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alloc.rs
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alloc.rs
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//! Memory allocation APIs.
//!
//! In a given program, the standard library has one “global” memory allocator
//! that is used for example by `Box<T>` and `Vec<T>`.
//!
//! Currently the default global allocator is unspecified. Libraries, however,
//! like `cdylib`s and `staticlib`s are guaranteed to use the [`System`] by
//! default.
//!
//! # The `#[global_allocator]` attribute
//!
//! This attribute allows configuring the choice of global allocator.
//! You can use this to implement a completely custom global allocator
//! to route all default allocation requests to a custom object.
//!
//! ```rust
//! use std::alloc::{GlobalAlloc, System, Layout};
//!
//! struct MyAllocator;
//!
//! unsafe impl GlobalAlloc for MyAllocator {
//! unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
//! System.alloc(layout)
//! }
//!
//! unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
//! System.dealloc(ptr, layout)
//! }
//! }
//!
//! #[global_allocator]
//! static GLOBAL: MyAllocator = MyAllocator;
//!
//! fn main() {
//! // This `Vec` will allocate memory through `GLOBAL` above
//! let mut v = Vec::new();
//! v.push(1);
//! }
//! ```
//!
//! The attribute is used on a `static` item whose type implements the
//! [`GlobalAlloc`] trait. This type can be provided by an external library:
//!
//! ```rust,ignore (demonstrates crates.io usage)
//! use jemallocator::Jemalloc;
//!
//! #[global_allocator]
//! static GLOBAL: Jemalloc = Jemalloc;
//!
//! fn main() {}
//! ```
//!
//! The `#[global_allocator]` can only be used once in a crate
//! or its recursive dependencies.
#![deny(unsafe_op_in_unsafe_fn)]
#![stable(feature = "alloc_module", since = "1.28.0")]
use core::intrinsics;
use core::ptr::NonNull;
use core::sync::atomic::{AtomicPtr, Ordering};
use core::{mem, ptr};
#[stable(feature = "alloc_module", since = "1.28.0")]
#[doc(inline)]
pub use alloc_crate::alloc::*;
/// The default memory allocator provided by the operating system.
///
/// This is based on `malloc` on Unix platforms and `HeapAlloc` on Windows,
/// plus related functions. However, it is not valid to mix use of the backing
/// system allocator with `System`, as this implementation may include extra
/// work, such as to serve alignment requests greater than the alignment
/// provided directly by the backing system allocator.
///
/// This type implements the `GlobalAlloc` trait and Rust programs by default
/// work as if they had this definition:
///
/// ```rust
/// use std::alloc::System;
///
/// #[global_allocator]
/// static A: System = System;
///
/// fn main() {
/// let a = Box::new(4); // Allocates from the system allocator.
/// println!("{a}");
/// }
/// ```
///
/// You can also define your own wrapper around `System` if you'd like, such as
/// keeping track of the number of all bytes allocated:
///
/// ```rust
/// use std::alloc::{System, GlobalAlloc, Layout};
/// use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
///
/// struct Counter;
///
/// static ALLOCATED: AtomicUsize = AtomicUsize::new(0);
///
/// unsafe impl GlobalAlloc for Counter {
/// unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
/// let ret = System.alloc(layout);
/// if !ret.is_null() {
/// ALLOCATED.fetch_add(layout.size(), SeqCst);
/// }
/// ret
/// }
///
/// unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
/// System.dealloc(ptr, layout);
/// ALLOCATED.fetch_sub(layout.size(), SeqCst);
/// }
/// }
///
/// #[global_allocator]
/// static A: Counter = Counter;
///
/// fn main() {
/// println!("allocated bytes before main: {}", ALLOCATED.load(SeqCst));
/// }
/// ```
///
/// It can also be used directly to allocate memory independently of whatever
/// global allocator has been selected for a Rust program. For example if a Rust
/// program opts in to using jemalloc as the global allocator, `System` will
/// still allocate memory using `malloc` and `HeapAlloc`.
#[stable(feature = "alloc_system_type", since = "1.28.0")]
#[derive(Debug, Default, Copy, Clone)]
pub struct System;
impl System {
#[inline]
fn alloc_impl(&self, layout: Layout, zeroed: bool) -> Result<NonNull<[u8]>, AllocError> {
match layout.size() {
0 => Ok(NonNull::slice_from_raw_parts(layout.dangling(), 0)),
// SAFETY: `layout` is non-zero in size,
size => unsafe {
let raw_ptr = if zeroed {
GlobalAlloc::alloc_zeroed(self, layout)
} else {
GlobalAlloc::alloc(self, layout)
};
let ptr = NonNull::new(raw_ptr).ok_or(AllocError)?;
Ok(NonNull::slice_from_raw_parts(ptr, size))
},
}
}
// SAFETY: Same as `Allocator::grow`
#[inline]
unsafe fn grow_impl(
&self,
ptr: NonNull<u8>,
old_layout: Layout,
new_layout: Layout,
zeroed: bool,
) -> Result<NonNull<[u8]>, AllocError> {
debug_assert!(
new_layout.size() >= old_layout.size(),
"`new_layout.size()` must be greater than or equal to `old_layout.size()`"
);
match old_layout.size() {
0 => self.alloc_impl(new_layout, zeroed),
// SAFETY: `new_size` is non-zero as `new_size` is greater than or equal to `old_size`
// as required by safety conditions and the `old_size == 0` case was handled in the
// previous match arm. Other conditions must be upheld by the caller
old_size if old_layout.align() == new_layout.align() => unsafe {
let new_size = new_layout.size();
// `realloc` probably checks for `new_size >= old_layout.size()` or something similar.
intrinsics::assume(new_size >= old_layout.size());
let raw_ptr = GlobalAlloc::realloc(self, ptr.as_ptr(), old_layout, new_size);
let ptr = NonNull::new(raw_ptr).ok_or(AllocError)?;
if zeroed {
raw_ptr.add(old_size).write_bytes(0, new_size - old_size);
}
Ok(NonNull::slice_from_raw_parts(ptr, new_size))
},
// SAFETY: because `new_layout.size()` must be greater than or equal to `old_size`,
// both the old and new memory allocation are valid for reads and writes for `old_size`
// bytes. Also, because the old allocation wasn't yet deallocated, it cannot overlap
// `new_ptr`. Thus, the call to `copy_nonoverlapping` is safe. The safety contract
// for `dealloc` must be upheld by the caller.
old_size => unsafe {
let new_ptr = self.alloc_impl(new_layout, zeroed)?;
ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_mut_ptr(), old_size);
Allocator::deallocate(self, ptr, old_layout);
Ok(new_ptr)
},
}
}
}
// The Allocator impl checks the layout size to be non-zero and forwards to the GlobalAlloc impl,
// which is in `std::sys::*::alloc`.
#[unstable(feature = "allocator_api", issue = "32838")]
unsafe impl Allocator for System {
#[inline]
fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
self.alloc_impl(layout, false)
}
#[inline]
fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
self.alloc_impl(layout, true)
}
#[inline]
unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout) {
if layout.size() != 0 {
// SAFETY: `layout` is non-zero in size,
// other conditions must be upheld by the caller
unsafe { GlobalAlloc::dealloc(self, ptr.as_ptr(), layout) }
}
}
#[inline]
unsafe fn grow(
&self,
ptr: NonNull<u8>,
old_layout: Layout,
new_layout: Layout,
) -> Result<NonNull<[u8]>, AllocError> {
// SAFETY: all conditions must be upheld by the caller
unsafe { self.grow_impl(ptr, old_layout, new_layout, false) }
}
#[inline]
unsafe fn grow_zeroed(
&self,
ptr: NonNull<u8>,
old_layout: Layout,
new_layout: Layout,
) -> Result<NonNull<[u8]>, AllocError> {
// SAFETY: all conditions must be upheld by the caller
unsafe { self.grow_impl(ptr, old_layout, new_layout, true) }
}
#[inline]
unsafe fn shrink(
&self,
ptr: NonNull<u8>,
old_layout: Layout,
new_layout: Layout,
) -> Result<NonNull<[u8]>, AllocError> {
debug_assert!(
new_layout.size() <= old_layout.size(),
"`new_layout.size()` must be smaller than or equal to `old_layout.size()`"
);
match new_layout.size() {
// SAFETY: conditions must be upheld by the caller
0 => unsafe {
Allocator::deallocate(self, ptr, old_layout);
Ok(NonNull::slice_from_raw_parts(new_layout.dangling(), 0))
},
// SAFETY: `new_size` is non-zero. Other conditions must be upheld by the caller
new_size if old_layout.align() == new_layout.align() => unsafe {
// `realloc` probably checks for `new_size <= old_layout.size()` or something similar.
intrinsics::assume(new_size <= old_layout.size());
let raw_ptr = GlobalAlloc::realloc(self, ptr.as_ptr(), old_layout, new_size);
let ptr = NonNull::new(raw_ptr).ok_or(AllocError)?;
Ok(NonNull::slice_from_raw_parts(ptr, new_size))
},
// SAFETY: because `new_size` must be smaller than or equal to `old_layout.size()`,
// both the old and new memory allocation are valid for reads and writes for `new_size`
// bytes. Also, because the old allocation wasn't yet deallocated, it cannot overlap
// `new_ptr`. Thus, the call to `copy_nonoverlapping` is safe. The safety contract
// for `dealloc` must be upheld by the caller.
new_size => unsafe {
let new_ptr = Allocator::allocate(self, new_layout)?;
ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_mut_ptr(), new_size);
Allocator::deallocate(self, ptr, old_layout);
Ok(new_ptr)
},
}
}
}
static HOOK: AtomicPtr<()> = AtomicPtr::new(ptr::null_mut());
/// Registers a custom allocation error hook, replacing any that was previously registered.
///
/// The allocation error hook is invoked when an infallible memory allocation fails, before
/// the runtime aborts. The default hook prints a message to standard error,
/// but this behavior can be customized with the [`set_alloc_error_hook`] and
/// [`take_alloc_error_hook`] functions.
///
/// The hook is provided with a `Layout` struct which contains information
/// about the allocation that failed.
///
/// The allocation error hook is a global resource.
///
/// # Examples
///
/// ```
/// #![feature(alloc_error_hook)]
///
/// use std::alloc::{Layout, set_alloc_error_hook};
///
/// fn custom_alloc_error_hook(layout: Layout) {
/// panic!("memory allocation of {} bytes failed", layout.size());
/// }
///
/// set_alloc_error_hook(custom_alloc_error_hook);
/// ```
#[unstable(feature = "alloc_error_hook", issue = "51245")]
pub fn set_alloc_error_hook(hook: fn(Layout)) {
HOOK.store(hook as *mut (), Ordering::SeqCst);
}
/// Unregisters the current allocation error hook, returning it.
///
/// *See also the function [`set_alloc_error_hook`].*
///
/// If no custom hook is registered, the default hook will be returned.
#[unstable(feature = "alloc_error_hook", issue = "51245")]
pub fn take_alloc_error_hook() -> fn(Layout) {
let hook = HOOK.swap(ptr::null_mut(), Ordering::SeqCst);
if hook.is_null() { default_alloc_error_hook } else { unsafe { mem::transmute(hook) } }
}
fn default_alloc_error_hook(layout: Layout) {
extern "Rust" {
// This symbol is emitted by rustc next to __rust_alloc_error_handler.
// Its value depends on the -Zoom={panic,abort} compiler option.
static __rust_alloc_error_handler_should_panic: u8;
}
#[allow(unused_unsafe)]
if unsafe { __rust_alloc_error_handler_should_panic != 0 } {
panic!("memory allocation of {} bytes failed\n", layout.size());
} else {
rtprintpanic!("memory allocation of {} bytes failed\n", layout.size());
}
}
#[cfg(not(test))]
#[doc(hidden)]
#[alloc_error_handler]
#[unstable(feature = "alloc_internals", issue = "none")]
pub fn rust_oom(layout: Layout) -> ! {
let hook = HOOK.load(Ordering::SeqCst);
let hook: fn(Layout) =
if hook.is_null() { default_alloc_error_hook } else { unsafe { mem::transmute(hook) } };
hook(layout);
crate::process::abort()
}
#[cfg(not(test))]
#[doc(hidden)]
#[allow(unused_attributes)]
#[unstable(feature = "alloc_internals", issue = "none")]
pub mod __default_lib_allocator {
use super::{GlobalAlloc, Layout, System};
// These magic symbol names are used as a fallback for implementing the
// `__rust_alloc` etc symbols (see `src/liballoc/alloc.rs`) when there is
// no `#[global_allocator]` attribute.
// for symbol names src/librustc_ast/expand/allocator.rs
// for signatures src/librustc_allocator/lib.rs
// linkage directives are provided as part of the current compiler allocator
// ABI
#[rustc_std_internal_symbol]
pub unsafe extern "C" fn __rdl_alloc(size: usize, align: usize) -> *mut u8 {
// SAFETY: see the guarantees expected by `Layout::from_size_align` and
// `GlobalAlloc::alloc`.
unsafe {
let layout = Layout::from_size_align_unchecked(size, align);
System.alloc(layout)
}
}
#[rustc_std_internal_symbol]
pub unsafe extern "C" fn __rdl_dealloc(ptr: *mut u8, size: usize, align: usize) {
// SAFETY: see the guarantees expected by `Layout::from_size_align` and
// `GlobalAlloc::dealloc`.
unsafe { System.dealloc(ptr, Layout::from_size_align_unchecked(size, align)) }
}
#[rustc_std_internal_symbol]
pub unsafe extern "C" fn __rdl_realloc(
ptr: *mut u8,
old_size: usize,
align: usize,
new_size: usize,
) -> *mut u8 {
// SAFETY: see the guarantees expected by `Layout::from_size_align` and
// `GlobalAlloc::realloc`.
unsafe {
let old_layout = Layout::from_size_align_unchecked(old_size, align);
System.realloc(ptr, old_layout, new_size)
}
}
#[rustc_std_internal_symbol]
pub unsafe extern "C" fn __rdl_alloc_zeroed(size: usize, align: usize) -> *mut u8 {
// SAFETY: see the guarantees expected by `Layout::from_size_align` and
// `GlobalAlloc::alloc_zeroed`.
unsafe {
let layout = Layout::from_size_align_unchecked(size, align);
System.alloc_zeroed(layout)
}
}
}