library/alloc/src/alloc.rs

//! Memory allocation APIs

#![stable(feature = "alloc_module", since = "1.28.0")]

#[cfg(not(test))]
use core::hint;

#[cfg(not(test))]
use core::ptr::{self, NonNull};

#[stable(feature = "alloc_module", since = "1.28.0")]
#[doc(inline)]
pub use core::alloc::*;

#[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
#[cfg_attr(not(bootstrap), lang = "global_alloc_ty")]
pub struct Global;

#[cfg(test)]
pub use std::alloc::Global;

/// Allocate 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 `alloc` 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]
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())
    }
}

/// Deallocate 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 `dealloc` 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]
pub unsafe fn dealloc(ptr: *mut u8, layout: Layout) {
    unsafe { __rust_dealloc(ptr, layout.size(), layout.align()) }
}

/// Reallocate 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 `realloc` method
/// 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]
pub unsafe fn realloc(ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
    unsafe { __rust_realloc(ptr, layout.size(), layout.align(), new_size) }
}

/// Allocate 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 `alloc_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, Layout};
///
/// unsafe {
///     let layout = Layout::new::<u16>();
///     let ptr = alloc_zeroed(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]
pub unsafe fn alloc_zeroed(layout: Layout) -> *mut u8 {
    unsafe { __rust_alloc_zeroed(layout.size(), layout.align()) }
}

#[cfg(not(test))]
impl Global {
    #[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 { 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]
    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]
    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 { dealloc(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 {
                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]
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) -> !;
}

/// Signal 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]
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"))]
    unsafe {
        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,
            )
        }
    }
}

#[cfg(not(no_global_oom_handling))]
/// Specialize clones into pre-allocated, uninitialized memory.
/// Used by `Box::clone` and `Rc`/`Arc::make_mut`.
pub(crate) trait WriteCloneIntoRaw: Sized {
    unsafe fn write_clone_into_raw(&self, target: *mut Self);
}

#[cfg(not(no_global_oom_handling))]
impl<T: Clone> WriteCloneIntoRaw for T {
    #[inline]
    default unsafe fn write_clone_into_raw(&self, target: *mut Self) {
        // Having allocated *first* may allow the optimizer to create
        // the cloned value in-place, skipping the local and move.
        unsafe { target.write(self.clone()) };
    }
}

#[cfg(not(no_global_oom_handling))]
impl<T: Copy> WriteCloneIntoRaw for T {
    #[inline]
    unsafe fn write_clone_into_raw(&self, target: *mut Self) {
        // We can always copy in-place, without ever involving a local value.
        unsafe { target.copy_from_nonoverlapping(self, 1) };
    }
}