sysconf(3) extensions.

[Brooks Davis]

On Wed, Nov 01, 2006 at 08:46:18PM +0100, Pawel Jakub Dawidek wrote:
> On Wed, Nov 01, 2006 at 01:24:57PM -0600, Brooks Davis wrote:
> > On Wed, Nov 01, 2006 at 08:06:06PM +0100, Pawel Jakub Dawidek wrote:
> > > Hi.
> > > 
> > > I'd like to add two non-standard value to the sysconf(3) functions,
> > > which can be found in both Solaris and Linux: _SC_PHYS_PAGES and
> > > _SC_AVPHYS_PAGES.
> > > 
> > > The patch is here:
> > > 
> > > 	http://people.freebsd.org/~pjd/patches/sysconf.patch
> > > 
> > > Can someone review it? Thanks.
> > 
> > What are they for?  My concern is that _SC_AVPHYS_PAGES isn't going to
> > semantically match other platforms since in a steady state the free count
> > is small on FreeBSD, but on other systems it swings quite a bit based on
> > load.
> 
> _SC_PHYS_PAGES is used by libzpool (a part of the ZFS file system).
> _SC_AVPHYS_PAGES I used more for completness.

OK.  I'd be somewhat inclined to leave _SC_AVPHYS_PAGES out in that
case, but I don't feel strongly about it.

> > +Note that it is possible for the product of this value and the value of
> > +.Li _SC_PAGE_SIZE
> > +to overflow.
> > 
> > This would be more clear if it said what was expected to overflow and
> > when.
> 
> The sysconf(3) functions return 'long' and on 32bit machine long is also
> 32bit. If we have more than 4GB of memory (PAE) and want to calculate
> physical memory in bytes doing _SC_PHYS_PAGES * _SC_PAGE_SIZE will
> overflow. But I'm not sure if I want to put this into manual page...
> Maybe as an example if there are or may be in the future other cases
> when this is possible?

That's the problem I expected.  Actually since it's a long not a u_long, it
would overflow at >2GB.  Maybe something like:

Note that it is possible that the product of this value and the value of
.Li _SC_PAGE_SIZE
will overflow a long in some configurations on a 32bit machine.

My worry is that your original phrasing may be unnecessarily subtle.

-- Brooks
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