You can only grow the stack automatically if you "prereserve" the virtual address space for it. Then it becomes just a question of "committing" the memory at time of need. Default options to mmap() should actually give you this (the OS finds/fills the pages for you on first pagefault), but Solaris/Linux differ on overcommit behaviour.

To "grow" a stack at runtime, if the virtual address space to just add mappings in the "right" place is unavailable (already used for something else, say), it means a stack switch. That isn't entirely impossible. But if done anywhere else but in the "base thread initialisation" (as seen in that example) it becomes complicated and hazardous. It's more like "coroutines" then, your current function "spawns" a func on a new callstack and on return, undoes that and cleans up. One can imagine things like thread-local signal handlers to "simulate" this transparently but that would become more of a piece of programming performance art. If there ever were an "unbelievable unix underbelly programming contest" (UUUPC) stuff like that might well make it.