[bug #59490] target may not be remade if prerequisite has no recipe

[Paul D. Smith]

Update of bug #59490 (project make):

  Status:None => Not A Bug  
 Open/Closed:Open => Closed 


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[bug #59490] target may not be remade if prerequisite has no recipe

[Greg Minshall]

Follow-up Comment #5, bug #59490 (project make):

thanks, both.

i apologize for my posted question being incomplete!

i'm very sympathetic to this:
> However, your makefile is lying to make; it says that the
> recipe for b1 builds just b1, but in fact it also builds c1
> "behind make's back".
(as, in general, i'd rather be a truth-teller).

i guess for public release, i had better not rely on "&:", until >= 4.3
make(1) become widely distributed.

thanks, by the way, for the fact that (afaict) if i have

b1 c1 &: a1
   
d1: c1
   

but, in a run,  is run, b1 changes, but c1 is *not* changed, then
d1 is *not* remade.

cheers.

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[bug #59490] target may not be remade if prerequisite has no recipe

[Paul D. Smith]

Follow-up Comment #4, bug #59490 (project make):

I've checked other implementations of make that aim to be POSIX compatible and
they also do not rebuild d1 in this situation.  I can't find any text in POSIX
that specifies how this should behave.  Indeed, the POSIX spec is very limited
in its discussion of targets that do not have commands.

In fact one other make implementation I found will never consider d1 to be out
of date no matter how many times you run it, if the c1 target doesn't have a
recipe.

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[bug #59490] target may not be remade if prerequisite has no recipe

[Paul D. Smith]

Follow-up Comment #3, bug #59490 (project make):

Your attached makefile shows the issue you're trying to get at, but the
example in your question is incomplete: it's missing the fact that (a) you run
"make d1" and (b) the file "c1" must already exist before the makefile runs. 
If "c1" doesn't exist, then "d1" will get rebuilt.

It's not actually the case that this is due to the directory cache, at least
not exactly.  I have a set of changes locally that "fix" the directory cache
problem by invalidating the cache every time make invokes a command of any
type.  However it doesn't change the behavior in this case.

The reason for the behavior is that make starts with d1, then decides it needs
c1.  Then it tries to build "b1", and succeeds.  However, your makefile is
lying to make; it says that the recipe for b1 builds just b1, but in fact it
also builds c1 "behind make's back".  Since make has no idea that c1 was
modified, it does not re-check the timestamp for c1.

Whether this is right or not, I'm not sure.  I'd have to re-check the manual
and the POSIX spec.  It's certainly the way GNU make has worked for 30 years
or so.

If you just want to avoid the problem, another alternative to using an empty
recipe (assuming you have GNU make 4.3) is to tell make the truth: that one
rule builds both targets.  You can do that with:


b1 c1 &: a1
touch b1
touch c1


(and remove the "c1: b1" prerequisite).

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[bug #59490] target may not be remade if prerequisite has no recipe

[Dmitry Goncharov]

Follow-up Comment #2, bug #59490 (project make):

Greg, you observe the effect of make fs cache.
In order to avoid this effect you need to tell make explicitly that c1 is to
be rebuilt when b1 changes.

e.g.

b1: a1
touch b1
c1: b1
touch c1
d1: c1
touch d1


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[bug #59490] target may not be remade if prerequisite has no recipe

[Greg Minshall]

Follow-up Comment #1, bug #59490 (project make):

though maybe it's not so simple?  e.g., if b is remade, but c doesn't change,
then probably d shouldn't change.

i guess my naive hope/expectation is that b will be remade, then c's date/time
will be compared with that of d to decided if d should be remade.

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[bug #59490] target may not be remade if prerequisite has no recipe

[Greg Minshall]

URL:
  

 Summary: target may not be remade if prerequisite has no
recipe
 Project: make
Submitted by: minshall
Submitted on: Fri 20 Nov 2020 06:52:47 AM UTC
Severity: 3 - Normal
  Item Group: Bug
  Status: None
 Privacy: Public
 Assigned to: None
 Open/Closed: Open
 Discussion Lock: Any
   Component Version: 4.3
Operating System: POSIX-Based
   Fixed Release: None
   Triage Status: None

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Details:

i have a feeling this is already known, and maybe accepted, behavior, but it
seems wrong, non-intuitive, to me.

if i have a dependency chain like
++
b1: a1
  touch b1
  touch c1
c1: b1
d1: c1
  touch d1
--

then, if b1 needs to be re-made, d1 will *not* be remade, even though c1's
prerequisite b1 is out of date.

if i change to 'c1: b1;', it works.

i find the behavior surprising, and i would think worthy of changing.  (i'm
guessing this is an optimization?)

with the attached Makefile, a 'make test' will show failure (or, success,
should it succeed).




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File Attachments:


---
Date: Fri 20 Nov 2020 06:52:47 AM UTC  Name: Makefile  Size: 2KiB   By:
minshall
make test


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