code queries

[Bran S]

On Wed, 15 Jul 2020 at 23:51, Ákos Uzonyi <uzonyi.akos at gmail.com> wrote:
>
> On Wed, 15 Jul 2020 at 19:27, Bran S <archsbran at gmail.com> wrote:
> > Could you please elaborate on this magic calculation that is going on here.
> > Link: https://github.com/strace/strace/blob/master/tests/tail_alloc.c#L16
> >
> >         const size_t len = (size + page_size - 1) & -page_size;
> >
> > I have tried running it in several variations, but I fail to see the
> > pattern by which value of the variable `len` changes.
> > Why is doing this required?
>
> If you are unsure why something is done differently than you would
> think, just try your version and see what happens. If you remove this
> "magic" calculation, munmap fails with EINVAL. Looking up the man
> page, you quickly realize that EINVAL means "addr" (in this case
> "tail_guard") is not aligned to a page boundary. So this magic
> calculation just rounds up "size" to be a multiple of page_size.

Thanks, I understood it now.