The thing that the docs refer to as "impose order" is not a single thing, but two. I would characterize a normal prerequisite as doing three things rather than two: - *update-marking: *cause a target to be marked out of date if the prereq is marked out of date - *require-existence: *require the prereq to be built successfully at least once before the target is built - *imposed-order: *require the prereq to be built before the target, if both are being built A prereq which truly was order-only would *exclusively* do the last one, imposing order. The "order-only" prereqs as they stand do both *imposed-order* and *require-existence*. For .PHONY targets, *require-existence* always mandates *update-marking*, which is why the overlap between those two cases is particularly confusing - a phony target which drops only update-marking is a contradiction in terms, so the natural assumption to make is that the effect of "order-only" prereqs is *not* that, and is instead something non-paradoxical.
I looked deeper into the list archives and saw that this discussion has come up before; Stefano Lattarini experienced the same problem in 2012 <https://lists.gnu.org/archive/html/bug-make/2012-06/msg00008.html> (thread starts 2012/06/11). Phillip Guenther said <https://lists.gnu.org/archive/html/bug-make/2012-06/msg00011.html> that this pure-order effect is not achievable with the structure of make, which presumably remains true. I agree with Lattarini's recommendations, which I'll reproduce below for convenience. The primary one was the suggestion of alternate names that might be used for what are currently called order-only prerequisites - "existence-only prerequisites" seems like the clearest one to me. Thanks, Jacob Lattarini: > - the name "order-only prerequisites" is IMHO poorly chosen, and don't > really correspond to their actual semantics; a more appropriate might > be "weak prerequisites" or "existence-only prerequisites"; > - the explanation given in the manual is imprecise and confusing; since > the explanation you've given here is OTOH very clear and concise, I > suggest you report it in the manual; > - it might make sense to state explicitly that order-only prereqs > which are phony behave in all ways like standard prerequisites. The referenced 'explanation you've given here' is from Paul Smith - presumably the same one that has responded to me? It was > No, that's not how order-only prerequisites work. An order-only > prerequisite is treated identically to a normal prerequisite except for > exactly one thing: when make checks to see if a target needs to be > remade, the time-last-modified values for any order-only prerequisites > are not considered. On Wed, May 18, 2022 at 11:14 AM Paul Smith <psm...@gnu.org> wrote: > On Wed, 2022-05-18 at 10:22 -0700, Jacob Kopczynski wrote: > > I believe I understand. The name "order-only" is highly misleading > > and should be changed - it does considerably more than "only" > > "order"; the only thing it does not do is check the timestamp. > > As described in the docs there are only two things a prerequisite can > do: impose order and effect the out-of-date decision of the target. > Order-only prerequisites do one of them (impose order) and don't do the > other (effect out-of-date decisions). > > What are the "considerably more" things that order-only prerequisites > do, besides impose order? > > I do agree that the docs need clarification though. >
