mirror of https://github.com/rust-lang/nomicon
Alexis Beingessner
9 years ago
committed by
Manish Goregaokar
parent
9f20f453b2
commit
1809c4defb
@ -0,0 +1,176 @@
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% Handling Zero-Sized Types
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It's time. We're going to fight the spectre that is zero-sized types. Safe Rust
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*never* needs to care about this, but Vec is very intensive on raw pointers and
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raw allocations, which are exactly the *only* two things that care about
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zero-sized types. We need to be careful of two things:
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* The raw allocator API has undefined behaviour if you pass in 0 for an
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allocation size.
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* raw pointer offsets are no-ops for zero-sized types, which will break our
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C-style pointer iterator.
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Thankfully we abstracted out pointer-iterators and allocating handling into
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RawValIter and RawVec respectively. How mysteriously convenient.
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## Allocating Zero-Sized Types
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So if the allocator API doesn't support zero-sized allocations, what on earth
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do we store as our allocation? Why, `heap::EMPTY` of course! Almost every operation
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with a ZST is a no-op since ZSTs have exactly one value, and therefore no state needs
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to be considered to store or load them. This actually extends to `ptr::read` and
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`ptr::write`: they won't actually look at the pointer at all. As such we *never* need
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to change the pointer.
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Note however that our previous reliance on running out of memory before overflow is
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no longer valid with zero-sized types. We must explicitly guard against capacity
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overflow for zero-sized types.
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Due to our current architecture, all this means is writing 3 guards, one in each
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method of RawVec.
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```rust,ignore
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impl<T> RawVec<T> {
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fn new() -> Self {
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unsafe {
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// !0 is usize::MAX. This branch should be stripped at compile time.
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let cap = if mem::size_of::<T>() == 0 { !0 } else { 0 };
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// heap::EMPTY doubles as "unallocated" and "zero-sized allocation"
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RawVec { ptr: Unique::new(heap::EMPTY as *mut T), cap: cap }
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}
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}
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fn grow(&mut self) {
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unsafe {
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let elem_size = mem::size_of::<T>();
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// since we set the capacity to usize::MAX when elem_size is
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// 0, getting to here necessarily means the Vec is overfull.
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assert!(elem_size != 0, "capacity overflow");
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let align = mem::align_of::<T>();
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let (new_cap, ptr) = if self.cap == 0 {
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let ptr = heap::allocate(elem_size, align);
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(1, ptr)
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} else {
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let new_cap = 2 * self.cap;
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let ptr = heap::reallocate(*self.ptr as *mut _,
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self.cap * elem_size,
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new_cap * elem_size,
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align);
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(new_cap, ptr)
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};
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// If allocate or reallocate fail, we'll get `null` back
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if ptr.is_null() { oom() }
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self.ptr = Unique::new(ptr as *mut _);
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self.cap = new_cap;
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}
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}
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}
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impl<T> Drop for RawVec<T> {
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fn drop(&mut self) {
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let elem_size = mem::size_of::<T>();
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// don't free zero-sized allocations, as they were never allocated.
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if self.cap != 0 && elem_size != 0 {
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let align = mem::align_of::<T>();
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let num_bytes = elem_size * self.cap;
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unsafe {
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heap::deallocate(*self.ptr as *mut _, num_bytes, align);
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}
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}
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}
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}
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```
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That's it. We support pushing and popping zero-sized types now. Our iterators
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(that aren't provided by slice Deref) are still busted, though.
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## Iterating Zero-Sized Types
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Zero-sized offsets are no-ops. This means that our current design will always
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initialize `start` and `end` as the same value, and our iterators will yield
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nothing. The current solution to this is to cast the pointers to integers,
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increment, and then cast them back:
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```rust,ignore
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impl<T> RawValIter<T> {
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unsafe fn new(slice: &[T]) -> Self {
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RawValIter {
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start: slice.as_ptr(),
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end: if mem::size_of::<T>() == 0 {
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((slice.as_ptr() as usize) + slice.len()) as *const _
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} else if slice.len() == 0 {
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slice.as_ptr()
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} else {
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slice.as_ptr().offset(slice.len() as isize)
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}
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}
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}
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}
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```
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Now we have a different bug. Instead of our iterators not running at all, our
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iterators now run *forever*. We need to do the same trick in our iterator impls.
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Also, our size_hint computation code will divide by 0 for ZSTs. Since we'll
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basically be treating the two pointers as if they point to bytes, we'll just
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map size 0 to divide by 1.
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```rust,ignore
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impl<T> Iterator for RawValIter<T> {
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type Item = T;
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fn next(&mut self) -> Option<T> {
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if self.start == self.end {
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None
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} else {
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unsafe {
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let result = ptr::read(self.start);
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self.start = if mem::size_of::<T>() == 0 {
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(self.start as usize + 1) as *const _
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} else {
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self.start.offset(1);
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}
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Some(result)
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}
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}
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}
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fn size_hint(&self) -> (usize, Option<usize>) {
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let elem_size = mem::size_of::<T>();
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let len = (self.end as usize - self.start as usize)
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/ if elem_size == 0 { 1 } else { elem_size };
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(len, Some(len))
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}
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}
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impl<T> DoubleEndedIterator for RawValIter<T> {
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fn next_back(&mut self) -> Option<T> {
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if self.start == self.end {
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None
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} else {
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unsafe {
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self.end = if mem::size_of::<T>() == 0 {
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(self.end as usize - 1) as *const _
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} else {
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self.end.offset(-1);
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}
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Some(ptr::read(self.end))
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}
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}
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}
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}
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```
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And that's it. Iteration works!
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