Trait GlobalAlloc

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 { ... }
}

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();
        };
        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.

Required Methods

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 may 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.

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 undefined behavior can result.

Provided Methods

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 can result in undefined behaviour. 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.

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, undefined behaviour can result.

(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