Struct capnp::message::SingleSegmentAllocator
source · pub struct SingleSegmentAllocator<'a> { /* private fields */ }
Expand description
An Allocator whose first and only segment is a backed by a user-provided buffer. If the segment fills up, subsequent allocations trigger panics.
The main purpose of this struct is to be used in situations where heap allocation is not available.
Recall that an Allocator
implementation must ensure that allocated segments are
initially zeroed. SingleSegmentAllocator
ensures that is the case by zeroing
the entire buffer upon initial construction, and then zeroing any potentially used
part of the buffer upon deallocate_segment()
.
You can reuse a SingleSegmentAllocator
by calling message::Builder::into_allocator()
,
or by initially passing it to message::Builder::new()
as a &mut SingleSegmentAllocator
.
Such reuse can save significant amounts of zeroing.
Implementations§
source§impl<'a> SingleSegmentAllocator<'a>
impl<'a> SingleSegmentAllocator<'a>
sourcepub fn new(segment: &'a mut [u8]) -> SingleSegmentAllocator<'a>
pub fn new(segment: &'a mut [u8]) -> SingleSegmentAllocator<'a>
Writes zeroes into the entire buffer and constructs a new allocator from it.
If the buffer is large, this operation could be relatively expensive. If you want to reuse
the same scratch space in a later message, you should reuse the entire
SingleSegmentAllocator
, to avoid paying this full cost again.
Trait Implementations§
source§impl<'a> Allocator for SingleSegmentAllocator<'a>
impl<'a> Allocator for SingleSegmentAllocator<'a>
source§fn allocate_segment(&mut self, minimum_size: u32) -> (*mut u8, u32)
fn allocate_segment(&mut self, minimum_size: u32) -> (*mut u8, u32)
minimum_size
words long (minimum_size * 8
bytes long). Allocator implementations
commonly allocate much more than the minimum, to reduce the total number of segments needed.
A reasonable strategy is to allocate the maximum of minimum_size
and twice the size of the
previous segment.source§unsafe fn deallocate_segment(
&mut self,
ptr: *mut u8,
_word_size: u32,
words_used: u32
)
unsafe fn deallocate_segment( &mut self, ptr: *mut u8, _word_size: u32, words_used: u32 )
word_size
is the length of the segment in words, as returned from allocate_segment()
.
words_used
is always less than or equal to word_size
, and indicates how many
words (contiguous from the start of the segment) were possibly written with non-zero values. Read more