Subject: Re: bin/34934: make sometimes fails to handle multiple targets on the left correctly
To: None <gnats-bugs@NetBSD.org>
From: David Holland <dholland@eecs.harvard.edu>
List: netbsd-bugs
Date: 10/29/2006 13:29:39
 >  If you have:
 >  
 >  a1 a1: b
        ^2
 >  	echo $@
 >  
 >  The make runs the rules twice, once for a1 and once for a2.
 >  This is the correct and required behaviour.
 >  (Otherwise the above could't output 'a1' and 'a2'.)

Yes.

It's not a matter of what the rules mean; it's a matter of when
timestamps can and cannot be cached. If you have

a1 a2: b
	echo foo > a1; echo bar > a2

then after running the recipe make is required to check and ascertain
that both a1 and a2 have been updated. Then it won't run the recipe a
second time. Whereas if you don't list both a1 and a2 on the left make
is allowed to assume that the one not listed isn't modified, and this
can cause things to go wrong later. (Lying to make rarely goes
unpunished.)

You'll note that make does actually do this; the problem is that
something breaks some of the time if a1 and a2 are in another
directory.

-- 
   - David A. Holland / dholland@eecs.harvard.edu