How Garbage Collectors Work: A Developer's Guide to Building One

Garbage collectors are runtime components that automatically reclaim memory occupied by objects a program no longer needs, preventing memory leaks and crashes. Most developers only notice them when something goes wrong, such as unexpected pauses or out-of-memory errors. One common approach is reference counting, used as the primary mechanism in CPython, where each object tracks how many references point to it and is freed the moment that count drops to zero. While elegant and simple, reference counting has known limitations, such as failing to handle circular references. Understanding the core algorithm is accessible enough that developers can build a basic garbage collector themselves, which helps demystify runtime memory management.