Forward to the list as it is a perfect answer!
-------- Message original --------
Sujet: Re: [Materm-devel] Small question about segfaults
Date: Mon, 20 Oct 2008 16:40:14 +0200
De: Frederik Deweerdt <[EMAIL PROTECTED]>
Pour :: jehan <[EMAIL PROTECTED]>
Références: <[EMAIL PROTECTED]>
<[EMAIL PROTECTED]>
<[EMAIL PROTECTED]>
[replying to you in private, I can't post to the list right now]
On Mon, Oct 20, 2008 at 04:35:06PM +0200, jehan wrote:
> Frederik Deweerdt writes:
>
> > From the program's point of view, all memory space is his, some
> > addresses are allocated, some not. But all the memory (eg. from 0 to
> > 0xffffffff on a 32bits arch) is potentially his.
>
> Ok, so what I understand here is that you confirm that the program won't
> segfault as long as I am not accessing another program's allocated memory,
> even if I get out my own allocated memory. Is that so?
Nope, if you get out your own allocated memory, you will segfault.
...but sometimes you won't :-)
The trick is as illustrated by this sample program:
int main()
{
char *p;
int i;
p = malloc(128); /* suppose that the libc allocated 4K for performance
purposes here */
for (i=0; i < 256; i++) {
p[i] = '0';
}
return 0;
}
This could segfault or not, depending on what the call to malloc did
behind the scenes.
Luckily a tool like valgrind would catch this for you.
Another point to clarify. From a program's point of view, he's alone
running in his address space (0-0xffffffff). The kernel set the memory
up in such a way.
So there's no way you could access another program's memory (except for
shared memory, like the one shmget provides)
Regards,
Frederik
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