Comment by adwn

When `free()` is called, the allocator internally marks that specific memory area as unused, but it doesn't necessarily return that area back to the OS, for two main reasons:

1. `malloc()` is usually called with sizes smaller than the sizes by which the allocator requests memory from the OS, which are at least page-sized (4096 bytes on x86/x86-64) and often much larger. After a `free()`, the freed memory can't be returned to the OS because it's only a small chunk in a larger OS allocation. Only after all memory within a page has been `free()`d, the allocator may, but doesn't have to, return that page back to the OS.

2. After a `free()`, the allocator wants to hang on to that memory area because the next `malloc()` is sure to follow soon.

This is a very simplified overview, and different allocators have different strategies for gathering new `malloc()`s in various areas and for returning areas back to the OS (or not).