I meant that the equivalent for mallloc would be that if you allocate 1mb buffer and a 2mb buffer, the kernel might return a 2mb buffer overlapping your earlier 1mb buffer - and be all like: "you asked for 1, you asked for 2 - and you've got 2 - if you wanted 3, you should've asked for 3". Afaik mallloc doesn't work like that - it assumes that you want more memory (and can fail or succeed etc).
I can see how the current stack/heap thing evolved - but I still think it's crazy :-)
All that's happening here is that userspace is moving its stack pointer into the heap it had previously allocated. Note that "moving the stack pointer" is not a kernel-mediated operation.
No of course, but the fact that you can "ask for more memory" by growing the stack onto your heap (rather than say, having the two start somewhere together and grow apart) - means that there's an asymmetry: mallloc will give you more ram or fail; growing the stack - can make your allocated memory overlap.
Your stack has to grow towards something. Sure, you can have it grow towards the bottom of the address space (which, due to wraparound, is also the top - where it will safely collide with the kernel addresses) but that only works for one stack - as soon as you create another thread, its thread has to grow towards something else.
I can see how the current stack/heap thing evolved - but I still think it's crazy :-)