alloc/alloc.rs
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//! Memory allocation APIs
#![stable(feature = "alloc_module", since = "1.28.0")]
#[stable(feature = "alloc_module", since = "1.28.0")]
#[doc(inline)]
pub use core::alloc::*;
#[cfg(not(test))]
use core::hint;
#[cfg(not(test))]
use core::ptr::{self, NonNull};
#[cfg(test)]
mod tests;
extern "Rust" {
// These are the magic symbols to call the global allocator. rustc generates
// them to call `__rg_alloc` etc. if there is a `#[global_allocator]` attribute
// (the code expanding that attribute macro generates those functions), or to call
// the default implementations in std (`__rdl_alloc` etc. in `library/std/src/alloc.rs`)
// otherwise.
// The rustc fork of LLVM 14 and earlier also special-cases these function names to be able to optimize them
// like `malloc`, `realloc`, and `free`, respectively.
#[rustc_allocator]
#[rustc_nounwind]
fn __rust_alloc(size: usize, align: usize) -> *mut u8;
#[rustc_deallocator]
#[rustc_nounwind]
fn __rust_dealloc(ptr: *mut u8, size: usize, align: usize);
#[rustc_reallocator]
#[rustc_nounwind]
fn __rust_realloc(ptr: *mut u8, old_size: usize, align: usize, new_size: usize) -> *mut u8;
#[rustc_allocator_zeroed]
#[rustc_nounwind]
fn __rust_alloc_zeroed(size: usize, align: usize) -> *mut u8;
static __rust_no_alloc_shim_is_unstable: u8;
}
/// The global memory allocator.
///
/// This type implements the [`Allocator`] trait by forwarding calls
/// to the allocator registered with the `#[global_allocator]` attribute
/// if there is one, or the `std` crate’s default.
///
/// Note: while this type is unstable, the functionality it provides can be
/// accessed through the [free functions in `alloc`](self#functions).
#[unstable(feature = "allocator_api", issue = "32838")]
#[derive(Copy, Clone, Default, Debug)]
#[cfg(not(test))]
// the compiler needs to know when a Box uses the global allocator vs a custom one
#[lang = "global_alloc_ty"]
pub struct Global;
#[cfg(test)]
pub use std::alloc::Global;
/// Allocates memory with the global allocator.
///
/// This function forwards calls to the [`GlobalAlloc::alloc`] method
/// of the allocator registered with the `#[global_allocator]` attribute
/// if there is one, or the `std` crate’s default.
///
/// This function is expected to be deprecated in favor of the `allocate` method
/// of the [`Global`] type when it and the [`Allocator`] trait become stable.
///
/// # Safety
///
/// See [`GlobalAlloc::alloc`].
///
/// # Examples
///
/// ```
/// use std::alloc::{alloc, dealloc, handle_alloc_error, Layout};
///
/// unsafe {
/// let layout = Layout::new::<u16>();
/// let ptr = alloc(layout);
/// if ptr.is_null() {
/// handle_alloc_error(layout);
/// }
///
/// *(ptr as *mut u16) = 42;
/// assert_eq!(*(ptr as *mut u16), 42);
///
/// dealloc(ptr, layout);
/// }
/// ```
#[stable(feature = "global_alloc", since = "1.28.0")]
#[must_use = "losing the pointer will leak memory"]
#[inline]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
pub unsafe fn alloc(layout: Layout) -> *mut u8 {
unsafe {
// Make sure we don't accidentally allow omitting the allocator shim in
// stable code until it is actually stabilized.
core::ptr::read_volatile(&__rust_no_alloc_shim_is_unstable);
__rust_alloc(layout.size(), layout.align())
}
}
/// Deallocates memory with the global allocator.
///
/// This function forwards calls to the [`GlobalAlloc::dealloc`] method
/// of the allocator registered with the `#[global_allocator]` attribute
/// if there is one, or the `std` crate’s default.
///
/// This function is expected to be deprecated in favor of the `deallocate` method
/// of the [`Global`] type when it and the [`Allocator`] trait become stable.
///
/// # Safety
///
/// See [`GlobalAlloc::dealloc`].
#[stable(feature = "global_alloc", since = "1.28.0")]
#[inline]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
pub unsafe fn dealloc(ptr: *mut u8, layout: Layout) {
unsafe { __rust_dealloc(ptr, layout.size(), layout.align()) }
}
/// Reallocates memory with the global allocator.
///
/// This function forwards calls to the [`GlobalAlloc::realloc`] method
/// of the allocator registered with the `#[global_allocator]` attribute
/// if there is one, or the `std` crate’s default.
///
/// This function is expected to be deprecated in favor of the `grow` and `shrink` methods
/// of the [`Global`] type when it and the [`Allocator`] trait become stable.
///
/// # Safety
///
/// See [`GlobalAlloc::realloc`].
#[stable(feature = "global_alloc", since = "1.28.0")]
#[must_use = "losing the pointer will leak memory"]
#[inline]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
pub unsafe fn realloc(ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
unsafe { __rust_realloc(ptr, layout.size(), layout.align(), new_size) }
}
/// Allocates zero-initialized memory with the global allocator.
///
/// This function forwards calls to the [`GlobalAlloc::alloc_zeroed`] method
/// of the allocator registered with the `#[global_allocator]` attribute
/// if there is one, or the `std` crate’s default.
///
/// This function is expected to be deprecated in favor of the `allocate_zeroed` method
/// of the [`Global`] type when it and the [`Allocator`] trait become stable.
///
/// # Safety
///
/// See [`GlobalAlloc::alloc_zeroed`].
///
/// # Examples
///
/// ```
/// use std::alloc::{alloc_zeroed, dealloc, handle_alloc_error, Layout};
///
/// unsafe {
/// let layout = Layout::new::<u16>();
/// let ptr = alloc_zeroed(layout);
/// if ptr.is_null() {
/// handle_alloc_error(layout);
/// }
///
/// assert_eq!(*(ptr as *mut u16), 0);
///
/// dealloc(ptr, layout);
/// }
/// ```
#[stable(feature = "global_alloc", since = "1.28.0")]
#[must_use = "losing the pointer will leak memory"]
#[inline]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
pub unsafe fn alloc_zeroed(layout: Layout) -> *mut u8 {
unsafe {
// Make sure we don't accidentally allow omitting the allocator shim in
// stable code until it is actually stabilized.
core::ptr::read_volatile(&__rust_no_alloc_shim_is_unstable);
__rust_alloc_zeroed(layout.size(), layout.align())
}
}
#[cfg(not(test))]
impl Global {
#[inline]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
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 { alloc_zeroed(layout) } else { alloc(layout) };
let ptr = NonNull::new(raw_ptr).ok_or(AllocError)?;
Ok(NonNull::slice_from_raw_parts(ptr, size))
},
}
}
// SAFETY: Same as `Allocator::grow`
#[inline]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
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 `old_size` is greater than or equal to `new_size`
// as required by safety conditions. 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.
hint::assert_unchecked(new_size >= old_layout.size());
let raw_ptr = realloc(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);
self.deallocate(ptr, old_layout);
Ok(new_ptr)
},
}
}
}
#[unstable(feature = "allocator_api", issue = "32838")]
#[cfg(not(test))]
unsafe impl Allocator for Global {
#[inline]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
self.alloc_impl(layout, false)
}
#[inline]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
fn allocate_zeroed(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
self.alloc_impl(layout, true)
}
#[inline]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
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 { dealloc(ptr.as_ptr(), layout) }
}
}
#[inline]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
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]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
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]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
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 {
self.deallocate(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.
hint::assert_unchecked(new_size <= old_layout.size());
let raw_ptr = realloc(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 = self.allocate(new_layout)?;
ptr::copy_nonoverlapping(ptr.as_ptr(), new_ptr.as_mut_ptr(), new_size);
self.deallocate(ptr, old_layout);
Ok(new_ptr)
},
}
}
}
/// The allocator for unique pointers.
#[cfg(all(not(no_global_oom_handling), not(test)))]
#[lang = "exchange_malloc"]
#[inline]
#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
let layout = unsafe { Layout::from_size_align_unchecked(size, align) };
match Global.allocate(layout) {
Ok(ptr) => ptr.as_mut_ptr(),
Err(_) => handle_alloc_error(layout),
}
}
// # Allocation error handler
#[cfg(not(no_global_oom_handling))]
extern "Rust" {
// This is the magic symbol to call the global alloc error handler. rustc generates
// it to call `__rg_oom` if there is a `#[alloc_error_handler]`, or to call the
// default implementations below (`__rdl_oom`) otherwise.
fn __rust_alloc_error_handler(size: usize, align: usize) -> !;
}
/// Signals a memory allocation error.
///
/// Callers of memory allocation APIs wishing to cease execution
/// in response to an allocation error are encouraged to call this function,
/// rather than directly invoking [`panic!`] or similar.
///
/// This function is guaranteed to diverge (not return normally with a value), but depending on
/// global configuration, it may either panic (resulting in unwinding or aborting as per
/// configuration for all panics), or abort the process (with no unwinding).
///
/// The default behavior is:
///
/// * If the binary links against `std` (typically the case), then
/// print a message to standard error and abort the process.
/// This behavior can be replaced with [`set_alloc_error_hook`] and [`take_alloc_error_hook`].
/// Future versions of Rust may panic by default instead.
///
/// * If the binary does not link against `std` (all of its crates are marked
/// [`#![no_std]`][no_std]), then call [`panic!`] with a message.
/// [The panic handler] applies as to any panic.
///
/// [`set_alloc_error_hook`]: ../../std/alloc/fn.set_alloc_error_hook.html
/// [`take_alloc_error_hook`]: ../../std/alloc/fn.take_alloc_error_hook.html
/// [The panic handler]: https://doc.rust-lang.org/reference/runtime.html#the-panic_handler-attribute
/// [no_std]: https://doc.rust-lang.org/reference/names/preludes.html#the-no_std-attribute
#[stable(feature = "global_alloc", since = "1.28.0")]
#[rustc_const_unstable(feature = "const_alloc_error", issue = "92523")]
#[cfg(all(not(no_global_oom_handling), not(test)))]
#[cold]
#[optimize(size)]
pub const fn handle_alloc_error(layout: Layout) -> ! {
const fn ct_error(_: Layout) -> ! {
panic!("allocation failed");
}
#[inline]
fn rt_error(layout: Layout) -> ! {
unsafe {
__rust_alloc_error_handler(layout.size(), layout.align());
}
}
#[cfg(not(feature = "panic_immediate_abort"))]
{
core::intrinsics::const_eval_select((layout,), ct_error, rt_error)
}
#[cfg(feature = "panic_immediate_abort")]
ct_error(layout)
}
// For alloc test `std::alloc::handle_alloc_error` can be used directly.
#[cfg(all(not(no_global_oom_handling), test))]
pub use std::alloc::handle_alloc_error;
#[cfg(all(not(no_global_oom_handling), not(test)))]
#[doc(hidden)]
#[allow(unused_attributes)]
#[unstable(feature = "alloc_internals", issue = "none")]
pub mod __alloc_error_handler {
// called via generated `__rust_alloc_error_handler` if there is no
// `#[alloc_error_handler]`.
#[rustc_std_internal_symbol]
pub unsafe fn __rdl_oom(size: usize, _align: usize) -> ! {
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;
}
if unsafe { __rust_alloc_error_handler_should_panic != 0 } {
panic!("memory allocation of {size} bytes failed")
} else {
core::panicking::panic_nounwind_fmt(
format_args!("memory allocation of {size} bytes failed"),
/* force_no_backtrace */ false,
)
}
}
}