GlobalAlloc in alloc::alloc - Rust
Description: A memory allocator that can be registered as the standard library’s default through the `#[global_allocator]` attribute.

## GlobalAlloc

# Trait GlobalAlloc

1.36.0 · Source

    pub unsafe trait GlobalAlloc {
        // Required methods
        unsafe fn alloc(&self, layout: Layout) -> *mut u8;
        unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout);

        // Provided methods
        unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 { ... }
        unsafe fn realloc(
            &self,
            ptr: *mut u8,
            layout: Layout,
            new_size: usize,
        ) -> *mut u8 { ... }
    }

Expand description

A memory allocator that can be registered as the standard library’s default through the `#[global_allocator]` attribute.

Some of the methods require that a memory block be currently allocated via an allocator. This means that:

- the starting address for that memory block was previously returned by a previous call to an allocation method such as `alloc`, and

- the memory block has not been subsequently deallocated, where blocks are deallocated either by being passed to a deallocation method such as `dealloc` or by being passed to a reallocation method that returns a non-null pointer.

## §Example

    use std::alloc::{GlobalAlloc, Layout};
    use std::cell::UnsafeCell;
    use std::ptr::null_mut;
    use std::sync::atomic::{AtomicUsize, Ordering::Relaxed};

    const ARENA_SIZE: usize = 128 * 1024;
    const MAX_SUPPORTED_ALIGN: usize = 4096;
    #[repr(C, align(4096))] // 4096 == MAX_SUPPORTED_ALIGN
    struct SimpleAllocator {
        arena: UnsafeCell<[u8; ARENA_SIZE]>,
        remaining: AtomicUsize, // we allocate from the top, counting down
    }

    #[global_allocator]
    static ALLOCATOR: SimpleAllocator = SimpleAllocator {
        arena: UnsafeCell::new([0x55; ARENA_SIZE]),
        remaining: AtomicUsize::new(ARENA_SIZE),
    };

    unsafe impl Sync for SimpleAllocator {}

    unsafe impl GlobalAlloc for SimpleAllocator {
        unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
            let size = layout.size();
            let align = layout.align();

            // `Layout` contract forbids making a `Layout` with align=0, or align not power of 2.
            // So we can safely use a mask to ensure alignment without worrying about UB.
            let align_mask_to_round_down = !(align - 1);

            if align > MAX_SUPPORTED_ALIGN {
                return null_mut();
            }

            let mut allocated = 0;
            if self
                .remaining
                .fetch_update(Relaxed, Relaxed, |mut remaining| {
                    if size > remaining {
                        return None;
                    }
                    remaining -= size;
                    remaining &= align_mask_to_round_down;
                    allocated = remaining;
                    Some(remaining)
                })
                .is_err()
            {
                return null_mut();
            };
            unsafe { self.arena.get().cast::<u8>().add(allocated) }
        }
        unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {}
    }

    fn main() {
        let _s = format!("allocating a string!");
        let currently = ALLOCATOR.remaining.load(Relaxed);
        println!("allocated so far: {}", ARENA_SIZE - currently);
    }

## §Safety

The `GlobalAlloc` trait is an `unsafe` trait for a number of reasons, and implementors must ensure that they adhere to these contracts:

- It’s undefined behavior if global allocators unwind. This restriction may be lifted in the future, but currently a panic from any of these functions may lead to memory unsafety.

- `Layout` queries and calculations in general must be correct. Callers of this trait are allowed to rely on the contracts defined on each method, and implementors must ensure such contracts remain true.

- You must not rely on allocations actually happening, even if there are explicit heap allocations in the source. The optimizer may detect unused allocations that it can either eliminate entirely or move to the stack and thus never invoke the allocator. The optimizer may further assume that allocation is infallible, so code that used to fail due to allocator failures may now suddenly work because the optimizer worked around the need for an allocation. More concretely, the following code example is unsound, irrespective of whether your custom allocator allows counting how many allocations have happened.

  ⓘ

      drop(Box::new(42));
      let number_of_heap_allocs = /* call private allocator API */;
      unsafe { std::hint::assert_unchecked(number_of_heap_allocs > 0); }

  Note that the optimizations mentioned above are not the only optimization that can be applied. You may generally not rely on heap allocations happening if they can be removed without changing program behavior. Whether allocations happen or not is not part of the program behavior, even if it could be detected via an allocator that tracks allocations by printing or otherwise having side effects.

## §Re-entrance

When implementing a global allocator one has to be careful not to create an infinitely recursive implementation by accident, as many constructs in the Rust standard library may allocate in their implementation. For example, on some platforms `std::sync::Mutex` may allocate, so using it is highly problematic in a global allocator.

Generally speaking for this reason one should stick to library features available through `core`, and avoid using `std` in a global allocator. A few features from `std` are guaranteed to not use `#[global_allocator]` to allocate:

- `std::thread_local`,
- `std::thread::current`,
- `std::thread::park` and `std::thread::Thread`’s `unpark` method and `Clone` implementation.

## Required Methods§

1.28.0 · Source

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

Allocates memory as described by the given `layout`.

Returns a pointer to newly-allocated memory, or null to indicate allocation failure.

##### §Safety

`layout` must have non-zero size. Attempting to allocate for a zero-sized `layout` will result in undefined behavior.

(Extension subtraits might provide more specific bounds on behavior, e.g., guarantee a sentinel address or a null pointer in response to a zero-size allocation request.)

The allocated block of memory may or may not be initialized.

##### §Errors

Returning a null pointer indicates that either memory is exhausted or `layout` does not meet this allocator’s size or alignment constraints.

Implementations are encouraged to return null on memory exhaustion rather than aborting, but this is not a strict requirement. (Specifically: it is legal to implement this trait atop an underlying native allocation library that aborts on memory exhaustion.)

Clients wishing to abort computation in response to an allocation error are encouraged to call the `handle_alloc_error` function, rather than directly invoking `panic!` or similar.

1.28.0 · Source

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

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

##### §Safety

The caller must ensure:

- `ptr` is a block of memory currently allocated via this allocator and,

- `layout` is the same layout that was used to allocate that block of memory.

Otherwise the behavior is undefined.

## Provided Methods§

1.28.0 · Source

#### 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.

##### §Safety

The caller has to ensure that `layout` has non-zero size. Like `alloc` zero sized `layout` will result in undefined behavior. However the allocated block of memory is guaranteed to be initialized.

##### §Errors

Returning a null pointer indicates that either memory is exhausted or `layout` does not meet allocator’s size or alignment constraints, just as in `alloc`.

Clients wishing to abort computation in response to an allocation error are encouraged to call the `handle_alloc_error` function, rather than directly invoking `panic!` or similar.

1.28.0 · Source

#### 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`.

If this returns a non-null pointer, then ownership of the memory block referenced by `ptr` has been transferred to this allocator. Any access to the old `ptr` is Undefined Behavior, even if the allocation remained in-place. The newly returned pointer is the only valid pointer for accessing this memory now.

The new memory block is allocated with `layout`, but with the `size` updated to `new_size` in bytes. This new layout must be used when deallocating the new memory block with `dealloc`. The range `0..min(layout.size(), new_size)` of the new memory block is guaranteed to have the same values as the original block.

If this method returns null, then ownership of the memory block has not been transferred to this allocator, and the contents of the memory block are unaltered.

##### §Safety

The caller must ensure that:

- `ptr` is allocated via this allocator,

- `layout` is the same layout that was used to allocate that block of memory,

- `new_size` is greater than zero.

- `new_size`, when rounded up to the nearest multiple of `layout.align()`, does not overflow `isize` (i.e., the rounded value must be less than or equal to `isize::MAX`).

If these are not followed, the behavior is undefined.

(Extension subtraits might provide more specific bounds on behavior, e.g., guarantee a sentinel address or a null pointer in response to a zero-size allocation request.)

##### §Errors

Returns null if the new layout does not meet the size and alignment constraints of the allocator, or if reallocation otherwise fails.

Implementations are encouraged to return null on memory exhaustion rather than panicking or aborting, but this is not a strict requirement. (Specifically: it is legal to implement this trait atop an underlying native allocation library that aborts on memory exhaustion.)

Clients wishing to abort computation in response to a reallocation error are encouraged to call the `handle_alloc_error` function, rather than directly invoking `panic!` or similar.

## Implementors§