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Date:	Fri, 9 Dec 2011 21:36:44 +0100
From:	Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To:	cygwin AT cygwin DOT com
Subject:	Re: gcc-4.5.3 segfaults wrt alloca
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On Dec  9 12:41, Ryan Johnson wrote:
> On 09/12/2011 12:07 PM, Eric Blake wrote:
> >On 12/09/2011 07:55 AM, Ryan Johnson wrote:
> >>On 09/12/2011 5:58 AM, Denis Excoffier wrote:
> >>>I use the latest packages and cygwin snapshots. The problem described
> >>>below began several snapshots in the past, around beginning of December.
> >>>
> >>>The following program, with static allocation of a reasonable amount
> >>>of data, segfaults, maybe in alloca(). With a smaller size
> >>>(eg 10000) it's ok. With new/malloc (even with 100 times more) it's
> >>>ok. With C or C++. 100% reproducible.
> >>>    unsigned int const SIZE = 689471;
> >>>    int foo[SIZE];
> >>Reasonable? You're trying to stack-allocate 2.5MB of data. Don't do that
> >>-- stack sizes are 2MB or less in most operating systems. Besides, doing
> >>anything useful with a buffer that size would completely drown out the
> >>overhead of calling malloc.
> >Not only that, but stack allocating more than 64k in a single function
> >is a recipe for bypassing the guard page and causing windows to silently
> >quit your program, rather than letting cygwin catch the guard page
> >access and convert it to normal SIGSEGV handling.  To be portable to all
> >OS, you should never stack allocate more than 4k in a single function.
> It's kind of interesting: when I ran that test case with my
> home-brew gcc-4.6, its alloca() explicitly walks through the
> proposed allocation in 4kB increments to ensure that a stack
> overflow triggers SIGSEGV right away, rather than allow silent data
> corruption later. I don't know if older versions also do this, but
> maybe that's why it used to "work" and now "doesn't work."

alloca works this way for ages, as far as I know.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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