heapmetadata

Heap metadata refers to information stored alongside allocated memory blocks in a computer's heap. When a program requests memory from the heap, the memory management system doesn't just return a pointer to the raw memory. Instead, it typically prepends or appends a small amount of data to the allocated block. This auxiliary data is known as heap metadata.

The purpose of heap metadata is to facilitate the operations of the memory allocator. This metadata can include crucial details such as the size of the allocated block, flags indicating whether the block is currently in use or free, and pointers to neighboring blocks. These pointers are essential for efficient memory deallocation and fragmentation management. For example, when a block is freed, its metadata can be used to merge it with adjacent free blocks, reducing external fragmentation.

Different memory allocators, such as glibc's ptmalloc or jemalloc, implement their heap metadata structures in various ways. Some might use simple linked lists of free blocks, while others employ more sophisticated data structures like trees or arrays for faster lookups. The design of heap metadata significantly impacts the performance of memory allocation and deallocation operations, as well as the overall memory usage of an application. Security vulnerabilities, such as buffer overflows, can sometimes be exploited by corrupting heap metadata, leading to crashes or arbitrary code execution. Therefore, protecting heap metadata is a critical aspect of memory safety.