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by dwattttt 53 days ago
> C programs can't do it because pointers to stack allocated objects may exist.

They sure shouldn't exist to the unused region of the stack though; if they do, that's a bug (because anything could claim that memory now). You should be free and clear to release stack pages past your current stack pointer.
2 comments
matheusmoreira 52 days ago
High level languages have entire runtime systems dedicated to managing resources like that. My language can allocate, grow, shrink and deallocate stacks dynamically. It has complete visibility into everything, and the stacks themselves are designed to be relocatable and position-independent.

In C it's impossible to even get the stack pointer without dropping to assembly or using compiler builtins. It's hard to know where the stack starts or even how big it is.

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dwattttt 52 days ago
I do agree with this, but just to be clear (for others), you don't need any runtime managing resource lifecycles to know that there shouldn't be pointers into free memory, such as the currently unused portion of the stack.
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simonask 52 days ago
There isn’t any operating system or compiler that does this today, and it probably isn’t worth it to pursue. Enlarging the stack via page fault is really expensive, so you would need really advanced heuristics to prevent repeatedly unmapping/remapping those pages.

The correct tool for myriad of small tasks is coroutines / green threads / async tasks, so why spend any energy optimizing threads for that purpose instead of what they are already good at?

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dwattttt 52 days ago
In the general case it's absolutely not worth it. In the context of "you want a large number of OS threads, and are willing to go to some effort", it's theoretically something you'd want to do; suppose the startup for a thread is measurably a high water mark for stack usage, after startup the steady state stack usage won't exceed 20% of that high mark, and you'd like as many threads/stacks as possible.

Coroutines / green threads / async tasks will all do this too, but there's something to be said for using/relying on the system scheduler instead of bringing your own in in addition.

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