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.
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); }
69 #[stable(feature = "global_alloc", since = "1.28.0")]
70 pub unsafe trait GlobalAlloc {
71 /// Allocate memory as described by the given `layout`.
73 /// Returns a pointer to newly-allocated memory,
74 /// or null to indicate allocation failure.
78 /// This function is unsafe because undefined behavior can result
79 /// if the caller does not ensure that `layout` has non-zero size.
81 /// (Extension subtraits might provide more specific bounds on
82 /// behavior, e.g., guarantee a sentinel address or a null pointer
83 /// in response to a zero-size allocation request.)
85 /// The allocated block of memory may or may not be initialized.
89 /// Returning a null pointer indicates that either memory is exhausted
90 /// or `layout` does not meet this allocator's size or alignment constraints.
92 /// Implementations are encouraged to return null on memory
93 /// exhaustion rather than aborting, but this is not
94 /// a strict requirement. (Specifically: it is *legal* to
95 /// implement this trait atop an underlying native allocation
96 /// library that aborts on memory exhaustion.)
98 /// Clients wishing to abort computation in response to an
99 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
100 /// rather than directly invoking `panic!` or similar.
102 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
103 #[stable(feature = "global_alloc", since = "1.28.0")]
104 unsafe fn alloc(&self, layout: Layout) -> *mut u8;
106 /// Deallocate the block of memory at the given `ptr` pointer with the given `layout`.
110 /// This function is unsafe because undefined behavior can result
111 /// if the caller does not ensure all of the following:
113 /// * `ptr` must denote a block of memory currently allocated via
116 /// * `layout` must be the same layout that was used
117 /// to allocate that block of memory,
118 #[stable(feature = "global_alloc", since = "1.28.0")]
119 unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout);
121 /// Behaves like `alloc`, but also ensures that the contents
122 /// are set to zero before being returned.
126 /// This function is unsafe for the same reasons that `alloc` is.
127 /// However the allocated block of memory is guaranteed to be initialized.
131 /// Returning a null pointer indicates that either memory is exhausted
132 /// or `layout` does not meet allocator's size or alignment constraints,
133 /// just as in `alloc`.
135 /// Clients wishing to abort computation in response to an
136 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
137 /// rather than directly invoking `panic!` or similar.
139 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
140 #[stable(feature = "global_alloc", since = "1.28.0")]
141 unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
142 let size = layout.size();
143 // SAFETY: the safety contract for `alloc` must be upheld by the caller.
144 let ptr = unsafe { self.alloc(layout) };
146 // SAFETY: as allocation succeeded, the region from `ptr`
147 // of size `size` is guaranteed to be valid for writes.
148 unsafe { ptr::write_bytes(ptr, 0, size) };
153 /// Shrink or grow a block of memory to the given `new_size`.
154 /// The block is described by the given `ptr` pointer and `layout`.
156 /// If this returns a non-null pointer, then ownership of the memory block
157 /// referenced by `ptr` has been transferred to this allocator.
158 /// The memory may or may not have been deallocated,
159 /// and should be considered unusable (unless of course it was
160 /// transferred back to the caller again via the return value of
161 /// this method). The new memory block is allocated with `layout`, but
162 /// with the `size` updated to `new_size`.
164 /// If this method returns null, then ownership of the memory
165 /// block has not been transferred to this allocator, and the
166 /// contents of the memory block are unaltered.
170 /// This function is unsafe because undefined behavior can result
171 /// if the caller does not ensure all of the following:
173 /// * `ptr` must be currently allocated via this allocator,
175 /// * `layout` must be the same layout that was used
176 /// to allocate that block of memory,
178 /// * `new_size` must be greater than zero.
180 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
181 /// must not overflow (i.e., the rounded value must be less than `usize::MAX`).
183 /// (Extension subtraits might provide more specific bounds on
184 /// behavior, e.g., guarantee a sentinel address or a null pointer
185 /// in response to a zero-size allocation request.)
189 /// Returns null if the new layout does not meet the size
190 /// and alignment constraints of the allocator, or if reallocation
193 /// Implementations are encouraged to return null on memory
194 /// exhaustion rather than panicking or aborting, but this is not
195 /// a strict requirement. (Specifically: it is *legal* to
196 /// implement this trait atop an underlying native allocation
197 /// library that aborts on memory exhaustion.)
199 /// Clients wishing to abort computation in response to a
200 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
201 /// rather than directly invoking `panic!` or similar.
203 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
204 #[stable(feature = "global_alloc", since = "1.28.0")]
205 unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
206 // SAFETY: the caller must ensure that the `new_size` does not overflow.
207 // `layout.align()` comes from a `Layout` and is thus guaranteed to be valid.
208 let new_layout = unsafe { Layout::from_size_align_unchecked(new_size, layout.align()) };
209 // SAFETY: the caller must ensure that `new_layout` is greater than zero.
210 let new_ptr = unsafe { self.alloc(new_layout) };
211 if !new_ptr.is_null() {
212 // SAFETY: the previously allocated block cannot overlap the newly allocated block.
213 // The safety contract for `dealloc` must be upheld by the caller.
215 ptr::copy_nonoverlapping(ptr, new_ptr, cmp::min(layout.size(), new_size));
216 self.dealloc(ptr, layout);