Re: Porting OpenBSD's sysctl hw.sensors framework to FreeBSD (was: Re: PERFORCE change 123040 for review)


On Sat, 7 Jul 2007, Constantine A. Murenin wrote:

On 07/07/2007 04:40, Andre Oppermann wrote:

Rui Paulo wrote:

Constantine A. Murenin wrote:

http://perforce.freebsd.org/chv.cgi?CH=123040

Change 123040 by cnst@dale on 2007/07/06 23:45:36

add new node to sysctl.h: HW_SENSORS / "hw.sensors"

Hmm. I thought new sysctl nodes or leafs were dynamic and you needn't to add any entry to sysctl.h.

Yes, all new sysctl nodes and leaves should be entirely dynamic. The (old) static ones should go away with 8.0 or so. IIRC there hasn't been any new static sysctl node since a loooong time.

I don't exactly consider April 2007 being a loooong time ago. ;)

So even first- and second-level nodes must be generated dynamically by 8.0?

While Andre's proposed goal of eliminating static sysctl nodes for 8.0 is laudable, a casual glimpse run found 261 references to top-level CTL_ constants in src outside of the kernel, so there's quite a bit of work to be done, as well as some rather serious ABI issues to address if we want that to happen. As such, while we can fix the consumers, I think simply eliminating support for static OIDs in 8.0 is likely unrealistic.

In general, it is desirable, however, for all new nodes to be created using OID_AUTO rather than hand-humbering in order to avoid introducing any further ABI dependencies on static OID allocation. All recent large node hierarchies have used automatic OID numbering, including the top-level compat, sysctl, and security hierarchies. I would like to see the hw.sensor subtree also use automatic OID numbering for the same reason.

While the sysctlbyname(3) API has its limitations, it is straight forward to use, and automatic OID numbering has both code cleanliness and compatibility benefits -- for example, when companies or organizations have heavily modified local FreeBSD installs, it prevents otherwise unresolvable collisions that would lead to ABI incompatibility.

Robert N M Watson
Computer Laboratory
University of Cambridge
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