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Document memory allocation APIs 

Add some docs where they were missing,
attempt to fix them where they were out of date.
This commit is contained in:
Simon Sapin 2018-05-31 18:23:42 +02:00
parent 0081d8826b
commit e9fd063edb
4 changed files with 245 additions and 50 deletions
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65
src/liballoc/alloc.rs
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@ -8,6 +8,8 @@
// option. This file may not be copied, modified, or distributed
// except according to those terms.
//! Memory allocation APIs
#![unstable(feature = "allocator_api",
reason = "the precise API and guarantees it provides may be tweaked \
slightly, especially to possibly take into account the \
@ -37,27 +39,80 @@ extern "Rust" {
fn __rust_alloc_zeroed(size: usize, align: usize) -> *mut u8;
}
/// The global memory allocator.
///
/// This type implements the [`Alloc`] trait by forwarding calls
/// to the allocator registered with the `#[global_allocator]` attribute
/// if there is one, or the `std` crate’s default.
#[derive(Copy, Clone, Default, Debug)]
pub struct 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 [`Alloc`] trait become stable.
///
/// # Safety
///
/// See [`GlobalAlloc::alloc`].
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub unsafe fn alloc(layout: Layout) -> *mut u8 {
__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 [`Alloc`] trait become stable.
///
/// # Safety
///
/// See [`GlobalAlloc::dealloc`].
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub unsafe fn dealloc(ptr: *mut u8, layout: Layout) {
__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 [`Alloc`] trait become stable.
///
/// # Safety
///
/// See [`GlobalAlloc::realloc`].
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub unsafe fn realloc(ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
__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 [`Alloc`] trait become stable.
///
/// # Safety
///
/// See [`GlobalAlloc::alloc_zeroed`].
#[unstable(feature = "allocator_api", issue = "32838")]
#[inline]
pub unsafe fn alloc_zeroed(layout: Layout) -> *mut u8 {
@ -123,6 +178,16 @@ pub(crate) unsafe fn box_free<T: ?Sized>(ptr: Unique<T>) {
}
}
/// Abort on memory allocation error or failure.
///
/// Callers of memory allocation APIs wishing to abort computation
/// in response to an allocation error are encouraged to call this function,
/// rather than directly invoking `panic!` or similar.
///
/// The default behavior of this function is to print a message to standard error
/// and abort the process.
/// It can be replaced with [`std::alloc::set_oom_hook`]
/// and [`std::alloc::take_oom_hook`].
#[rustc_allocator_nounwind]
pub fn oom(layout: Layout) -> ! {
#[allow(improper_ctypes)]

1
src/liballoc_system/lib.rs
View File

@ -44,6 +44,7 @@ const MIN_ALIGN: usize = 16;
use core::alloc::{Alloc, GlobalAlloc, AllocErr, Layout};
use core::ptr::NonNull;
/// The default memory allocator provided by the operating system.
#[unstable(feature = "allocator_api", issue = "32838")]
pub struct System;

227
src/libcore/alloc.rs
View File

@ -8,6 +8,8 @@
// option. This file may not be copied, modified, or distributed
// except according to those terms.
//! Memory allocation APIs
#![unstable(feature = "allocator_api",
reason = "the precise API and guarantees it provides may be tweaked \
slightly, especially to possibly take into account the \
@ -315,8 +317,9 @@ impl Layout {
}
}
/// The parameters given to `Layout::from_size_align` do not satisfy
/// its documented constraints.
/// The parameters given to `Layout::from_size_align`
/// or some other `Layout` constructor
/// do not satisfy its documented constraints.
#[derive(Clone, PartialEq, Eq, Debug)]
pub struct LayoutErr {
private: ()
@ -329,8 +332,8 @@ impl fmt::Display for LayoutErr {
}
}
/// The `AllocErr` error specifies whether an allocation failure is
/// specifically due to resource exhaustion or if it is due to
/// The `AllocErr` error indicates an allocation failure
/// that may be due to resource exhaustion or to
/// something wrong when combining the given input arguments with this
/// allocator.
#[derive(Clone, PartialEq, Eq, Debug)]
@ -346,6 +349,7 @@ impl fmt::Display for AllocErr {
/// The `CannotReallocInPlace` error is used when `grow_in_place` or
/// `shrink_in_place` were unable to reuse the given memory block for
/// a requested layout.
// FIXME: should this be in libcore or liballoc?
#[derive(Clone, PartialEq, Eq, Debug)]
pub struct CannotReallocInPlace;
@ -363,6 +367,7 @@ impl fmt::Display for CannotReallocInPlace {
}
/// Augments `AllocErr` with a CapacityOverflow variant.
// FIXME: should this be in libcore or liballoc?
#[derive(Clone, PartialEq, Eq, Debug)]
#[unstable(feature = "try_reserve", reason = "new API", issue="48043")]
pub enum CollectionAllocErr {
@ -389,27 +394,125 @@ impl From<LayoutErr> for CollectionAllocErr {
}
}
/// A memory allocator that can be registered to be the one backing `std::alloc::Global`
/// A memory allocator that can be registered as the standard library’s default
/// though the `#[global_allocator]` attributes.
///
/// Some of the methods require that a memory block be *currently
/// allocated* via an allocator. This means that:
///
/// * the starting address for that memory block was previously
/// returned by a previous call to an allocation method
/// such as `alloc`, and
///
/// * the memory block has not been subsequently deallocated, where
/// blocks are deallocated either by being passed to a deallocation
/// method such as `dealloc` or by being
/// passed to a reallocation method that returns a non-null pointer.
///
///
/// # Example
///
/// ```no_run
/// use std::alloc::{GlobalAlloc, Layout, alloc};
/// use std::ptr::null_mut;
///
/// struct MyAllocator;
///
/// unsafe impl GlobalAlloc for MyAllocator {
/// unsafe fn alloc(&self, _layout: Layout) -> *mut u8 { null_mut() }
/// unsafe fn dealloc(&self, _ptr: *mut u8, _layout: Layout) {}
/// }
///
/// #[global_allocator]
/// static A: MyAllocator = MyAllocator;
///
/// fn main() {
/// unsafe {
/// assert!(alloc(Layout::new::<u32>()).is_null())
/// }
/// }
/// ```
///
/// # Unsafety
///
/// The `GlobalAlloc` trait is an `unsafe` trait for a number of reasons, and
/// implementors must ensure that they adhere to these contracts:
///
/// * Pointers returned from allocation functions must point to valid memory and
/// retain their validity until at least the instance of `GlobalAlloc` is dropped
/// itself.
///
/// * It's undefined behavior if global allocators unwind. This restriction may
/// be lifted in the future, but currently a panic from any of these
/// functions may lead to memory unsafety.
///
/// * `Layout` queries and calculations in general must be correct. Callers of
/// this trait are allowed to rely on the contracts defined on each method,
/// and implementors must ensure such contracts remain true.
pub unsafe trait GlobalAlloc {
/// Allocate memory as described by the given `layout`.
///
/// Returns a pointer to newly-allocated memory,
/// or NULL to indicate allocation failure.
/// or null to indicate allocation failure.
///
/// # Safety
///
/// **FIXME:** what are the exact requirements?
/// This function is unsafe because undefined behavior can result
/// if the caller does not ensure that `layout` has non-zero size.
///
/// (Extension subtraits might provide more specific bounds on
/// behavior, e.g. guarantee a sentinel address or a null pointer
/// in response to a zero-size allocation request.)
///
/// The allocated block of memory may or may not be initialized.
///
/// # Errors
///
/// Returning a null pointer indicates that either memory is exhausted
/// or `layout` does not meet allocator's size or alignment constraints.
///
/// Implementations are encouraged to return null on memory
/// exhaustion rather than aborting, but this is not
/// a strict requirement. (Specifically: it is *legal* to
/// implement this trait atop an underlying native allocation
/// library that aborts on memory exhaustion.)
///
/// Clients wishing to abort computation in response to an
/// allocation error are encouraged to call the [`oom`] function,
/// rather than directly invoking `panic!` or similar.
unsafe fn alloc(&self, layout: Layout) -> *mut u8;
/// Deallocate the block of memory at the given `ptr` pointer with the given `layout`.
///
/// # Safety
///
/// **FIXME:** what are the exact requirements?
/// In particular around layout *fit*. (See docs for the `Alloc` trait.)
/// This function is unsafe because undefined behavior can result
/// if the caller does not ensure all of the following:
///
/// * `ptr` must denote a block of memory currently allocated via
/// this allocator,
///
/// * `layout` must be the same layout that was used
/// to allocated that block of memory,
unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout);
/// Behaves like `alloc`, but also ensures that the contents
/// are set to zero before being returned.
///
/// # Safety
///
/// This function is unsafe for the same reasons that `alloc` is.
/// However the allocated block of memory is guaranteed to be initialized.
///
/// # Errors
///
/// Returning a null pointer indicates that either memory is exhausted
/// or `layout` does not meet allocator's size or alignment constraints,
/// just as in `alloc`.
///
/// Clients wishing to abort computation in response to an
/// allocation error are encouraged to call the [`oom`] function,
/// rather than directly invoking `panic!` or similar.
unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut u8 {
let size = layout.size();
let ptr = self.alloc(layout);
@ -422,19 +525,51 @@ pub unsafe trait GlobalAlloc {
/// Shink or grow a block of memory to the given `new_size`.
/// The block is described by the given `ptr` pointer and `layout`.
///
/// Return a new pointer (which may or may not be the same as `ptr`),
/// or NULL to indicate reallocation failure.
/// If this returns a non-null pointer, then ownership of the memory block
/// referenced by `ptr` has been transferred to this alloctor.
/// The memory may or may not have been deallocated,
/// and should be considered unusable (unless of course it was
/// transferred back to the caller again via the return value of
/// this method).
///
/// If reallocation is successful, the old `ptr` pointer is considered
/// to have been deallocated.
/// If this method returns null, then ownership of the memory
/// block has not been transferred to this allocator, and the
/// contents of the memory block are unaltered.
///
/// # Safety
///
/// `new_size`, when rounded up to the nearest multiple of `old_layout.align()`,
/// must not overflow (i.e. the rounded value must be less than `usize::MAX`).
/// This function is unsafe because undefined behavior can result
/// if the caller does not ensure all of the following:
///
/// **FIXME:** what are the exact requirements?
/// In particular around layout *fit*. (See docs for the `Alloc` trait.)
/// * `ptr` must be currently allocated via this allocator,
///
/// * `layout` must be the same layout that was used
/// to allocated that block of memory,
///
/// * `new_size` must be greater than zero.
///
/// * `new_size`, when rounded up to the nearest multiple of `layout.align()`,
/// must not overflow (i.e. the rounded value must be less than `usize::MAX`).
///
/// (Extension subtraits might provide more specific bounds on
/// behavior, e.g. guarantee a sentinel address or a null pointer
/// in response to a zero-size allocation request.)
///
/// # Errors
///
/// Returns null if the new layout does not meet the size
/// and alignment constraints of the allocator, or if reallocation
/// otherwise fails.
///
/// Implementations are encouraged to return null on memory
/// exhaustion rather than panicking or aborting, but this is not
/// a strict requirement. (Specifically: it is *legal* to
/// implement this trait atop an underlying native allocation
/// library that aborts on memory exhaustion.)
///
/// Clients wishing to abort computation in response to a
/// reallocation error are encouraged to call the [`oom`] function,
/// rather than directly invoking `panic!` or similar.
unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
let new_layout = Layout::from_size_align_unchecked(new_size, layout.align());
let new_ptr = self.alloc(new_layout);
@ -523,27 +658,21 @@ pub unsafe trait GlobalAlloc {
/// retain their validity until at least the instance of `Alloc` is dropped
/// itself.
///
/// * It's undefined behavior if global allocators unwind. This restriction may
/// be lifted in the future, but currently a panic from any of these
/// functions may lead to memory unsafety. Note that as of the time of this
/// writing allocators *not* intending to be global allocators can still panic
/// in their implementation without violating memory safety.
///
/// * `Layout` queries and calculations in general must be correct. Callers of
/// this trait are allowed to rely on the contracts defined on each method,
/// and implementors must ensure such contracts remain true.
///
/// Note that this list may get tweaked over time as clarifications are made in
/// the future. Additionally global allocators may gain unique requirements for
/// how to safely implement one in the future as well.
/// the future.
pub unsafe trait Alloc {
// (Note: existing allocators have unspecified but well-defined
// (Note: some existing allocators have unspecified but well-defined
// behavior in response to a zero size allocation request ;
// e.g. in C, `malloc` of 0 will either return a null pointer or a
// unique pointer, but will not have arbitrary undefined
// behavior. Rust should consider revising the alloc::heap crate
// to reflect this reality.)
// behavior.
// However in jemalloc for example,
// `mallocx(0)` is documented as undefined behavior.)
/// Returns a pointer meeting the size and alignment guarantees of
/// `layout`.
@ -579,8 +708,8 @@ pub unsafe trait Alloc {
/// library that aborts on memory exhaustion.)
///
/// Clients wishing to abort computation in response to an
/// allocation error are encouraged to call the allocator's `oom`
/// method, rather than directly invoking `panic!` or similar.
/// allocation error are encouraged to call the `oom` function,
/// rather than directly invoking `panic!` or similar.
unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr>;
/// Deallocate the memory referenced by `ptr`.
@ -686,9 +815,9 @@ pub unsafe trait Alloc {
/// implement this trait atop an underlying native allocation
/// library that aborts on memory exhaustion.)
///
/// Clients wishing to abort computation in response to an
/// reallocation error are encouraged to call the allocator's `oom`
/// method, rather than directly invoking `panic!` or similar.
/// Clients wishing to abort computation in response to a
/// reallocation error are encouraged to call the [`oom`] function,
/// rather than directly invoking `panic!` or similar.
unsafe fn realloc(&mut self,
ptr: NonNull<u8>,
layout: Layout,
@ -731,8 +860,8 @@ pub unsafe trait Alloc {
/// constraints, just as in `alloc`.
///
/// Clients wishing to abort computation in response to an
/// allocation error are encouraged to call the allocator's `oom`
/// method, rather than directly invoking `panic!` or similar.
/// allocation error are encouraged to call the [`oom`] function,
/// rather than directly invoking `panic!` or similar.
unsafe fn alloc_zeroed(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
let size = layout.size();
let p = self.alloc(layout);
@ -757,8 +886,8 @@ pub unsafe trait Alloc {
/// constraints, just as in `alloc`.
///
/// Clients wishing to abort computation in response to an
/// allocation error are encouraged to call the allocator's `oom`
/// method, rather than directly invoking `panic!` or similar.
/// allocation error are encouraged to call the [`oom`] function,
/// rather than directly invoking `panic!` or similar.
unsafe fn alloc_excess(&mut self, layout: Layout) -> Result<Excess, AllocErr> {
let usable_size = self.usable_size(&layout);
self.alloc(layout).map(|p| Excess(p, usable_size.1))
@ -778,9 +907,9 @@ pub unsafe trait Alloc {
/// `layout` does not meet allocator's size or alignment
/// constraints, just as in `realloc`.
///
/// Clients wishing to abort computation in response to an
/// reallocation error are encouraged to call the allocator's `oom`
/// method, rather than directly invoking `panic!` or similar.
/// Clients wishing to abort computation in response to a
/// reallocation error are encouraged to call the [`oom`] function,
/// rather than directly invoking `panic!` or similar.
unsafe fn realloc_excess(&mut self,
ptr: NonNull<u8>,
layout: Layout,
@ -823,7 +952,7 @@ pub unsafe trait Alloc {
/// could fit `layout`.
///
/// Note that one cannot pass `CannotReallocInPlace` to the `oom`
/// method; clients are expected either to be able to recover from
/// function; clients are expected either to be able to recover from
/// `grow_in_place` failures without aborting, or to fall back on
/// another reallocation method before resorting to an abort.
unsafe fn grow_in_place(&mut self,
@ -878,7 +1007,7 @@ pub unsafe trait Alloc {
/// could fit `layout`.
///
/// Note that one cannot pass `CannotReallocInPlace` to the `oom`
/// method; clients are expected either to be able to recover from
/// function; clients are expected either to be able to recover from
/// `shrink_in_place` failures without aborting, or to fall back
/// on another reallocation method before resorting to an abort.
unsafe fn shrink_in_place(&mut self,
@ -926,8 +1055,8 @@ pub unsafe trait Alloc {
/// will *not* yield undefined behavior.
///
/// Clients wishing to abort computation in response to an
/// allocation error are encouraged to call the allocator's `oom`
/// method, rather than directly invoking `panic!` or similar.
/// allocation error are encouraged to call the [`oom`] function,
/// rather than directly invoking `panic!` or similar.
fn alloc_one<T>(&mut self) -> Result<NonNull<T>, AllocErr>
where Self: Sized
{
@ -993,8 +1122,8 @@ pub unsafe trait Alloc {
/// Always returns `Err` on arithmetic overflow.
///
/// Clients wishing to abort computation in response to an
/// allocation error are encouraged to call the allocator's `oom`
/// method, rather than directly invoking `panic!` or similar.
/// allocation error are encouraged to call the [`oom`] function,
/// rather than directly invoking `panic!` or similar.
fn alloc_array<T>(&mut self, n: usize) -> Result<NonNull<T>, AllocErr>
where Self: Sized
{
@ -1037,9 +1166,9 @@ pub unsafe trait Alloc {
///
/// Always returns `Err` on arithmetic overflow.
///
/// Clients wishing to abort computation in response to an
/// reallocation error are encouraged to call the allocator's `oom`
/// method, rather than directly invoking `panic!` or similar.
/// Clients wishing to abort computation in response to a
/// reallocation error are encouraged to call the [`oom`] function,
/// rather than directly invoking `panic!` or similar.
unsafe fn realloc_array<T>(&mut self,
ptr: NonNull<T>,
n_old: usize,

2
src/libstd/alloc.rs
View File

@ -8,7 +8,7 @@
// option. This file may not be copied, modified, or distributed
// except according to those terms.
//! dox
//! Memory allocation APIs
#![unstable(issue = "32838", feature = "allocator_api")]