In principle, this memory allocator is roughly equivalent to Doug Lea's dlmalloc with fine-grained locking. malloc: Uses a freelist binned by chunk size, with a bitmap to optimize searching for the smallest non-empty bin which can satisfy an allocation. If no free chunks are available, it creates a new chunk of the requested size and attempts to merge it with any existing free chunk immediately below the newly created chunk. Whether the chunk was obtained from a bin or newly created, it's likely to be larger than the requested allocation. malloc always finishes its work by passing the new chunk to realloc, which will split it into two chunks and free the tail portion.