# 1.28.0[−][src]Trait alloc::alloc::GlobalAlloc

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

        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 though the `#[global_allocator]` attributes.

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, alloc};
    use std::ptr::null_mut;

    struct MyAllocator;

    unsafe impl GlobalAlloc for MyAllocator {
        unsafe fn alloc(&self, _layout: Layout) -> *mut u8 { null_mut() }
        unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {}
    }

    #[global_allocator]
    static A: MyAllocator = MyAllocator;

    fn main() {
        unsafe {
            assert!(alloc(Layout::new::<u32>()).is_null())
        }
    }

# Unsafety

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.

## Required methods

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

Allocate memory as described by the given `layout`.

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

# Safety

This function is unsafe because undefined behavior can result if the caller does not ensure that `layout` has non-zero size.

(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 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)`

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

# Safety

This function is unsafe because undefined behavior can result if the caller does not ensure all of the following:

- `ptr` must denote a block of memory currently allocated via this allocator,

- `layout` must be the same layout that was used to allocated that block of memory,

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## 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

This function is unsafe for the same reasons that `alloc` is. 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`

Shrink or grow a block of memory to the given `new_size`. 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. The memory may or may not have been deallocated, and should be considered unusable (unless of course it was transferred back to the caller again via the return value of this method).

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

This function is unsafe because undefined behavior can result if the caller does not ensure all of the following:

- `ptr` must be currently allocated via this allocator,

- `layout` must be the same layout that was used to allocated that block of memory,

- `new_size` must be greater than zero.

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

(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.

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## Implementors

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