Editing C dynamic memory allocation (section)

===Heap-based===
{{see also|sbrk}}
Implementation of legacy allocators was commonly done using the [[Data_segment#Heap|heap segment]].  The allocator would usually expand and contract the heap to fulfill allocation requests.

The heap method suffers from a few inherent flaws:

* A linear allocator can only shrink if the last allocation is released. Even if largely unused, the heap can get "stuck" at a very large size because of a small but long-lived allocation at its tip which could waste any amount of address space, although some allocators on some systems may be able to release entirely empty intermediate pages to the OS.
* A linear allocator is sensitive to [[fragmentation (computer)|fragmentation]]. A good allocator will attempt to track and reuse free slots through the entire heap, but as allocation sizes and lifetimes get mixed it can be difficult and expensive to find or coalesce free segments large enough to hold new allocation requests.
* A linear allocator has extremely poor concurrency characteristics, as the heap segment is per-process every thread has to synchronise on allocation, and concurrent allocations from threads which may have very different work loads amplifies the previous two issues.

{{anchor|dlmalloc}}