1 //! Memory allocation APIs.
3 //! In a given program, the standard library has one “global” memory allocator
4 //! that is used for example by `Box<T>` and `Vec<T>`.
6 //! Currently the default global allocator is unspecified. Libraries, however,
7 //! like `cdylib`s and `staticlib`s are guaranteed to use the [`System`] by
10 //! # The `#[global_allocator]` attribute
12 //! This attribute allows configuring the choice of global allocator.
13 //! You can use this to implement a completely custom global allocator
14 //! to route all default allocation requests to a custom object.
17 //! use std::alloc::{GlobalAlloc, System, Layout};
19 //! struct MyAllocator;
21 //! unsafe impl GlobalAlloc for MyAllocator {
22 //! unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
23 //! System.alloc(layout)
26 //! unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
27 //! System.dealloc(ptr, layout)
31 //! #[global_allocator]
32 //! static GLOBAL: MyAllocator = MyAllocator;
35 //! // This `Vec` will allocate memory through `GLOBAL` above
36 //! let mut v = Vec::new();
41 //! The attribute is used on a `static` item whose type implements the
42 //! [`GlobalAlloc`] trait. This type can be provided by an external library:
44 //! ```rust,ignore (demonstrates crates.io usage)
45 //! extern crate jemallocator;
47 //! use jemallocator::Jemalloc;
49 //! #[global_allocator]
50 //! static GLOBAL: Jemalloc = Jemalloc;
55 //! The `#[global_allocator]` can only be used once in a crate
56 //! or its recursive dependencies.
58 #![deny(unsafe_op_in_unsafe_fn)]
59 #![stable(feature = "alloc_module", since = "1.28.0")]
62 use core::ptr::NonNull;
63 use core::sync::atomic::{AtomicPtr, Ordering};
66 #[stable(feature = "alloc_module", since = "1.28.0")]
68 pub use alloc_crate::alloc::*;
70 /// The default memory allocator provided by the operating system.
72 /// This is based on `malloc` on Unix platforms and `HeapAlloc` on Windows,
73 /// plus related functions.
75 /// This type implements the `GlobalAlloc` trait and Rust programs by default
76 /// work as if they had this definition:
79 /// use std::alloc::System;
81 /// #[global_allocator]
82 /// static A: System = System;
85 /// let a = Box::new(4); // Allocates from the system allocator.
86 /// println!("{}", a);
90 /// You can also define your own wrapper around `System` if you'd like, such as
91 /// keeping track of the number of all bytes allocated:
94 /// use std::alloc::{System, GlobalAlloc, Layout};
95 /// use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
99 /// static ALLOCATED: AtomicUsize = AtomicUsize::new(0);
101 /// unsafe impl GlobalAlloc for Counter {
102 /// unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
103 /// let ret = System.alloc(layout);
104 /// if !ret.is_null() {
105 /// ALLOCATED.fetch_add(layout.size(), SeqCst);
110 /// unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
111 /// System.dealloc(ptr, layout);
112 /// ALLOCATED.fetch_sub(layout.size(), SeqCst);
116 /// #[global_allocator]
117 /// static A: Counter = Counter;
120 /// println!("allocated bytes before main: {}", ALLOCATED.load(SeqCst));
124 /// It can also be used directly to allocate memory independently of whatever
125 /// global allocator has been selected for a Rust program. For example if a Rust
126 /// program opts in to using jemalloc as the global allocator, `System` will
127 /// still allocate memory using `malloc` and `HeapAlloc`.
128 #[stable(feature = "alloc_system_type", since = "1.28.0")]
129 #[derive(Debug, Default, Copy, Clone)]
134 fn alloc_impl(&self, layout: Layout, zeroed: bool) -> Result<NonNull<[u8]>, AllocError> {
135 match layout.size() {
136 0 => Ok(NonNull::slice_from_raw_parts(layout.dangling(), 0)),
137 // SAFETY: `layout` is non-zero in size,
139 let raw_ptr = if zeroed {
140 GlobalAlloc::alloc_zeroed(self, layout)
142 GlobalAlloc::alloc(self, layout)
144 let ptr = NonNull::new(raw_ptr).ok_or(AllocError)?;
145 Ok(NonNull::slice_from_raw_parts(ptr, size))
150 // SAFETY: Same as `Allocator::grow`
158 ) -> Result<NonNull<[u8]>, AllocError> {
160 new_layout.size() >= old_layout.size(),
161 "`new_layout.size()` must be greater than or equal to `old_layout.size()`"
164 match old_layout.size() {
165 0 => self.alloc_impl(new_layout, zeroed),
167 // SAFETY: `new_size` is non-zero as `new_size` is greater than or equal to `old_size`
168 // as required by safety conditions and the `old_size == 0` case was handled in the
169 // previous match arm. Other conditions must be upheld by the caller
170 old_size if old_layout.align() == new_layout.align() => unsafe {
171 let new_size = new_layout.size();
173 // `realloc` probably checks for `new_size >= old_layout.size()` or something similar.
174 intrinsics::assume(new_size >= old_layout.size());
176 let raw_ptr = GlobalAlloc::realloc(self, ptr.as_ptr(), old_layout, new_size);
177 let ptr = NonNull::new(raw_ptr).ok_or(AllocError)?;
179 raw_ptr.add(old_size).write_bytes(0, new_size - old_size);
181 Ok(NonNull::slice_from_raw_parts(ptr, new_size))
184 // SAFETY: because `new_layout.size()` must be greater than or equal to `old_size`,
185 // both the old and new memory allocation are valid for reads and writes for `old_size`
186 // bytes. Also, because the old allocation wasn't yet deallocated, it cannot overlap
187 // `new_ptr`. Thus, the call to `copy_nonoverlapping` is safe. The safety contract
188 // for `dealloc` must be upheld by the caller.
190 let new_ptr = self.alloc_impl(new_layout, zeroed)?;
191 ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_mut_ptr(), old_size);
192 Allocator::deallocate(&self, ptr, old_layout);
199 // The Allocator impl checks the layout size to be non-zero and forwards to the GlobalAlloc impl,
200 // which is in `std::sys::*::alloc`.
201 #[unstable(feature = "allocator_api", issue = "32838")]
202 unsafe impl Allocator for System {
204 fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
205 self.alloc_impl(layout, false)
209 fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
210 self.alloc_impl(layout, true)
214 unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout) {
215 if layout.size() != 0 {
216 // SAFETY: `layout` is non-zero in size,
217 // other conditions must be upheld by the caller
218 unsafe { GlobalAlloc::dealloc(self, ptr.as_ptr(), layout) }
228 ) -> Result<NonNull<[u8]>, AllocError> {
229 // SAFETY: all conditions must be upheld by the caller
230 unsafe { self.grow_impl(ptr, old_layout, new_layout, false) }
234 unsafe fn grow_zeroed(
239 ) -> Result<NonNull<[u8]>, AllocError> {
240 // SAFETY: all conditions must be upheld by the caller
241 unsafe { self.grow_impl(ptr, old_layout, new_layout, true) }
250 ) -> Result<NonNull<[u8]>, AllocError> {
252 new_layout.size() <= old_layout.size(),
253 "`new_layout.size()` must be smaller than or equal to `old_layout.size()`"
256 match new_layout.size() {
257 // SAFETY: conditions must be upheld by the caller
259 Allocator::deallocate(&self, ptr, old_layout);
260 Ok(NonNull::slice_from_raw_parts(new_layout.dangling(), 0))
263 // SAFETY: `new_size` is non-zero. Other conditions must be upheld by the caller
264 new_size if old_layout.align() == new_layout.align() => unsafe {
265 // `realloc` probably checks for `new_size <= old_layout.size()` or something similar.
266 intrinsics::assume(new_size <= old_layout.size());
268 let raw_ptr = GlobalAlloc::realloc(self, ptr.as_ptr(), old_layout, new_size);
269 let ptr = NonNull::new(raw_ptr).ok_or(AllocError)?;
270 Ok(NonNull::slice_from_raw_parts(ptr, new_size))
273 // SAFETY: because `new_size` must be smaller than or equal to `old_layout.size()`,
274 // both the old and new memory allocation are valid for reads and writes for `new_size`
275 // bytes. Also, because the old allocation wasn't yet deallocated, it cannot overlap
276 // `new_ptr`. Thus, the call to `copy_nonoverlapping` is safe. The safety contract
277 // for `dealloc` must be upheld by the caller.
279 let new_ptr = Allocator::allocate(&self, new_layout)?;
280 ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_mut_ptr(), new_size);
281 Allocator::deallocate(&self, ptr, old_layout);
288 static HOOK: AtomicPtr<()> = AtomicPtr::new(ptr::null_mut());
290 /// Registers a custom allocation error hook, replacing any that was previously registered.
292 /// The allocation error hook is invoked when an infallible memory allocation fails, before
293 /// the runtime aborts. The default hook prints a message to standard error,
294 /// but this behavior can be customized with the [`set_alloc_error_hook`] and
295 /// [`take_alloc_error_hook`] functions.
297 /// The hook is provided with a `Layout` struct which contains information
298 /// about the allocation that failed.
300 /// The allocation error hook is a global resource.
301 #[unstable(feature = "alloc_error_hook", issue = "51245")]
302 pub fn set_alloc_error_hook(hook: fn(Layout)) {
303 HOOK.store(hook as *mut (), Ordering::SeqCst);
306 /// Unregisters the current allocation error hook, returning it.
308 /// *See also the function [`set_alloc_error_hook`].*
310 /// If no custom hook is registered, the default hook will be returned.
311 #[unstable(feature = "alloc_error_hook", issue = "51245")]
312 pub fn take_alloc_error_hook() -> fn(Layout) {
313 let hook = HOOK.swap(ptr::null_mut(), Ordering::SeqCst);
314 if hook.is_null() { default_alloc_error_hook } else { unsafe { mem::transmute(hook) } }
317 fn default_alloc_error_hook(layout: Layout) {
318 rtprintpanic!("memory allocation of {} bytes failed\n", layout.size());
323 #[alloc_error_handler]
324 #[unstable(feature = "alloc_internals", issue = "none")]
325 pub fn rust_oom(layout: Layout) -> ! {
326 let hook = HOOK.load(Ordering::SeqCst);
327 let hook: fn(Layout) =
328 if hook.is_null() { default_alloc_error_hook } else { unsafe { mem::transmute(hook) } };
330 crate::process::abort()
335 #[allow(unused_attributes)]
336 #[unstable(feature = "alloc_internals", issue = "none")]
337 pub mod __default_lib_allocator {
338 use super::{GlobalAlloc, Layout, System};
339 // These magic symbol names are used as a fallback for implementing the
340 // `__rust_alloc` etc symbols (see `src/liballoc/alloc.rs`) when there is
341 // no `#[global_allocator]` attribute.
343 // for symbol names src/librustc_ast/expand/allocator.rs
344 // for signatures src/librustc_allocator/lib.rs
346 // linkage directives are provided as part of the current compiler allocator
349 #[rustc_std_internal_symbol]
350 pub unsafe extern "C" fn __rdl_alloc(size: usize, align: usize) -> *mut u8 {
351 // SAFETY: see the guarantees expected by `Layout::from_size_align` and
352 // `GlobalAlloc::alloc`.
354 let layout = Layout::from_size_align_unchecked(size, align);
359 #[rustc_std_internal_symbol]
360 pub unsafe extern "C" fn __rdl_dealloc(ptr: *mut u8, size: usize, align: usize) {
361 // SAFETY: see the guarantees expected by `Layout::from_size_align` and
362 // `GlobalAlloc::dealloc`.
363 unsafe { System.dealloc(ptr, Layout::from_size_align_unchecked(size, align)) }
366 #[rustc_std_internal_symbol]
367 pub unsafe extern "C" fn __rdl_realloc(
373 // SAFETY: see the guarantees expected by `Layout::from_size_align` and
374 // `GlobalAlloc::realloc`.
376 let old_layout = Layout::from_size_align_unchecked(old_size, align);
377 System.realloc(ptr, old_layout, new_size)
381 #[rustc_std_internal_symbol]
382 pub unsafe extern "C" fn __rdl_alloc_zeroed(size: usize, align: usize) -> *mut u8 {
383 // SAFETY: see the guarantees expected by `Layout::from_size_align` and
384 // `GlobalAlloc::alloc_zeroed`.
386 let layout = Layout::from_size_align_unchecked(size, align);
387 System.alloc_zeroed(layout)