std\sys\alloc/

windows.rs

//! Implements `System` on Windows.
//!
//! All pointers returned by this allocator have, in addition to the guarantees of `GlobalAlloc`, the
//! following properties:
//!
//! If the pointer was allocated or reallocated with a `layout` specifying an alignment <= `MIN_ALIGN`
//! the pointer will be aligned to at least `MIN_ALIGN` and point to the start of the allocated block.
//!
//! If the pointer was allocated or reallocated with a `layout` specifying an alignment > `MIN_ALIGN`
//! the pointer will be aligned to the specified alignment and not point to the start of the allocated block.
//! Instead there will be a header readable directly before the returned pointer, containing the actual
//! location of the start of the block.

use super::{MIN_ALIGN, realloc_fallback};
use crate::alloc::Layout;
use crate::ffi::c_void;
use crate::mem::MaybeUninit;
use crate::ptr;
use crate::sys::c;

#[cfg(test)]
mod tests;

// Heap memory management on Windows is done by using the system Heap API (heapapi.h)
// See https://docs.microsoft.com/windows/win32/api/heapapi/

// Flag to indicate that the memory returned by `HeapAlloc` should be zeroed.
const HEAP_ZERO_MEMORY: u32 = 0x00000008;

// Get a handle to the default heap of the current process, or null if the operation fails.
//
// SAFETY: Successful calls to this function within the same process are assumed to
// always return the same handle, which remains valid for the entire lifetime of the process.
//
// See https://docs.microsoft.com/windows/win32/api/heapapi/nf-heapapi-getprocessheap
windows_link::link!("kernel32.dll" "system" fn GetProcessHeap() -> c::HANDLE);

// Allocate a block of `dwBytes` bytes of memory from a given heap `hHeap`.
// The allocated memory may be uninitialized, or zeroed if `dwFlags` is
// set to `HEAP_ZERO_MEMORY`.
//
// Returns a pointer to the newly-allocated memory or null if the operation fails.
// The returned pointer will be aligned to at least `MIN_ALIGN`.
//
// SAFETY:
//  - `hHeap` must be a non-null handle returned by `GetProcessHeap`.
//  - `dwFlags` must be set to either zero or `HEAP_ZERO_MEMORY`.
//
// Note that `dwBytes` is allowed to be zero, contrary to some other allocators.
//
// See https://docs.microsoft.com/windows/win32/api/heapapi/nf-heapapi-heapalloc
windows_link::link!("kernel32.dll" "system" fn HeapAlloc(hheap: c::HANDLE, dwflags: u32, dwbytes: usize) -> *mut c_void);

// Reallocate a block of memory behind a given pointer `lpMem` from a given heap `hHeap`,
// to a block of at least `dwBytes` bytes, either shrinking the block in place,
// or allocating at a new location, copying memory, and freeing the original location.
//
// Returns a pointer to the reallocated memory or null if the operation fails.
// The returned pointer will be aligned to at least `MIN_ALIGN`.
// If the operation fails the given block will never have been freed.
//
// SAFETY:
//  - `hHeap` must be a non-null handle returned by `GetProcessHeap`.
//  - `dwFlags` must be set to zero.
//  - `lpMem` must be a non-null pointer to an allocated block returned by `HeapAlloc` or
//     `HeapReAlloc`, that has not already been freed.
// If the block was successfully reallocated at a new location, pointers pointing to
// the freed memory, such as `lpMem`, must not be dereferenced ever again.
//
// Note that `dwBytes` is allowed to be zero, contrary to some other allocators.
//
// See https://docs.microsoft.com/windows/win32/api/heapapi/nf-heapapi-heaprealloc
windows_link::link!("kernel32.dll" "system" fn HeapReAlloc(
    hheap: c::HANDLE,
    dwflags : u32,
    lpmem: *const c_void,
    dwbytes: usize
) -> *mut c_void);

// Free a block of memory behind a given pointer `lpMem` from a given heap `hHeap`.
// Returns a nonzero value if the operation is successful, and zero if the operation fails.
//
// SAFETY:
//  - `hHeap` must be a non-null handle returned by `GetProcessHeap`.
//  - `dwFlags` must be set to zero.
//  - `lpMem` must be a pointer to an allocated block returned by `HeapAlloc` or `HeapReAlloc`,
//     that has not already been freed.
// If the block was successfully freed, pointers pointing to the freed memory, such as `lpMem`,
// must not be dereferenced ever again.
//
// Note that `lpMem` is allowed to be null, which will not cause the operation to fail.
//
// See https://docs.microsoft.com/windows/win32/api/heapapi/nf-heapapi-heapfree
windows_link::link!("kernel32.dll" "system" fn HeapFree(hheap: c::HANDLE, dwflags: u32, lpmem: *const c_void) -> c::BOOL);

fn get_process_heap() -> *mut c_void {
    // SAFETY: GetProcessHeap simply returns a valid handle or NULL so is always safe to call.
    unsafe { GetProcessHeap() }
}

#[inline(never)]
fn process_heap_alloc(
    _heap: MaybeUninit<c::HANDLE>, // We pass this argument to match the ABI of `HeapAlloc`,
    flags: u32,
    bytes: usize,
) -> *mut c_void {
    let heap = get_process_heap();
    if core::intrinsics::unlikely(heap.is_null()) {
        return ptr::null_mut();
    }
    // SAFETY: `heap` is a non-null handle returned by `GetProcessHeap`.
    unsafe { HeapAlloc(heap, flags, bytes) }
}

// Header containing a pointer to the start of an allocated block.
// SAFETY: Size and alignment must be <= `MIN_ALIGN`.
#[repr(C)]
struct Header(*mut u8);

// Allocate a block of optionally zeroed memory for a given `layout`.
// SAFETY: Returns a pointer satisfying the guarantees of `System` about allocated pointers,
// or null if the operation fails. If this returns non-null `HEAP` will have been successfully
// initialized.
#[inline]
unsafe fn allocate(layout: Layout, zeroed: bool) -> *mut u8 {
    // Allocated memory will be either zeroed or uninitialized.
    let flags = if zeroed { HEAP_ZERO_MEMORY } else { 0 };

    if layout.align() <= MIN_ALIGN {
        // The returned pointer points to the start of an allocated block.
        process_heap_alloc(MaybeUninit::uninit(), flags, layout.size()) as *mut u8
    } else {
        // Allocate extra padding in order to be able to satisfy the alignment.
        // This addition does not overflow due to `Layout` type invariants,
        // `size()` is at most `isize::MAX` while
        // `align()` is at most `1 << (bits in usize - 2)` if `size()` is non-zero.
        let total = layout.align() + layout.size();

        let ptr = process_heap_alloc(MaybeUninit::uninit(), flags, total) as *mut u8;
        if ptr.is_null() {
            // Allocation has failed.
            return ptr::null_mut();
        }

        // Create a correctly aligned pointer offset from the start of the allocated block,
        // and write a header before it.

        let offset = layout.align() - (ptr.addr() & (layout.align() - 1));
        // SAFETY: `MIN_ALIGN` <= `offset` <= `layout.align()` and the size of the allocated
        // block is `layout.align() + layout.size()`. `aligned` will thus be a correctly aligned
        // pointer inside the allocated block with at least `layout.size()` bytes after it and at
        // least `MIN_ALIGN` bytes of padding before it.
        let aligned = unsafe { ptr.add(offset) };
        // SAFETY: Because the size and alignment of a header is <= `MIN_ALIGN` and `aligned`
        // is aligned to at least `MIN_ALIGN` and has at least `MIN_ALIGN` bytes of padding before
        // it, it is safe to write a header directly before it.
        unsafe { ptr::write((aligned as *mut Header).sub(1), Header(ptr)) };

        // SAFETY: The returned pointer does not point to the start of an allocated block,
        // but there is a header readable directly before it containing the location of the start
        // of the block.
        aligned
    }
}

pub unsafe fn alloc(layout: Layout) -> *mut u8 {
    // SAFETY: Pointers returned by `allocate` satisfy the guarantees of `System`
    let zeroed = false;
    unsafe { allocate(layout, zeroed) }
}

#[inline]
pub unsafe fn alloc_zeroed(layout: Layout) -> *mut u8 {
    // SAFETY: Pointers returned by `allocate` satisfy the guarantees of `System`
    let zeroed = true;
    unsafe { allocate(layout, zeroed) }
}

#[inline]
pub unsafe fn dealloc(ptr: *mut u8, layout: Layout) {
    let block = {
        if layout.align() <= MIN_ALIGN {
            ptr
        } else {
            // The location of the start of the block is stored in the padding before `ptr`.

            // SAFETY: Because of the contract of `System`, `ptr` is guaranteed to be non-null
            // and have a header readable directly before it.
            unsafe { ptr::read((ptr as *mut Header).sub(1)).0 }
        }
    };

    // because `ptr` has been successfully allocated with this allocator,
    // there must be a valid process heap.
    let heap = get_process_heap();

    // SAFETY: `heap` is a non-null handle returned by `GetProcessHeap`,
    // `block` is a pointer to the start of an allocated block.
    unsafe { HeapFree(heap, 0, block.cast::<c_void>()) };
}

#[inline]
pub unsafe fn realloc(ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
    if layout.align() <= MIN_ALIGN {
        // because `ptr` has been successfully allocated with this allocator,
        // there must be a valid process heap.
        let heap = get_process_heap();

        // SAFETY: `heap` is a non-null handle returned by `GetProcessHeap`,
        // `ptr` is a pointer to the start of an allocated block.
        // The returned pointer points to the start of an allocated block.
        unsafe { HeapReAlloc(heap, 0, ptr.cast::<c_void>(), new_size).cast::<u8>() }
    } else {
        // SAFETY: `realloc_fallback` is implemented using `dealloc` and `alloc`, which will
        // correctly handle `ptr` and return a pointer satisfying the guarantees of `System`
        unsafe { realloc_fallback(ptr, layout, new_size) }
    }
}