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On 08/08/2010 12:43 AM, Ulrich Drepper wrote:
> On 08/07/2010 06:59 AM, Robert Nichols wrote:
>> Pages newly allocated by the kernel will be zeroed. They begin life as
>> a copy-on-write mmap() of /dev/zero.
>
> Mostly true although /dev/zero hasn't played a role in this for many
> years now.
>
> Anonymous memory returned by mmap must be cleared. Memory provided by
> sbrk can be cleared and it is on Linux.
>
> This is all rather problematic nowadays since it means many unnecessary
> memory operations, in general. There have been lots of talks about
> relaxing the rules for sbrk and adding an mmap flag to avoid the
> clearing. This can easily be accommodated in the userlevel
> implementation and lead to big improvements.
>
>
>
>> Once you have used and freed
>> memory from those pages, however, that memory will not be re-zeroed.
>
> It's only guaranteed to be cleared upon reused, not directly after they
> are freed.
>
>
>> If a subsequent malloc() happens to grab that same memory you will see
>> the old contents. It will, however, be data written there by the
>> current process.
>
> Perhaps a bit strong: no memory freed with free() must be assumed to be
> cleared. Only when the memory is returned to the kernel will it before
> the next use be cleared.
It is probably worth mentioning that in general free() does *NOT* return memory
back to the kernel.
so, in general, freeing and then malloc() -- if malloc() happens to chose memory
previously used by the application, and free'd, then the newly malloc'd memory
would have the previous contents.
- -Greg
> Everything else would be a big performance issue.
>
> You can see it yourself by using MALLOC_PERTURB_. It's really a
> debugging tool to find call site which depend on malloc clearing memory
> and use memory after fgreeing. But it obviously it's also useful for
> scrubbing memory.
>
- --
+---------------------------------------------------------------------+
Please also check the log file at "/dev/null" for additional information.
(from /var/log/Xorg.setup.log)
| Greg Hosler ghosler@xxxxxxxxxx |
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