The task of fulfilling an allocation request consists of locating a block of unused memory of sufficient size. Memory requests are satisfied by allocating portions from a large pool of memory called the
heap or
free store. At any given time, some parts of the heap are in use, while some are "free" (unused) and thus available for future allocations. In the C language, the function which allocates memory from the heap is called and the function which takes previously allocated memory and marks it as "free" (to be used by future allocations) is called . Several issues complicate the implementation, such as
external fragmentation, which arises when there are many small gaps between allocated memory blocks, which invalidates their use for an allocation request. The allocator's
metadata can also inflate the size of (individually) small allocations. This is often managed by
chunking. The memory management system must track outstanding allocations to ensure that they do not overlap and that no memory is ever "lost" (i.e. that there are no "
memory leaks").
Efficiency The specific dynamic memory allocation algorithm implemented can impact performance significantly. A study conducted in 1994 by
Digital Equipment Corporation illustrates the
overheads involved for a variety of allocators. The lowest average
instruction path length required to allocate a single memory slot was 52 (as measured with an instruction level
profiler on a variety of software).
Implementations Since the precise location of the allocation is not known in advance, the memory is accessed indirectly, usually through a
pointer reference. The specific algorithm used to organize the memory area and allocate and deallocate chunks is interlinked with the
kernel, and may use any of the following methods:
Fixed-size blocks allocation Fixed-size blocks allocation, also called memory pool allocation, uses a
free list of fixed-size blocks of memory (often all of the same size). This works well for simple
embedded systems where no large objects need to be allocated but suffers from
fragmentation especially with long memory addresses. However, due to the significantly reduced overhead, this method can substantially improve performance for objects that need frequent allocation and deallocation, and so it is often used in
video games.
Buddy blocks In this system, memory is allocated into several pools of memory instead of just one, where each pool represents blocks of memory of a certain
power of two in size, or blocks of some other convenient size progression. All blocks of a particular size are kept in a sorted
linked list or
tree and all new blocks that are formed during allocation are added to their respective memory pools for later use. If a smaller size is requested than is available, the smallest available size is selected and split. One of the resulting parts is selected, and the process repeats until the request is complete. When a block is allocated, the allocator will start with the smallest sufficiently large block to avoid needlessly breaking blocks. When a block is freed, it is compared to its buddy. If they are both free, they are combined and placed in the correspondingly larger-sized buddy-block list.
Slab allocation This memory allocation mechanism preallocates memory chunks suitable to fit objects of a certain type or size. These chunks are called caches and the allocator only has to keep track of a list of free cache slots. Constructing an object will use any one of the free cache slots and destructing an object will add a slot back to the free cache slot list. This technique alleviates memory fragmentation and is efficient as there is no need to search for a suitable portion of memory, as any open slot will suffice.
Stack allocation Many
Unix-like systems as well as
Microsoft Windows implement a function called for dynamically allocating stack memory in a way similar to the heap-based . A compiler typically translates it to inlined instructions manipulating the stack pointer. Although there is no need of manually freeing memory allocated this way as it is automatically freed when the function that called returns, there exists a risk of overflow. And since alloca is an
ad hoc expansion seen in many systems but never in
POSIX or the C standard, its behavior in case of a stack overflow is undefined. A safer version of alloca called , which reports errors, exists on Microsoft Windows. It requires the use of .
gnulib provides an equivalent interface, albeit instead of throwing an SEH exception on overflow, it delegates to malloc when an overlarge size is detected. A similar feature can be emulated using manual accounting and size-checking, such as in the uses of in glibc. == Automated memory management ==