Struct Segment 

pub struct Segment {
    pub next: *mut Segment,
    pub owner: usize,
    pub foreign: AtomicPtr<u8>,
    pub foreign_bytes: AtomicUsize,
    pub foreign_live: AtomicUsize,
    pub spans: [SpanMeta; 64],
    /* private fields */
}

The header at the base of every segment.

Fields

next: *mut Segment

Intrusive list of a heap’s segments — an allocator cannot use a Vec to track its own memory without recursing into itself.

owner: usize

The shard that owns every span here. Foreign frees find their way home through this.

foreign: AtomicPtr<u8>

Slots freed by a thread other than the owner, as a lock-free stack of slot addresses. See [push_foreign] for why this is push-only.

foreign_bytes: AtomicUsize

Slot bytes parked on foreign, so the accounting can price the list without walking it. Bytes rather than a count: one list carries slots of several classes, so a count cannot be converted back.

AtomicUsize rather than AtomicU64 because 32-bit targets (Cortex-M among them) have no 64-bit atomic, and a pending foreign-free list cannot exceed the address space anyway.

foreign_live: AtomicUsize

Of those, the bytes callers actually asked for.

The owner’s live/rounding counters still include everything on this list, because the thread that freed it cannot touch another thread’s counters. Snapshots move the amount across so it is counted once — see Heap::snapshot.

spans: [SpanMeta; 64]

Per-span bookkeeping, indexed by span number. Index 0 describes the header span itself and is never assigned a class.

Implementations

impl Segment

pub unsafe fn init(base: NonNull<u8>, owner: usize) -> NonNull<Segment>

Initialise a freshly mapped segment in place.

Safety

base must be a live, writable, 4 MiB-aligned mapping of SEGMENT_BYTES bytes that nothing else references.

pub fn span_base(&self, index: usize) -> *mut u8

Base address of span index within this segment.

pub fn is_valid(&self) -> bool

Check the header is one of ours. A false result means a pointer reached dealloc that this allocator never handed out.