A memory allocator is a crucial software component that manages memory allocation and deallocation for programs. It acts as an intermediary between programs and the operating system, efficiently distributing memory blocks from a larger memory pool called the heap. The allocator keeps track of which memory blocks are currently in use and which are available for future allocation requests.
The memory allocation process follows several key steps. First, a program requests memory using functions like malloc. The allocator then searches through its free blocks using different strategies. First-fit uses the first block that's large enough, best-fit finds the smallest suitable block to minimize waste, and worst-fit uses the largest block. Once a suitable block is found, it's marked as allocated and a pointer to its location is returned to the program.
Memory deallocation is equally important as allocation. When a program calls free with a pointer, the allocator marks that block as available. A crucial optimization called coalescing then occurs - the allocator checks if adjacent blocks are also free and merges them into larger contiguous blocks. This process prevents external fragmentation, where memory becomes divided into small unusable pieces, and ensures that larger allocation requests can be satisfied efficiently.
Memory allocators face two main fragmentation problems. External fragmentation occurs when free memory exists but is scattered in small pieces that cannot satisfy larger allocation requests, even though the total free space might be sufficient. Internal fragmentation happens when an allocated block is larger than what the program actually needs, resulting in wasted space within the block. Allocators use techniques like coalescing to reduce external fragmentation and block splitting to minimize internal fragmentation.
Advanced memory allocators use sophisticated techniques for better performance. Free list management organizes available blocks in linked lists, often segregated by size for faster lookup. The buddy system uses power-of-2 block sizes, making splitting and merging operations very efficient and reducing external fragmentation. Slab allocation pre-allocates pools of fixed-size objects, eliminating allocation overhead and is commonly used in operating system kernels for managing frequently allocated data structures.