Return the correct value for sysconf(_SC_PAGESIZE)

Corinna Vinschen
Thu Nov 17 09:55:00 GMT 2016

On Nov 16 15:51, Erik Bray wrote:
> On Wed, Nov 16, 2016 at 3:01 PM, Eric Blake <> wrote:
> > On 11/16/2016 07:56 AM, Erik Bray wrote:
> >
> >>> There is no good reason to use the non-POSIXy page size.  It doesn't
> >>> help you in the least for any pagesize-related functionality.  Mmap
> >>> as well as malloc and friends only work with _SC_PAGESIZE sized pages.
> >>>
> >>> It sounds as if you're looking for a solution for which there's no
> >>> problem...
> >>
> >>
> >> FWIW the background here is that I'm working on porting psutil [1] to
> >> Cygwin, and trying to accomplish as much as *possible* through the
> >> POSIX interfaces without having to fall back on the Windows API.  It's
> >> actually a great exercise in what is and isn't possible with Cygwin :)
> >>
> >> In this case I was trying to compute process memory usage from
> >> /proc/<pid>/statm which gives values in page counts, so I need the
> >> page size (the actual page size) to compute the values in bytes.
> >
> > If /proc/<pid>/statm is reporting memory in multiples that are not the
> > POSIX _SC_PAGESIZE, that is a bug in the statm file emulation that
> > should be fixed there.
> So then something like that attached (untested) patch should work,
> though it made statm inconsistent with what is reported in
> /proc/<pid>/status which reports memory in multiples of page size.  So
> that has to be patched as well.

Patches applied as obvious, thank you.

> This of course leads to memory reporting that is inconsistent with
> what Windows says.  But I guess if that's "normal" in Cygwin (for the
> reasons stated by Corinna) then it's an acceptable trade-off?

It is.  From the POSIX POV we have a 64K page size, from the Windows
POV we have a bastard of 4K pages which can only be allocated in
chunks of 16.  The latter simply doesn't fly well with POSIX

Since native Windows tools don't see the Cygwin internal assumptions,
it doesn't matter, unless you mix Windows and Cygwin memory functions.
Which, honestly, should only be done by Cygwin itself.


Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat
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