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Heap is a linear
block of memory which is used to handle pointer allocation and deallocation.
Heap performs two operations, allocate and free. Allocate operation takes input
as size in bytes and returns pointer to block of memory of defined size. If no
memory exists, it returns null pointer. Free operation is used to free the
allocated block. Pascal uses new and dispose, where as C+ uses new and delete
for allocate and free operation respectively. C language uses malloc and free
as a part of standard library
stdlib.h for
allocation and dellocating memory. The prototype of these functions are as
follows
void *malloc
(unsigned size);
void free (void
*ptr);
One way of
implementing heap is maintaining a circular list of free blocks, from which memory
can be drawn through malloc function and returned through free function. Though
this is very simple to implement and maintain, it has few disadvantages. One of
the disadvantage is that, the pointer to free block may not be the one given to
malloc. Possibility of user giving invalid pointer corrupts the heap. Secondly
there can be small fragments of free blocks. This has to be compacted so that
large blocks of continuous memory are available for malloc.
More efficient
way of heap implementation is using circular linked list which keeps track of
both allocated and free blocks. Heap consists of nodes (blocks) which has
information of size of used area and size of free area followed by user space
and free space as shown below.
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