Re: [Valgrind-users] Errors with gcc -static

[Nicholas N. <n.n...@gm...>]

On Sat, Jul 18, 2009 at 12:55 AM, John Reiser<jr...@bi...> wrote:
> Nicholas Nethercote wrote:
>
> 0)  sbrk() can *decrease* process address space.  No zero fill is done
>    for a decrease, not even the fragment on the high end of the last page
>    that is beyond the new highest address.  For maximum safety and
>    portability, then the bytes in the last page that reside above [the new]
>    sbrk(0) should be considered to be uninitialized, but in practice
>    it is exceedingly likely that they will retain their previous contents.
>
> 1)  If an increase is large enough to require new whole pages,
>    then those new whole pages (like all new pages) are zero-filled
>    by the operating system.  So if sbrk(0) already is page aligned,
>    then sbrk(PAGE_SIZE) *does* zero-fill the new memory.
>
> 2)  Any increase that lies within an existing allocated page
>    is not changed.  So if (x= sbrk(0)) is not page aligned, then
>    sbrk(PAGE_SIZE) yields ((PAGE_SIZE -1) & -x) bytes which
>    keep their existing contents, and an additional PAGE_SIZE
>    bytes which are zeroed.  ((PAGE_SIZE -1) & x) of them are
>    "covered" by the sbrk(), and the rest of them come along for
>    the ride because the operating system deals only in whole pages.
>    Again, for maximum safety and portability, then anything that
>    lives above [the new] sbrk(0) should be considered uninitialized,
>    but in practice will retain previous contents [zero in this case.]
>
> Also remember that sbrk(...) can fail  :-)

Thanks, John.  Is this info specific to Linux/glibc?  AFAICT sbrk/brk
isn't even POSIX, which makes it even harder to determine what it
should do.

Nick