Since standard OS stacks are contiguous and unmovable, you can't grow them once they run out of space. However while the address space for the maximum stack siz

Since standard OS stacks are contiguous and unmovable, you can't grow them once they run out of space. However while the address space for the maximum stack size gets reserved, each page only requires backing memory once it's first used. So as far as physical memory consumption is concerned, typical stacks act as growable with a fixed maximum size. Since address space is huge on 64-bit systems, choosing a large stack size is cheap on such systems (at least of they allow over-commit). For the main thread, a system can also try to keep other allocations far from the stack without committing to any particular size (heap grows upwards from the bottom of the address space, stack downwards from the top). But this doesn't scale to multiple threads and leads to an unpredictable maximum stack size, so I prefer the fixed reserved space approach.