On Sat, Jan 17, 2004 at 10:49:10PM +0000, Tom Hughes wrote:
> In message <200...@ra...>
> Balazs Rauznitz <ba...@ra...> wrote:
>
> > There's only one thing I cannot figure out: I get this error in my
> > program:
> >
> > ==7819== Warning: client switching stacks? %esp: 0xBFFFF4FC --> 0xBFC2EBB4
> > ==7819== 1 errors in context 1 of 24:
> > ==7819== Invalid write of size 4
> > ==7819== at 0x8049F69: create_cache_file (test.c:480)
> > ==7819== Address 0xBFC2EBD0 is on thread 1's stack
> >
> > I'm not using any threads; my program crunches numbers. I get these
> > errors after malloc-ing a huge chunk of memory; that's what triggers
> > the 'client switching stacks' message. Is it a glitch in valgrind or am
> > I compiling my program with incorrect switches ?
>
> Thread 1 is the main thread, so you always have that, so it isn't odd
> that valgrind is talking about an address being on thread 1's stack.
>
> Are you sure you mallocing this very large chunk of memory and not
> allocating it on the stack? perhaps with alloca? That might cause
> valgrind to think the client had switched stacks, which might in turn
> cause it to think the large chunk of memory was not addressable.
I'm both mallocing a huge chunk and allocating memory on the stack. The
error masseges can be reproduced just with this:
main()
{
int i[1000000], j;
j = 0;
}
I just read this in the docs:
Warning: client switching stacks?
Valgrind spotted such a large change in the stack pointer, %esp, that
it guesses the client is switching to a different stack. At this point
it makes a kludgey guess where the base of the new stack is, and sets
memory permissions accordingly. You may get many bogus error messages
following this, if Valgrind guesses wrong. At the moment "large change"
is defined as a change of more that 2000000 in the value of the %esp
(stack pointer) register.
I guess that explains. Do you have suggestions how to avoid such errors
? Is there are gcc flag to work around this problem ? Is it bad
programming practice to have so huge stack ? Maybe it's harmless...
Balazs
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