Memory collision in Stacks

Question

So I understand what a stack overflow is, when memory collides (and the title of this website) but what I do not understand is why new entries to the stack are in a decremental memory address. Why are they not in a random memory address, would it not make more sense so that memory collision is not an issue? I am guessing there is some sort of optimizing reason behind that?

** EDIT **

What I did not realize is a stack is given x amount of address space. Makes sense now but brings me to a follow-up question. Can I explicitly state how much memory I want to allocate to a stack?

http://en.wikipedia.org/wiki/Stack_overflow – eandersson May 30 '13 at 14:03 — eandersson, May 30 '13 at 14:03

score 3 · Accepted Answer · answered May 30 '13 at 14:06

3

"Memory collides" would better suit the term of "buffer overflow", where you write outside of the predestined space, but where it is likely to be within a different allocated memory block.

A stack overflow is not about writing outside of one's memory allocation into another memory allocation. It's just about writing outside of one's stack memory allocation. Most likely outside of the stack there's a guard memory page, that is not allocated for anything and which causes a fault on a read or write attempt.

And assigning a random address for each value pushed on the stack makes it hard to find data on the stack (and it's not a stack anymore). When the compiler or programmer knows that subsequent elements occupy subsequent addresses, then it's easy to compute those addresses just from the base pointer of the stack frame.

answered May 30 '13 at 14:06

Adrian Panasiuk

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it makes sense I did not realize it's allocated x amount of memory at compile time. But is there a way to explicitly state how much memory to allocate? – Jacob Pollack May 30 '13 at 14:17
No, it's allocated at runtime and you can do it like this http://stackoverflow.com/a/2279084/111160 – Adrian Panasiuk May 30 '13 at 14:26
But hitting a stack overflow with a correctly designed and coded program is unlikely, it is caused by a bug most of the time – Adrian Panasiuk May 30 '13 at 14:27

score 1 · Answer 2 · answered May 30 '13 at 14:05

The answer to this question is probably complex, but basically stack operations are considered to be very primitive functions that the processor does as part of normal execution of code. (Saving return addresses and other stuff.)

So where do you put the memory management code? Where do you track the allocated addresses or add code to allocate new addresses? There really isn't anywhere to do this as these are basic operations performed by the processor itself.

Similar to the memory that holds the code itself, the stack is assumed to be setup before the code runs (and pointed to by the stack register). There really isn't any place to add complex memory management to stack memory. And so, yes, if not enough memory was provided, the stack will overflow.

score 1 · Answer 3 · answered May 30 '13 at 14:10

Stack overflow is when you have used up all available stack space. The space available for the stack is, in most cases just an arbitrary limit chosen by the system designers. It is possible to alter this, but on modern systems, it's not really an issue - code that needs several megabytes of stack, unless the system is REALLY huge, is probably not correctly designed.

The stack grows towards zero from "custom" - it has to go in a defined direction or it would be very hard to follow what is going on, and lower adddress is just as good as higher address. It used to be that stack and heap grew towards each other, which would allow code that uses a lot of stack and not so much heap to work in the same amount of memory as something that uses a smaller amount of stack and a larger amount of heap. But these days, there is typically enough memory (space) that the heap can be defined to be somewhere completely separate from the stack. Instead the stack overflow is detected by having a region of "reserved" memory just at the top of the stack that is not usable - so the OS gets a "trap" for using memory that isn't available, and the application can be killed.

Memory collision in Stacks

3 Answers3