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.
56 #[stable(feature = "global_alloc", since = "1.28.0")]
57 pub unsafe trait GlobalAlloc {
58 /// Allocate memory as described by the given `layout`.
60 /// Returns a pointer to newly-allocated memory,
61 /// or null to indicate allocation failure.
65 /// This function is unsafe because undefined behavior can result
66 /// if the caller does not ensure that `layout` has non-zero size.
68 /// (Extension subtraits might provide more specific bounds on
69 /// behavior, e.g., guarantee a sentinel address or a null pointer
70 /// in response to a zero-size allocation request.)
72 /// The allocated block of memory may or may not be initialized.
76 /// Returning a null pointer indicates that either memory is exhausted
77 /// or `layout` does not meet this allocator's size or alignment constraints.
79 /// Implementations are encouraged to return null on memory
80 /// exhaustion rather than aborting, but this is not
81 /// a strict requirement. (Specifically: it is *legal* to
82 /// implement this trait atop an underlying native allocation
83 /// library that aborts on memory exhaustion.)
85 /// Clients wishing to abort computation in response to an
86 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
87 /// rather than directly invoking `panic!` or similar.
89 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
90 #[stable(feature = "global_alloc", since = "1.28.0")]
91 unsafe fn alloc(&self, layout: Layout) -> *mut u8;
93 /// Deallocate the block of memory at the given `ptr` pointer with the given `layout`.
97 /// This function is unsafe because undefined behavior can result
98 /// if the caller does not ensure all of the following:
100 /// * `ptr` must denote a block of memory currently allocated via
103 /// * `layout` must be the same layout that was used
104 /// to allocate that block of memory,
105 #[stable(feature = "global_alloc", since = "1.28.0")]
106 unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout);
108 /// Behaves like `alloc`, but also ensures that the contents
109 /// are set to zero before being returned.
113 /// This function is unsafe for the same reasons that `alloc` is.
114 /// However the allocated block of memory is guaranteed to be initialized.
118 /// Returning a null pointer indicates that either memory is exhausted
119 /// or `layout` does not meet allocator's size or alignment constraints,
120 /// just as in `alloc`.
122 /// Clients wishing to abort computation in response to an
123 /// allocation error are encouraged to call the [`handle_alloc_error`] function,
124 /// rather than directly invoking `panic!` or similar.
126 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
127 #[stable(feature = "global_alloc", since = "1.28.0")]
128 unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
129 let size = layout.size();
130 // SAFETY: the safety contract for `alloc` must be upheld by the caller.
131 let ptr = unsafe { self.alloc(layout) };
133 // SAFETY: as allocation succeeded, the region from `ptr`
134 // of size `size` is guaranteed to be valid for writes.
135 unsafe { ptr::write_bytes(ptr, 0, size) };
140 /// Shrink or grow a block of memory to the given `new_size`.
141 /// The block is described by the given `ptr` pointer and `layout`.
143 /// If this returns a non-null pointer, then ownership of the memory block
144 /// referenced by `ptr` has been transferred to this allocator.
145 /// The memory may or may not have been deallocated,
146 /// and should be considered unusable (unless of course it was
147 /// transferred back to the caller again via the return value of
148 /// this method). The new memory block is allocated with `layout`, but
149 /// with the `size` updated to `new_size`.
151 /// If this method returns null, then ownership of the memory
152 /// block has not been transferred to this allocator, and the
153 /// contents of the memory block are unaltered.
157 /// This function is unsafe because undefined behavior can result
158 /// if the caller does not ensure all of the following:
160 /// * `ptr` must be currently allocated via this allocator,
162 /// * `layout` must be the same layout that was used
163 /// to allocate that block of memory,
165 /// * `new_size` must be greater than zero.
167 /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
168 /// must not overflow (i.e., the rounded value must be less than `usize::MAX`).
170 /// (Extension subtraits might provide more specific bounds on
171 /// behavior, e.g., guarantee a sentinel address or a null pointer
172 /// in response to a zero-size allocation request.)
176 /// Returns null if the new layout does not meet the size
177 /// and alignment constraints of the allocator, or if reallocation
180 /// Implementations are encouraged to return null on memory
181 /// exhaustion rather than panicking or aborting, but this is not
182 /// a strict requirement. (Specifically: it is *legal* to
183 /// implement this trait atop an underlying native allocation
184 /// library that aborts on memory exhaustion.)
186 /// Clients wishing to abort computation in response to a
187 /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
188 /// rather than directly invoking `panic!` or similar.
190 /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
191 #[stable(feature = "global_alloc", since = "1.28.0")]
192 unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
193 // SAFETY: the caller must ensure that the `new_size` does not overflow.
194 // `layout.align()` comes from a `Layout` and is thus guaranteed to be valid.
195 let new_layout = unsafe { Layout::from_size_align_unchecked(new_size, layout.align()) };
196 // SAFETY: the caller must ensure that `new_layout` is greater than zero.
197 let new_ptr = unsafe { self.alloc(new_layout) };
198 if !new_ptr.is_null() {
199 // SAFETY: the previously allocated block cannot overlap the newly allocated block.
200 // The safety contract for `dealloc` must be upheld by the caller.
202 ptr::copy_nonoverlapping(ptr, new_ptr, cmp::min(layout.size(), new_size));
203 self.dealloc(ptr, layout);