Re: [conntrack-tools 6/6] build: fix dependency-tracking of yacc-generated header

[Jeremy Sowden <jeremy@azazel.net>]

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On 2021-09-25, at 17:49:27 +0200, Jan Engelhardt wrote:
> On Saturday 2021-09-25 17:10, Jeremy Sowden wrote:
> > List it as a built source in order to force make to create it before
> > compilation.  Otherwise, a parallel make can end up attempting to
> > compile the output of lex before yacc has finished generating its
> > own output:
> >
> > --- a/src/Makefile.am
> > +++ b/src/Makefile.am
> > @@ -6,6 +6,7 @@ endif
> >
> >  AM_YFLAGS = -d
> >
> > +BUILT_SOURCES = read_config_yy.h
> >  MAINTAINERCLEANFILES = read_config_yy.c read_config_yy.h read_config_lex.c
>
> I have a strong reason to believe that you could just write
>
>  read_config_yy.h: read_config_yy.y
>
> (detail https://lists.gnu.org/archive/html/automake/2021-09/msg00011.html )

Automake complains:

  src/Makefile.am:65: warning: user target 'read_config_yy.h' defined here ...
  automake: ... overrides Automake target 'read_config_yy.h' defined here

From the documentation (https://www.gnu.org/software/automake/manual/automake.html#Extending):

  Note that Automake does not make any distinction between rules with
  commands and rules that only specify dependencies. So it is not
  possible to append new dependencies to an automake-defined target
  without redefining the entire rule.

In this case, adding the rule you suggest to Makefile.am suppresses the
rule that automake would normally generate:

  read_config_yy.h: read_config_yy.c
          @if test ! -f $@; then rm -f read_config_yy.c; else :; fi
          @if test ! -f $@; then $(MAKE) $(AM_MAKEFLAGS) read_config_yy.c; else :; fi

J.

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