core/alloc/

global.rs

use crate::alloc::Layout;
use crate::{cmp, 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::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.
///
///   ```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 {
    /// 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.
    ///
    /// [`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;

    /// 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.
    #[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
    ///
    /// The caller has to ensure that `layout` has non-zero size. Like `alloc`
    /// zero sized `layout` can 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.
    ///
    /// [`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
    }

    /// 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 behavior 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.
    ///
    /// [`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
    }
}