HEAP MEMORY

Heap memory, also known as dynamic memory, is an alternative to local stack memory. Local memory is so automatic that it is allocated automatically on function call and it is deallocated automatically when a function exits. Heap memory is different in every way. The programmer explicitly requests the allocation of a memory block of a particular size, and the block continues to be allocated until the programmer explicitly requests that it be deallocated. Nothing happens automatically. So the programmer has much greater control of memory, but with greater responsibility since the memory must now be actively managed.

The advantages of heap memory:

• Lifetime:
  Because the programmer now controls exactly when memory is allocated and deallocated, it is possible to build a data structure in memory, and return that data structure to the caller. This was never possible with local memory which was automatically deallocated when the function exited.
• Size:
  The size of allocated memory can be controlled with more detail. For example, a string buffer can be allocated at run-time which is exactly the right size to hold a particular string. With local memory, the code is more likely to declare a buffer size 1000 and hope for the best.

The disadvantages of heap memory:

• More Work:
  Heap allocation needs to be arranged explicitly in the code which is just more work.
• More Bugs:
  Because it's now done explicitly in the code, realistically on occasion the allocation will be done incorrectly leading to memory bugs. Local memory is constrained, but at least it's never wrong.

However, there are many problems that can only be solved with heap memory, so that's that way it has to be. In languages with garbage collectors such as LISP and Java, the above disadvantages are mostly eliminated. The garbage collector takes over most of the responsibility.

HEAP ALLOCATION

The heap is a large area of memory available for use by the program. The program can request blocks of memory for its use within the heap. In order to allocate a block of some size, the program makes an explicit request by calling the heap allocation function. The allocation function reserves a block of memory of the requested size in the heap and returns a pointer to it.

Suppose a program makes three allocation requests to 25 allocate memory to hold three separate images in the heap each of which takes 1024 bytes of memory.

Each allocation request reserves a contiguous area of the requested size in the heap and returns a pointer to that new block to the program. Since each block is always referred to by a pointer, the block always plays the role of an object that the pointer can points to and the program always manipulates its heap blocks through pointers. The heap block pointers are sometimes known as base addresspointers since by convention they point to the base, lowest address byte, of the block.

In this example, the three blocks have been allocated contiguously starting at the bottom of the heap, and each block is 1024 bytes in size as requested. In reality, the heap manager can allocate the blocks wherever it wants in the heap so long as the blocks do not overlap and they are at least the requested size. At any particular moment, some areas in the heap have been allocated to the program, and so are in use. Other areas have yet to be committed and so are free and are available to satisfy allocation requests. The heap manager has its own, private data structures to record what areas of the heap are committed to what purpose at any moment The heap manager satisfies each allocation request from the pool of free memory and updates its private data structures to record which areas of the heap are in use.

HEAP DEALLOCATION

When the program is finished using a block of memory, it makes an explicit deallocation request to indicate to the heap manager that the program is now finished with that block.

The heap manager updates its private data structures to show that the area of memory occupied by the block is free again and so may be reused to satisfy future allocation requests.

After the deallocation, the pointer continues to point to the now deallocated block. The program must not access the deallocated object. The pointer is there, but it must not be used. Sometimes the code will set the pointer to NULL immediately after the deallocation to make explicit the fact that it is no longer valid.

HEAP - SUMMARY

• The heap is an area of memory available to allocate blocks of memory for the program.
• There is some heap manager library code which manages the heap for the program. The programmer makes requests to the heap manager, which in turn manages the internals of the heap. In C, the heap is managed by the ANSI library functions malloc(),free(), and realloc().
• The heap manager uses its own private data structures to keep track of which blocks in the heap are free and which blocks are currently in use by the program and how large those blocks are. Initially, all of the heap is free.
• The heap may be of a fixed size, or it may appear to be of a fixed but extremely large size backed by virtual memory. In either case, it is possible for the heap to get full if all of its memory has been allocated and so it cannot satisfy an allocation request. The allocation function will communicate this runtime condition in some way to the program - usually by returning a NULL pointer or raising a language specific runtime exception.
• The allocation function requests a block in the heap of a particular size. The heap manager selects an area of memory to use to satisfy the request, marks that area as in use in its private data structures, and returns a pointer to the heap block.
  The caller is now free to use that memory by dereferencing the pointer. The block is guaranteed to be reserved for the sole use of the caller - the heap will not hand out that same area of memory to some other caller. The block does not move around inside the heap - its location and size are fixed once it is allocated.
  Generally, when a block is allocated, its contents are random. The new owner is responsible for setting the memory to something meaningful. Sometimes there is variation on the memory allocation function which sets the block to all zeros (calloc() in C).
• The deallocation function is the opposite of the allocation function. The program makes a single deallocation call to return a block of memory to the heap free area for later reuse. Each block should only be deallocated once. The deallocation function takes as its argument a pointer to a heap block previously furnished by the allocation function. The pointer must be exactly the same pointer returned earlier by the allocation function, not just any pointer into the block. After the deallocation, the program must treat the pointer as bad and not access the deallocated object.