What malloc Actually Does
The anatomy of an allocator: metadata, bins, coalescing, and why the interface is so small.
The interface is three functions; the job is enormous
malloc, free, realloc hide a subsystem that must satisfy conflicting goals at once:
- throughput — allocate fast (the common case should be a few instructions);
- memory efficiency — metadata and fragmentation must not eat the heap;
- low fragmentation — long-running programs mix sizes unpredictably;
- thread safety — modern allocators do this without a global lock.
Real allocators (glibc's, musl's, jemalloc) chunk memory into size classes, keep free lists per class, coalesce neighbors on free, and mmap very large requests directly.
The tradeoff triangle
Fast + compact + general: pick two. Arena allocators choose fast + compact for one lifetime batch and give up general free. Pools choose fast for one size and give up flexibility. Understanding why each design gives something up is the engineering content of this module — code alone does not teach it.
Alignment is part of the contract
Every allocation is aligned for any fundamental type (typically 16 bytes). Your own allocators must honor that, or every struct cast through the result is a latent crash.