[PD] What exactly is a "stack overflow" ?

[Mathieu Bouchard]

On Mon, 17 Dec 2007, Martin Peach wrote:

> The cpu signals an overflow whenever the stack space runs out (the program
> tries to access the stack beyond its boundaries)

With Pd, this is never what happens on its own. Instead, just before 
processing each message, a check of the count of nested message 
processings in made (messages sent that are still being processed, not 
counting those scheduled by [delay] and stuff).

Win16 would typically have a 512 byte limit for the stack. This is too 
little for today's needs, but it could be increased up to 64k if you 
really wanted. In contrast, Linux can typically grow the stack to 512 megs 
or more, and I don't see why Win32 wouldn't be able to (though I have idea 
what Win32 does about it -- it probably has a reasonable stack size). 512 
megs is too much for the protection that Pd needs.

Older versions of Pd checked stack size instead, but it was a problem with 
large on-stack allocations. What Linux (or OSX) does when reaching maximum 
stack size (which you can intentionally lower) is raise a signal, using 
kill(). It normally doesn't give you any sane way of handling this, so 
this is why applications that want to stay alive after a so-called stack 
overflow have to do it themselves and never involve the OS in it.

(If you want to escape an OS-based stack overflow, you can, you need to 
do things dependent on both which OS and which CPU type, and I wouldn't be 
surprised if it also depended on which kernel version. I never actually 
did it, but I'm pretty sure that it's doable, just that it's too scary).

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| Mathieu Bouchard - tél:+1.514.383.3801, Montréal QC Canada