Re: Cygwin.dll crash, alloca and custom stack

["Bitmead, Chris" <Chris dot Bitmead at ag dot gov dot au>]

>Your demonstration code didn't use any Windows system calls.  You were
>calling cygwin functions.

And printf doesn't use a Windows system call to write its output??

I fully admit to being in the dark about whether it is Windows or cygwin
that is being annoying here. 

>I notice that you save your stack pointer in
>an automatic variable (saved on the stack), reset the stack pointer, and
>then get it back again.  You are relying on the fact that the frame
>pointer doesn't change in that scenario.

A valid observation. I may need to push the fp onto the new stack and pop it
off before restoring.

>It is really not nonsensible for an OS to assume that it has control of
>the stack.

Lots of claims flying around, but no justification. An OS specified stack
can provide automatic extension to applications that are interested in that
feature, but apart from that I don't see why it should give a rip what an
app does with its stack.

>I already did earlier.  Let's not loop.

You said that Windows has the TIB area. Ok, but if I restore the system
stack before talking to windows, I don't see any problem.

>And what happens when the compiler decides to allocate more memory on
>the stack than what seems possible from code inspection, like when
>there's a signal?

Well you've heard of sigaltstack() right?  That's an API where you call
malloc()
to create some stack space. But I guess that API is "bad" because
it means that the OS is no longer "controlling" where the stack is right?

>And, I assume that there will be no recursion in your program?  Because if
there
>is you're guaranteed heap corruption.

No, each function call has its own scheme environment and therefore its own
stack. So on a recursive call a new environment is allocated and thus a new
stack is allocated. Of course, most scheme recursion is tail calls and that
will be implemented with longjmp, thus re-using the same stack. By
allocating my own stacks I can longjmp down the stack (conceptually), which
is normally a no-no, but I've shown to work fine if you allocate your own
chain of stacks that are not garbage collected till they have no references.






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