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."
Ryan
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