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use crate::alloc::Layout;
use crate::cmp;
use crate::ptr;
/// 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::{Acquire, SeqCst},
/// };
///
/// 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(SeqCst, SeqCst, |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(Acquire);
/// 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.
///
/// ```rust,ignore (unsound and has placeholders)
/// 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.
#[stable(feature = "global_alloc", since = "1.28.0")]
pub unsafe trait GlobalAlloc {
/// 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 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.
///
/// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
#[stable(feature = "global_alloc", since = "1.28.0")]
unsafe fn alloc(&self, layout: Layout) -> *mut u8;
/// 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 allocate that block of memory.
#[stable(feature = "global_alloc", since = "1.28.0")]
unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout);
/// 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.
///
/// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
#[stable(feature = "global_alloc", since = "1.28.0")]
unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
let size = layout.size();
// SAFETY: the safety contract for `alloc` must be upheld by the caller.
let ptr = unsafe { self.alloc(layout) };
if !ptr.is_null() {
// SAFETY: as allocation succeeded, the region from `ptr`
// of size `size` is guaranteed to be valid for writes.
unsafe { ptr::write_bytes(ptr, 0, size) };
}
ptr
}
/// Shrink or grow 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
///
/// 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 allocate 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 isize (i.e., the rounded value must be less than or
/// equal to `isize::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.
///
/// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
#[stable(feature = "global_alloc", since = "1.28.0")]
unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
// SAFETY: the caller must ensure that the `new_size` does not overflow.
// `layout.align()` comes from a `Layout` and is thus guaranteed to be valid.
let new_layout = unsafe { Layout::from_size_align_unchecked(new_size, layout.align()) };
// SAFETY: the caller must ensure that `new_layout` is greater than zero.
let new_ptr = unsafe { self.alloc(new_layout) };
if !new_ptr.is_null() {
// SAFETY: the previously allocated block cannot overlap the newly allocated block.
// The safety contract for `dealloc` must be upheld by the caller.
unsafe {
ptr::copy_nonoverlapping(ptr, new_ptr, cmp::min(layout.size(), new_size));
self.dealloc(ptr, layout);
}
}
new_ptr
}
}