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Diffstat (limited to 'sys_util/src/alloc.rs')
-rw-r--r-- | sys_util/src/alloc.rs | 119 |
1 files changed, 119 insertions, 0 deletions
diff --git a/sys_util/src/alloc.rs b/sys_util/src/alloc.rs new file mode 100644 index 0000000..ad73065 --- /dev/null +++ b/sys_util/src/alloc.rs @@ -0,0 +1,119 @@ +use std::alloc::{alloc, alloc_zeroed, dealloc, Layout}; + +/// A contiguous memory allocation with a specified size and alignment, with a +/// Drop impl to perform the deallocation. +/// +/// Conceptually this is like a Box<[u8]> but for which we can select a minimum +/// required alignment at the time of allocation. +/// +/// # Example +/// +/// ``` +/// use std::alloc::Layout; +/// use std::mem; +/// use sys_util::LayoutAllocation; +/// +/// #[repr(C)] +/// struct Header { +/// q: usize, +/// entries: [Entry; 0], // flexible array member +/// } +/// +/// #[repr(C)] +/// struct Entry { +/// e: usize, +/// } +/// +/// fn demo(num_entries: usize) { +/// let size = mem::size_of::<Header>() + num_entries * mem::size_of::<Entry>(); +/// let layout = Layout::from_size_align(size, mem::align_of::<Header>()).unwrap(); +/// let mut allocation = LayoutAllocation::zeroed(layout); +/// +/// // Safe to obtain an exclusive reference because there are no other +/// // references to the allocation yet and all-zero is a valid bit pattern for +/// // our header. +/// let header = unsafe { allocation.as_mut::<Header>() }; +/// } +/// ``` +pub struct LayoutAllocation { + ptr: *mut u8, + layout: Layout, +} + +impl LayoutAllocation { + /// Allocates memory with the specified size and alignment. The content is + /// not initialized. + /// + /// Uninitialized data is not safe to read. Further, it is not safe to + /// obtain a reference to data potentially holding a bit pattern + /// incompatible with its type, for example an uninitialized bool or enum. + pub fn uninitialized(layout: Layout) -> Self { + let ptr = if layout.size() > 0 { + unsafe { + // Safe as long as we guarantee layout.size() > 0. + alloc(layout) + } + } else { + layout.align() as *mut u8 + }; + LayoutAllocation { ptr, layout } + } + + /// Allocates memory with the specified size and alignment and initializes + /// the content to all zero-bytes. + /// + /// Note that zeroing the memory does not necessarily make it safe to obtain + /// a reference to the allocation. Depending on the intended type T, + /// all-zero may or may not be a legal bit pattern for that type. For + /// example obtaining a reference would immediately be undefined behavior if + /// one of the fields has type NonZeroUsize. + pub fn zeroed(layout: Layout) -> Self { + let ptr = if layout.size() > 0 { + unsafe { + // Safe as long as we guarantee layout.size() > 0. + alloc_zeroed(layout) + } + } else { + layout.align() as *mut u8 + }; + LayoutAllocation { ptr, layout } + } + + /// Returns a raw pointer to the allocated data. + pub fn as_ptr<T>(&self) -> *mut T { + self.ptr as *mut T + } + + /// Returns a shared reference to the allocated data. + /// + /// # Safety + /// + /// Caller is responsible for ensuring that the data behind this pointer has + /// been initialized as much as necessary and that there are no already + /// existing mutable references to any part of the data. + pub unsafe fn as_ref<T>(&self) -> &T { + &*self.as_ptr() + } + + /// Returns an exclusive reference to the allocated data. + /// + /// # Safety + /// + /// Caller is responsible for ensuring that the data behind this pointer has + /// been initialized as much as necessary and that there are no already + /// existing references to any part of the data. + pub unsafe fn as_mut<T>(&mut self) -> &mut T { + &mut *self.as_ptr() + } +} + +impl Drop for LayoutAllocation { + fn drop(&mut self) { + if self.layout.size() > 0 { + unsafe { + // Safe as long as we guarantee layout.size() > 0. + dealloc(self.ptr, self.layout); + } + } + } +} |