src/libcore/alloc/global.rs

   1 use crate::alloc::Layout;
   2 use crate::cmp;
   3 use crate::ptr;
   4
   5 /// A memory allocator that can be registered as the standard library’s default
   6 /// through the `#[global_allocator]` attribute.
   7 ///
   8 /// Some of the methods require that a memory block be *currently
   9 /// allocated* via an allocator. This means that:
  10 ///
  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
  14 ///
  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.
  19 ///
  20 ///
  21 /// # Example
  22 ///
  23 /// ```no_run
  24 /// use std::alloc::{GlobalAlloc, Layout, alloc};
  25 /// use std::ptr::null_mut;
  26 ///
  27 /// struct MyAllocator;
  28 ///
  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) {}
  32 /// }
  33 ///
  34 /// #[global_allocator]
  35 /// static A: MyAllocator = MyAllocator;
  36 ///
  37 /// fn main() {
  38 ///     unsafe {
  39 ///         assert!(alloc(Layout::new::<u32>()).is_null())
  40 ///     }
  41 /// }
  42 /// ```
  43 ///
  44 /// # Safety
  45 ///
  46 /// The `GlobalAlloc` trait is an `unsafe` trait for a number of reasons, and
  47 /// implementors must ensure that they adhere to these contracts:
  48 ///
  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.
  52 ///
  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`.
  59     ///
  60     /// Returns a pointer to newly-allocated memory,
  61     /// or null to indicate allocation failure.
  62     ///
  63     /// # Safety
  64     ///
  65     /// This function is unsafe because undefined behavior can result
  66     /// if the caller does not ensure that `layout` has non-zero size.
  67     ///
  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.)
  71     ///
  72     /// The allocated block of memory may or may not be initialized.
  73     ///
  74     /// # Errors
  75     ///
  76     /// Returning a null pointer indicates that either memory is exhausted
  77     /// or `layout` does not meet this allocator's size or alignment constraints.
  78     ///
  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.)
  84     ///
  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.
  88     ///
  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;
  92
  93     /// Deallocate the block of memory at the given `ptr` pointer with the given `layout`.
  94     ///
  95     /// # Safety
  96     ///
  97     /// This function is unsafe because undefined behavior can result
  98     /// if the caller does not ensure all of the following:
  99     ///
 100     /// * `ptr` must denote a block of memory currently allocated via
 101     ///   this allocator,
 102     ///
 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);
 107
 108     /// Behaves like `alloc`, but also ensures that the contents
 109     /// are set to zero before being returned.
 110     ///
 111     /// # Safety
 112     ///
 113     /// This function is unsafe for the same reasons that `alloc` is.
 114     /// However the allocated block of memory is guaranteed to be initialized.
 115     ///
 116     /// # Errors
 117     ///
 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`.
 121     ///
 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.
 125     ///
 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         let ptr = self.alloc(layout);
 131         if !ptr.is_null() {
 132             ptr::write_bytes(ptr, 0, size);
 133         }
 134         ptr
 135     }
 136
 137     /// Shrink or grow a block of memory to the given `new_size`.
 138     /// The block is described by the given `ptr` pointer and `layout`.
 139     ///
 140     /// If this returns a non-null pointer, then ownership of the memory block
 141     /// referenced by `ptr` has been transferred to this allocator.
 142     /// The memory may or may not have been deallocated,
 143     /// and should be considered unusable (unless of course it was
 144     /// transferred back to the caller again via the return value of
 145     /// this method). The new memory block is allocated with `layout`, but
 146     /// with the `size` updated to `new_size`.
 147     ///
 148     /// If this method returns null, then ownership of the memory
 149     /// block has not been transferred to this allocator, and the
 150     /// contents of the memory block are unaltered.
 151     ///
 152     /// # Safety
 153     ///
 154     /// This function is unsafe because undefined behavior can result
 155     /// if the caller does not ensure all of the following:
 156     ///
 157     /// * `ptr` must be currently allocated via this allocator,
 158     ///
 159     /// * `layout` must be the same layout that was used
 160     ///   to allocate that block of memory,
 161     ///
 162     /// * `new_size` must be greater than zero.
 163     ///
 164     /// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
 165     ///   must not overflow (i.e., the rounded value must be less than `usize::MAX`).
 166     ///
 167     /// (Extension subtraits might provide more specific bounds on
 168     /// behavior, e.g., guarantee a sentinel address or a null pointer
 169     /// in response to a zero-size allocation request.)
 170     ///
 171     /// # Errors
 172     ///
 173     /// Returns null if the new layout does not meet the size
 174     /// and alignment constraints of the allocator, or if reallocation
 175     /// otherwise fails.
 176     ///
 177     /// Implementations are encouraged to return null on memory
 178     /// exhaustion rather than panicking or aborting, but this is not
 179     /// a strict requirement. (Specifically: it is *legal* to
 180     /// implement this trait atop an underlying native allocation
 181     /// library that aborts on memory exhaustion.)
 182     ///
 183     /// Clients wishing to abort computation in response to a
 184     /// reallocation error are encouraged to call the [`handle_alloc_error`] function,
 185     /// rather than directly invoking `panic!` or similar.
 186     ///
 187     /// [`handle_alloc_error`]: ../../alloc/alloc/fn.handle_alloc_error.html
 188     #[stable(feature = "global_alloc", since = "1.28.0")]
 189     unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
 190         let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
 191         let new_ptr = self.alloc(new_layout);
 192         if !new_ptr.is_null() {
 193             ptr::copy_nonoverlapping(ptr, new_ptr, cmp::min(layout.size(), new_size));
 194             self.dealloc(ptr, layout);
 195         }
 196         new_ptr
 197     }
 198 }