A free list is a data structure used for dynamic memory allocation. It contains addresses of available memory locations. Memory allocation can fail if the list is empty or if there are no large enough memory blocks. Different types of memory management systems exist, such as the first fit system and buddy allocation. Garbage collection can consolidate free spaces to prevent fragmentation.
A free list is a data structure that contains the addresses of computer memory locations available for use by a running program when using dynamic memory allocation. The list becomes necessary when a program needs to allocate space from an area of free memory called the heap. The implementation of a free list can be a simple linked list or it could be a more complex data structure such as a sort tree Most high-level computer programming languages handle the free list automatically, eliminating the need for a free list manual.
When a program requires space to store information during program execution, it must request a specific amount of memory from the underlying operating system. The locations of memory blocks that can be used are stored in the list. For the allocation to be successful, the requested amount of memory must be available in one or more of these blocks. When a pointer to an appropriate memory location is returned, that list item is removed.
After a program has finished using memory, it can deallocate it—that is, move the pointer to the memory block back into the free list, where it will be available the next time an allocation is made. Memory allocation may fail because the list is empty or because there are no large enough memory blocks to satisfy the program’s request.
The simplest form of memory management is called a first fit system. This system maintains a single list of free memory locations. When a memory request is sent, the list is traversed and the first block that is large enough is returned. If the block is more than double the requested size, it is halved and the unused half is added to the list. This method trades simple encoding for the risk of having fragmented memory areas that may never be returned to the list.
A different form of memory management is called a buddy allocation system. Unlike the first fit system, buddy allocation maintains several free lists, each of which contains open blocks of only one particular size. This means that when received an allocation request, the list containing just enough blocks to satisfy the request is consulted and an open position is returned. If no free blocks smaller than twice the requested size are available, a larger block is split in two to satisfy the requirements.
The term “free list” can refer to either a single linked list of memory addresses, or a much more complex type of data structure. Different types of sort trees, when kept simple and balanced, can help increase the speed of sorting. finding open blocks of memory at the expense of complicating the source code. A linked list may be slower than a specialized sort tree but creates programming code that is much easier to read, debug, and modify.
Some programming languages and operating systems use a special mechanism called garbage collection, a process that can help take the different entries in a free list and consolidate the free spaces so that they are contiguous. the effect of preventing fragmentation and allowing larger blocks of memory to be allocated.
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