### `unsafe fn grow_in_place(
&mut self,
ptr: NonNull<u8>,
layout: Layout,
new_size: usize
) -> Result<(), CannotReallocInPlace>`[src]

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

Attempts to extend the allocation referenced by `ptr` to fit `new_size`. Read more

#### `unsafe fn shrink_in_place(
&mut self,
ptr: NonNull<u8>,
layout: Layout,
new_size: usize
) -> Result<(), CannotReallocInPlace>`[src]

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

Attempts to shrink the allocation referenced by `ptr` to fit `new_size`. Read more

#### `fn alloc_one<T>(&mut self) -> Result<NonNull<T>, AllocErr>`[src]

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

Allocates a block suitable for holding an instance of `T`. Read more

#### `unsafe fn dealloc_one<T>(&mut self, ptr: NonNull<T>)`[src]

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

Deallocates a block suitable for holding an instance of `T`. Read more

#### `fn alloc_array<T>(&mut self, n: usize) -> Result<NonNull<T>, AllocErr>`[src]

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

Allocates a block suitable for holding `n` instances of `T`. Read more

#### `unsafe fn realloc_array<T>(
&mut self,
ptr: NonNull<T>,
n_old: usize,
n_new: usize
) -> Result<NonNull<T>, AllocErr>`[src]

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

Reallocates a block previously suitable for holding `n_old` instances of `T`, returning a block suitable for holding `n_new` instances of `T`. Read more

#### `unsafe fn dealloc_array<T>(
&mut self,
ptr: NonNull<T>,
n: usize
) -> Result<(), AllocErr>`[src]

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

Deallocates a block suitable for holding `n` instances of `T`. Read more

## Auto Trait Implementations

### `impl UnwindSafe for System`

### `impl RefUnwindSafe for System`

### `impl Unpin for System`

### `impl Send for System`

### `impl Sync for System`

## Blanket Implementations

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

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

Performs the conversion.

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

#### `fn into(self) -> U`[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.

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

####
ⓘImportant traits for &'_ mut I

### Important traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<'_, R: Read + ?Sized> Read for &'_ mut Rimpl<'_, W: Write + ?Sized> Write for &'_ mut W`
`fn borrow(&self) -> &T`[src]

Immutably borrows from an owned value. Read more

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

####
ⓘImportant traits for &'_ mut I

### Important traits for &'_ mut I

`impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<'_, R: Read + ?Sized> Read for &'_ mut Rimpl<'_, W: Write + ?Sized> Write for &'_ mut W`
`fn borrow_mut(&mut self) -> &mut T`[src]

Mutably borrows from an owned value. Read more

### `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> 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