Re: Target-specific variable in subdirectory problem

[Benjamin Cama]

Hi Paul,

Le mercredi 02 août 2017 à 15:16 -0400, Paul Smith a écrit :
> On Wed, 2017-08-02 at 20:03 +0100, Sven C. Dack wrote:
> > On 02/08/17 11:19, Benjamin Cama wrote:
> > > Hi,
> > > 
> > > I may be doing something wrong, but the following Makefile gives me
> > > strange results: the target-specific variable does not apply when
> > > used
> > > for a target in a subdirectory.
> > > 
> > >           test-%: FOO = BAR
> > >           test-%:
> > >                echo $(FOO)
> > >           
> > > E.g.:
> > > 
> > >           $ make test-stem
> > >           echo BAR
> > >           BAR
> > >           $ make subdir/test-stem
> > >           echo
> 
> Pattern rules have a special dispensation for handling patterns with a
> directory prefix, described here:
> 
> https://www.gnu.org/software/make/manual/html_node/Pattern-Match.html
> 
> > When the target pattern does not contain a slash (and it usually does
> > not), directory names in the file names are removed from the file
> > name before it is compared with the target prefix and suffix. After
> > the comparison of the file name to the target pattern, the directory
> > names, along with the slash that ends them, are added on to the
> > prerequisite file names generated from the pattern rule’s
> > prerequisite patterns and the file name.
> 
> That's how "subdir/test-stem" can match a pattern "test-%" which you
> normally would not expect it to match.

Thanks for the reference. I actually kind of expected that it would
match in subdirs in my particular case, so my assumptions were wrong.
During my tries to isolate the problem, I indeed stumbled upon
non-pattern rules strictly matching the filenames, which then made me
cautious about first wanting to report this as a “bug”, when it could
have been a misinterpretation on my side, which it seems it is.

> However, pattern-specific variables do not have this special
> dispensation.  They are simple pattern matches with no special rules
> for dealing with directories.  So, "subdir/test-stem" doesn't match the
> pattern "test-%" for a pattern-specific variable.

Ah, thanks for the explanation of the difference of behavior! This
explains what I see.

> It's possible that this is a mis-feature and that pattern-specific
> variables should have this special capability as well, but they were
> not intended to do so when originally created.  You could file an
> enhancement request on Savannah if you like:
> 
>   https://savannah.gnu.org/bugs/?func=additem&group=make

It indeed kind of look strange that the pattern-specific feature does
not work in some particular case. I do not know what would be the
“right” thing to do, though; I am not sure I am skilled enough to know.

Concerning my real use case, it is just to define target-specific
variables for some recipes which are located in subdirectories. I am not
looking for the target-specific variable feature in filenames strictly
matching the target in the current directory. I looked strange that no
variable was defined when in a subdirectory.

Anyway, it seems that a global variable would solve my problem, and
indeed it looks a bit more right this way: I can still get the stem in
my recipe (with $*), and everything works as expected. It's just that
the global namespace is “polluted” by this variable which is used only
in my particular recipe. And this is where I may be mistaken on the use
of target-specific variable: I tend to use them as a kind of “private
namespace” feature, when I define something that will be used only in
one particular recipe. But then there is no difference with global
variable in this case, apart from namespace pollution. I now understand
that this feature is better suited to overriding a global variable in
some specific recipe: is this right?

To further enhance my understanding of the “right” usage of Makefiles,
would you advise me to explicitly specify the subdirectory in the target
definition? Because I feel that counting on pattern rules to match in
subdirectory is kind of hackish, now. Like:

        $(SUBDIR)/target-%:
          …

And in my particular case, I actually know the $(SUBDIR), but would it
be right then to call make with:

        make SUBDIR=foo foo/target-1234

? It looks redundant to me.

Thanks,
-- 
Benjamin Cama - Tél : 258