1 use crate::alloc::Layout;
5 /// A memory allocator that can be registered as the standard library’s default
6 /// through the `#[global_allocator]` attribute.
8 /// Some of the methods require that a memory block be *currently
9 /// allocated* via an allocator. This means that:
11 /// * the starting address for that memory block was previously
12 /// returned by a previous call to an allocation method
13 /// such as `alloc`, and
15 /// * the memory block has not been subsequently deallocated, where
16 /// blocks are deallocated either by being passed to a deallocation
17 /// method such as `dealloc` or by being
18 /// passed to a reallocation method that returns a non-null pointer.
24 /// use std::alloc::{GlobalAlloc, Layout, alloc};
25 /// use std::ptr::null_mut;
27 /// struct MyAllocator;
29 /// unsafe impl GlobalAlloc for MyAllocator {
30 /// unsafe fn alloc(&self, _layout: Layout) -> *mut u8 { null_mut() }
31 /// unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {}
34 /// #[global_allocator]
35 /// static A: MyAllocator = MyAllocator;
39 /// assert!(alloc(Layout::new::<u32>()).is_null())
46 /// The `GlobalAlloc` trait is an `unsafe` trait for a number of reasons, and
47 /// implementors must ensure that they adhere to these contracts:
49 /// * It's undefined behavior if global allocators unwind. This restriction may
50 /// be lifted in the future, but currently a panic from any of these
51 /// functions may lead to memory unsafety.
53 /// * `Layout` queries and calculations in general must be correct. Callers of
54 /// this trait are allowed to rely on the contracts defined on each method,
55 /// and implementors must ensure such contracts remain true.
57 /// * You may not rely on allocations actually happening, even if there are explicit
58 /// heap allocations in the source. The optimizer may detect allocation/deallocation
59 /// pairs that it can instead move to stack allocations/deallocations and thus never
60 /// invoke the allocator here.
61 /// More concretely, the following code example is unsound, irrespective of whether your
62 /// custom allocator allows counting how many allocations have happened.
64 /// ```rust,ignore (unsound and has placeholders)
65 /// drop(Box::new(42));
66 /// let number_of_heap_allocs = /* call private allocator API */;
67 /// unsafe { std::intrinsics::assume(number_of_heap_allocs > 0); }
70 /// Note that allocation/deallocation pairs being moved to the stack is not the only
71 /// optimization that can be applied. You may generally not rely on heap allocations
72 /// happening, if they can be removed without changing program behaviour.
73 /// Whether allocations happen or not is not part of the program behaviour, even if it
74 /// could be detected via an allocator that tracks allocations by printing or otherwise
75 /// having side effects.
76 #[stable(feature = "global_alloc", since = "1.28.0")]
77 pub unsafe trait GlobalAlloc {
78 /// Allocate memory as described by the given `layout`.
80 /// Returns a pointer to newly-allocated memory,
81 /// or null to indicate allocation failure.
85 /// This function is unsafe because undefined behavior can result
86 /// if the caller does not ensure that `layout` has non-zero size.
88 /// (Extension subtraits might provide more specific bounds on
89 /// behavior, e.g., guarantee a sentinel address or a null pointer
90 /// in response to a zero-size allocation request.)
92 /// The allocated block of memory may or may not be initialized.
96 /// Returning a null pointer indicates that either memory is exhausted
97 /// or `layout` does not meet this allocator's size or alignment constraints.
99 /// Implementations are encouraged to return null on memory
100 /// exhaustion rather than aborting, but this is not
101 /// a strict requirement. (Specifically: it is *legal* to
102 /// implement this trait atop an underlying native allocation
103 /// library that aborts on memory exhaustion.)
105 /// Clients wishing to abort computation in response to an
106 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
107 /// rather than directly invoking `panic!` or similar.
109 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
110 #[stable(feature = "global_alloc", since = "1.28.0")]
111 unsafe fn alloc(&self, layout: Layout) -> *mut u8;
113 /// Deallocate the block of memory at the given `ptr` pointer with the given `layout`.
117 /// This function is unsafe because undefined behavior can result
118 /// if the caller does not ensure all of the following:
120 /// * `ptr` must denote a block of memory currently allocated via
123 /// * `layout` must be the same layout that was used
124 /// to allocate that block of memory,
125 #[stable(feature = "global_alloc", since = "1.28.0")]
126 unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout);
128 /// Behaves like `alloc`, but also ensures that the contents
129 /// are set to zero before being returned.
133 /// This function is unsafe for the same reasons that `alloc` is.
134 /// However the allocated block of memory is guaranteed to be initialized.
138 /// Returning a null pointer indicates that either memory is exhausted
139 /// or `layout` does not meet allocator's size or alignment constraints,
140 /// just as in `alloc`.
142 /// Clients wishing to abort computation in response to an
143 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
144 /// rather than directly invoking `panic!` or similar.
146 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
147 #[stable(feature = "global_alloc", since = "1.28.0")]
148 unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
149 let size = layout.size();
150 // SAFETY: the safety contract for `alloc` must be upheld by the caller.
151 let ptr = unsafe { self.alloc(layout) };
153 // SAFETY: as allocation succeeded, the region from `ptr`
154 // of size `size` is guaranteed to be valid for writes.
155 unsafe { ptr::write_bytes(ptr, 0, size) };
160 /// Shrink or grow a block of memory to the given `new_size`.
161 /// The block is described by the given `ptr` pointer and `layout`.
163 /// If this returns a non-null pointer, then ownership of the memory block
164 /// referenced by `ptr` has been transferred to this allocator.
165 /// The memory may or may not have been deallocated,
166 /// and should be considered unusable (unless of course it was
167 /// transferred back to the caller again via the return value of
168 /// this method). The new memory block is allocated with `layout`, but
169 /// with the `size` updated to `new_size`.
171 /// If this method returns null, then ownership of the memory
172 /// block has not been transferred to this allocator, and the
173 /// contents of the memory block are unaltered.
177 /// This function is unsafe because undefined behavior can result
178 /// if the caller does not ensure all of the following:
180 /// * `ptr` must be currently allocated via this allocator,
182 /// * `layout` must be the same layout that was used
183 /// to allocate that block of memory,
185 /// * `new_size` must be greater than zero.
187 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
188 /// must not overflow (i.e., the rounded value must be less than `usize::MAX`).
190 /// (Extension subtraits might provide more specific bounds on
191 /// behavior, e.g., guarantee a sentinel address or a null pointer
192 /// in response to a zero-size allocation request.)
196 /// Returns null if the new layout does not meet the size
197 /// and alignment constraints of the allocator, or if reallocation
200 /// Implementations are encouraged to return null on memory
201 /// exhaustion rather than panicking or aborting, but this is not
202 /// a strict requirement. (Specifically: it is *legal* to
203 /// implement this trait atop an underlying native allocation
204 /// library that aborts on memory exhaustion.)
206 /// Clients wishing to abort computation in response to a
207 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
208 /// rather than directly invoking `panic!` or similar.
210 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
211 #[stable(feature = "global_alloc", since = "1.28.0")]
212 unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
213 // SAFETY: the caller must ensure that the `new_size` does not overflow.
214 // `layout.align()` comes from a `Layout` and is thus guaranteed to be valid.
215 let new_layout = unsafe { Layout::from_size_align_unchecked(new_size, layout.align()) };
216 // SAFETY: the caller must ensure that `new_layout` is greater than zero.
217 let new_ptr = unsafe { self.alloc(new_layout) };
218 if !new_ptr.is_null() {
219 // SAFETY: the previously allocated block cannot overlap the newly allocated block.
220 // The safety contract for `dealloc` must be upheld by the caller.
222 ptr::copy_nonoverlapping(ptr, new_ptr, cmp::min(layout.size(), new_size));
223 self.dealloc(ptr, layout);