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Re: trouble with order rules in implicit rules

From: Noel Yap
Subject: Re: trouble with order rules in implicit rules
Date: Fri, 16 Apr 2004 14:57:32 -0400
User-agent: Mozilla Thunderbird 0.5 (Windows/20040212)

I think I've got it (de ja vu, actually, I think I ran into this some time ago 
and came to the same conclusion):  Don't use order rules in implicit rules.  
Rather, use order rules only in static rules.

If I change the makefile to:

.PHONY: install
install: foo/bar/baz

        mkdir -p $(@)

foo/%: ./%
        @echo $(<)
        touch $(@)

foo/bar/%: bar/%
        @echo $(<)
        touch $(@)

foo/bar/baz: | foo/bar/.


everything works perfectly.  This sounds to me, though, that there's still a 
bug lurking somewhere.


Noel Yap wrote:
I think I see. If I switch the two rules and foo doesn't exist, everything works fine. But if foo exists and foo/bar doesn't, the same error occurs even if the rule for foo/bar/% comes first. If this is the defined behaviour, what needs to be done to get the action to be fired in the latter scenario?


Paul D. Smith wrote:

%% Noel Yap <address@hidden> writes:

  ny> I had thought that the better-matching rule would be the one used.

I don't know what "better-matching" means, but the way make chooses a
pattern rule is well-defined; from section "Introduction to Pattern

  The order in which pattern rules appear in the makefile is important
since this is the order in which they are considered.  Of equally
applicable rules, only the first one found is used.

I guess that's a little confusing, since "equally applicable" isn't
defined anywhere.  But, it refers to the last sentence:

Note however, that a rule whose prerequisites actually exist or are
mentioned always takes priority over a rule with prerequisites that must
be made by chaining other implicit rules.

Anyway, the short answer is that for basic pattern rules (not chained
pattern rules) the first one appearing in the makefile is always used.

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