Re: suggested rule for remaking config.h is wrong

[Eric Blake]

On 04/23/2013 09:18 PM, Peter Eisentraut wrote:
> In the manual section "Automatic Remaking", the following rule is
> suggested:
> 
>      config.h: stamp-h
>      stamp-h: config.h.in config.status
>              ./config.status
> 
> This actually doesn't work so well.  It should be
> 
>      config.h: stamp-h ;
>      stamp-h: config.h.in config.status
>              ./config.status

Wow - I've been hunting for this very bug for 48 hours now due to an
off-list bug report!

I can definitely confirm that with no recipe, make -d reports the
following on the first run:

     Prerequisite `stamp-h' is older than target `config.h'.
    No recipe for `config.h' and no prerequisites actually changed.
    No need to remake target `config.h'.

and only on the second run does it change its mind:

     Prerequisite `stamp-h' is newer than target `config.h'.
    Must remake target `config.h'.
    Successfully remade target file `config.h'.

Meanwhile, adding the empty recipe (via the trailing semicolon) forces
make to always behave as in the second run pre-patch.

> 
> If you look at the makefiles generated by Automake, it agrees.

Well, the file I looked at has a lot more than an empty recipe:

config.h: stamp-h1
        @if test ! -f $@; then rm -f stamp-h1; else :; fi
        @if test ! -f $@; then $(MAKE) $(AM_MAKEFLAGS) stamp-h1; else :; fi

stamp-h1: $(srcdir)/config.h.in $(top_builddir)/config.status
        @rm -f stamp-h1
        cd $(top_builddir) && $(SHELL) ./config.status config.h
$(srcdir)/config.h.in:  $(am__configure_deps)
        ($(am__cd) $(top_srcdir) && $(AUTOHEADER))
        rm -f stamp-h1
        touch $@

but the point remains that automake is not generating an empty recipe file.

> 
> This is with GNU Make 3.81.

My testing was with make 3.82.

> 
> The rule for config.h.in: stamp-h.in is probably similarly affected, but
> I don't have verification for that.
> 

Indeed possible; should I go ahead and suggest a semicolon at both sites
in the manual without waiting for confirmation of whether it makes a
difference?

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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