bug#13104: Extending automake rules w.r.t dependencies

[Nick Bowler]

Hello,

On 2012-12-06 16:41 +0100, Theophile Ranquet wrote:
> I am seeking advice for a workaround of what seems like a limitation
> in Automake (1.12.2).
> 
> We have recently added figures to Bison's documentation. These figures
> are included by our texinfo source by using "@image", which requires
> different image formats depending on what we want to produce. For
> convenience's sake, these figures are generated from Graphviz DOT
> files, so we tried adding the following to our doc/local.mk:
> 
> doc/bison.dvi:  $(FIGS_DOT:.dot=.eps)
> doc/bison.html: $(FIGS_DOT:.dot=.png)
> doc/bison.pdf:  $(FIGS_DOT:.dot=.pdf)
> 
> However, these overwrite Automake's rules. This seems a bit similar,
> in that regard, to the issue discussed here:
> http://lists.gnu.org/archive/html/automake/2012-11/msg00015.html

The usual workaround to add prerequisites to an automake-generated
rule is to use a one-off make variable for the target name.  For
example,

  doc_bison_dvi = doc/bison.dvi
  $(doc_bison_dvi): $(FIGS_DOT:.dot=.eps)

and so on.  Automake does a simple string comparison of the target name
to decide whether or the generated rule is overridden, so using a
variable will hide the "real" target name from this check.  Note that
this is only appropriate for rules with no commands.

> (Note that in another branch we are using multiple Makefiles, and in
> that branch adding the previous lines to our doc/Makefile.am has the
> desired effect, though I am not sure why... is this a bug or a
> feature?)

Hard to say without looking a concrete example, but the most likely
explanation is that the Automake-generated rules are different in the
recursive case, such that your custom rules don't actually override
anything.

Hope that helps,
-- 
Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)