In a program, you'll dynamically request memory buffers or blocks of a particular size from the runtime environment using malloc(), realloc(), or calloc(), and then you'll release them back to the runtime environment when they're no longer required using free().
The memory allocator ensures that your requests are satisfied by managing a region of the program's memory area known as the heap. In this heap, it tracks all of the information — such as the size of the original block — about the blocks and heap buffers that it has allocated to your program, in order that it can make the memory available to you during subsequent allocation requests. When a block is released, it places it on a list of available blocks called a free list. It usually keeps the information about a block in the header that precedes the block itself in memory.
The runtime environment grows the size of the heap when it no longer has enough memory available to satisfy allocation requests, and it returns memory from the heap to the system when the program releases memory.
The basic heap allocation mechanism is broken up into two separate pieces, a chunk-based small block allocator and a list-based large block allocator. By configuring specific parameters, you can select the various sizes for the chunks in the chunk allocator and also the boundary between the small and the large allocator.