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 //! [`System`]: struct.System.html
12 //! # The `#[global_allocator]` attribute
14 //! This attribute allows configuring the choice of global allocator.
15 //! You can use this to implement a completely custom global allocator
16 //! to route all default allocation requests to a custom object.
19 //! use std::alloc::{GlobalAlloc, System, Layout};
21 //! struct MyAllocator;
23 //! unsafe impl GlobalAlloc for MyAllocator {
24 //! unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
25 //! System.alloc(layout)
28 //! unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
29 //! System.dealloc(ptr, layout)
33 //! #[global_allocator]
34 //! static GLOBAL: MyAllocator = MyAllocator;
37 //! // This `Vec` will allocate memory through `GLOBAL` above
38 //! let mut v = Vec::new();
43 //! The attribute is used on a `static` item whose type implements the
44 //! [`GlobalAlloc`] trait. This type can be provided by an external library:
46 //! [`GlobalAlloc`]: ../../core/alloc/trait.GlobalAlloc.html
48 //! ```rust,ignore (demonstrates crates.io usage)
49 //! extern crate jemallocator;
51 //! use jemallocator::Jemalloc;
53 //! #[global_allocator]
54 //! static GLOBAL: Jemalloc = Jemalloc;
59 //! The `#[global_allocator]` can only be used once in a crate
60 //! or its recursive dependencies.
62 #![stable(feature = "alloc_module", since = "1.28.0")]
64 use core::ptr::NonNull;
65 use core::sync::atomic::{AtomicPtr, Ordering};
68 use crate::sys_common::util::dumb_print;
70 #[stable(feature = "alloc_module", since = "1.28.0")]
72 pub use alloc_crate::alloc::*;
74 /// The default memory allocator provided by the operating system.
76 /// This is based on `malloc` on Unix platforms and `HeapAlloc` on Windows,
77 /// plus related functions.
79 /// This type implements the `GlobalAlloc` trait and Rust programs by default
80 /// work as if they had this definition:
83 /// use std::alloc::System;
85 /// #[global_allocator]
86 /// static A: System = System;
89 /// let a = Box::new(4); // Allocates from the system allocator.
90 /// println!("{}", a);
94 /// You can also define your own wrapper around `System` if you'd like, such as
95 /// keeping track of the number of all bytes allocated:
98 /// use std::alloc::{System, GlobalAlloc, Layout};
99 /// use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
103 /// static ALLOCATED: AtomicUsize = AtomicUsize::new(0);
105 /// unsafe impl GlobalAlloc for Counter {
106 /// unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
107 /// let ret = System.alloc(layout);
108 /// if !ret.is_null() {
109 /// ALLOCATED.fetch_add(layout.size(), SeqCst);
114 /// unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
115 /// System.dealloc(ptr, layout);
116 /// ALLOCATED.fetch_sub(layout.size(), SeqCst);
120 /// #[global_allocator]
121 /// static A: Counter = Counter;
124 /// println!("allocated bytes before main: {}", ALLOCATED.load(SeqCst));
128 /// It can also be used directly to allocate memory independently of whatever
129 /// global allocator has been selected for a Rust program. For example if a Rust
130 /// program opts in to using jemalloc as the global allocator, `System` will
131 /// still allocate memory using `malloc` and `HeapAlloc`.
132 #[stable(feature = "alloc_system_type", since = "1.28.0")]
133 #[derive(Debug, Default, Copy, Clone)]
136 // The AllocRef impl just forwards to the GlobalAlloc impl, which is in `std::sys::*::alloc`.
137 #[unstable(feature = "allocator_api", issue = "32838")]
138 unsafe impl AllocRef for System {
140 unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
141 NonNull::new(GlobalAlloc::alloc(self, layout)).ok_or(AllocErr)
145 unsafe fn alloc_zeroed(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
146 NonNull::new(GlobalAlloc::alloc_zeroed(self, layout)).ok_or(AllocErr)
150 unsafe fn dealloc(&mut self, ptr: NonNull<u8>, layout: Layout) {
151 GlobalAlloc::dealloc(self, ptr.as_ptr(), layout)
160 ) -> Result<NonNull<u8>, AllocErr> {
161 NonNull::new(GlobalAlloc::realloc(self, ptr.as_ptr(), layout, new_size)).ok_or(AllocErr)
165 static HOOK: AtomicPtr<()> = AtomicPtr::new(ptr::null_mut());
167 /// Registers a custom allocation error hook, replacing any that was previously registered.
169 /// The allocation error hook is invoked when an infallible memory allocation fails, before
170 /// the runtime aborts. The default hook prints a message to standard error,
171 /// but this behavior can be customized with the [`set_alloc_error_hook`] and
172 /// [`take_alloc_error_hook`] functions.
174 /// The hook is provided with a `Layout` struct which contains information
175 /// about the allocation that failed.
177 /// The allocation error hook is a global resource.
179 /// [`set_alloc_error_hook`]: fn.set_alloc_error_hook.html
180 /// [`take_alloc_error_hook`]: fn.take_alloc_error_hook.html
181 #[unstable(feature = "alloc_error_hook", issue = "51245")]
182 pub fn set_alloc_error_hook(hook: fn(Layout)) {
183 HOOK.store(hook as *mut (), Ordering::SeqCst);
186 /// Unregisters the current allocation error hook, returning it.
188 /// *See also the function [`set_alloc_error_hook`].*
190 /// If no custom hook is registered, the default hook will be returned.
192 /// [`set_alloc_error_hook`]: fn.set_alloc_error_hook.html
193 #[unstable(feature = "alloc_error_hook", issue = "51245")]
194 pub fn take_alloc_error_hook() -> fn(Layout) {
195 let hook = HOOK.swap(ptr::null_mut(), Ordering::SeqCst);
196 if hook.is_null() { default_alloc_error_hook } else { unsafe { mem::transmute(hook) } }
199 fn default_alloc_error_hook(layout: Layout) {
200 dumb_print(format_args!("memory allocation of {} bytes failed", layout.size()));
205 #[alloc_error_handler]
206 #[unstable(feature = "alloc_internals", issue = "none")]
207 pub fn rust_oom(layout: Layout) -> ! {
208 let hook = HOOK.load(Ordering::SeqCst);
209 let hook: fn(Layout) =
210 if hook.is_null() { default_alloc_error_hook } else { unsafe { mem::transmute(hook) } };
213 crate::sys::abort_internal();
219 #[allow(unused_attributes)]
220 #[unstable(feature = "alloc_internals", issue = "none")]
221 pub mod __default_lib_allocator {
222 use super::{GlobalAlloc, Layout, System};
223 // These magic symbol names are used as a fallback for implementing the
224 // `__rust_alloc` etc symbols (see `src/liballoc/alloc.rs) when there is
225 // no `#[global_allocator]` attribute.
227 // for symbol names src/librustc/middle/allocator.rs
228 // for signatures src/librustc_allocator/lib.rs
230 // linkage directives are provided as part of the current compiler allocator
233 #[rustc_std_internal_symbol]
234 pub unsafe extern "C" fn __rdl_alloc(size: usize, align: usize) -> *mut u8 {
235 let layout = Layout::from_size_align_unchecked(size, align);
239 #[rustc_std_internal_symbol]
240 pub unsafe extern "C" fn __rdl_dealloc(ptr: *mut u8, size: usize, align: usize) {
241 System.dealloc(ptr, Layout::from_size_align_unchecked(size, align))
244 #[rustc_std_internal_symbol]
245 pub unsafe extern "C" fn __rdl_realloc(
251 let old_layout = Layout::from_size_align_unchecked(old_size, align);
252 System.realloc(ptr, old_layout, new_size)
255 #[rustc_std_internal_symbol]
256 pub unsafe extern "C" fn __rdl_alloc_zeroed(size: usize, align: usize) -> *mut u8 {
257 let layout = Layout::from_size_align_unchecked(size, align);
258 System.alloc_zeroed(layout)