Dynamic memory allocation

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Dynamic memory allocation is the allocation of memory storage for use by a computer program during its runtime. It is a way of distributing ownership of limited memory resources among many applications. Importantly, the amount of memory allocated is determined by the program at the time of allocation and need not be known in advance. A dynamic allocation exists until it is explicitly released, either by the programmer or by a garbage collector. This is notably different from automatic and static memory allocation, which require advance knowledge of the required amount of memory and have a fixed duration. It is said that an object so allocated has dynamic lifetime.

[edit] Details

The task of fulfilling an allocation request, which involves finding a block of unused memory of sufficient size, is complicated by the need to avoid both internal and external fragmentation while keeping both allocation and deallocation efficient. Also, the allocator's metadata can inflate the size of (individually) small allocations; chunking attempts to reduce this effect.

Usually, memory is allocated from a large pool of unused memory area called the heap (also called the free store). Since the precise location of the allocation is not known in advance, the memory is accessed indirectly, usually via a reference. The precise algorithm used to organize the memory area and allocate and deallocate chunks is hidden behind an abstract interface and may use any of the methods described below.

[edit] Implementations

[edit] Fixed-Size-Blocks Allocation

This scheme uses a free list of fixed-size blocks of memory (often all of the same size). This works well for simple embedded systems.

[edit] Buddy Blocks

In this system, memory is allocated from a large block of memory that is a power of two in size. If the block is more than twice as large as desired, it is broken in two. One of the halves is selected, and the process repeats (checking the size again and splitting if needed) until the block is just large enough.

All the blocks of a particular size are kept in a sorted linked list or tree. When a block is freed, it is compared to its buddy. If they are both free, they are combined and placed in the next-largest size buddy-block list. (When a block is allocated, the allocator will start with the smallest sufficiently large block avoiding needlessly breaking blocks)

Buddy block allocators are used both in real-time operating systems and in general-purpose operating systems (such as Microsoft Windows and Linux).

[edit] References

• Donald Knuth. Fundamental Algorithms, Third Edition. Addison-Wesley, 1997. ISBN 0-201-89683-4. Section 2.4: Dynamic Storage Allocation, pp.435–456.
• Herbert Glarner's visualized Dynamic Storage Allocation, describing efficient techniques
• Simple Memory Allocation Algorithms on OSDEV Community

[edit] See also

• Allocation algorithms
• malloc
• bsdmalloc
• mtmalloc
• umem alloc
• Vmalloc
• Slab allocation
• mmap
• hazard pointer
• Heap Layers
• Hoard memory allocator
• Memory pool
• obstack
• Stack-based memory allocation