Re: pattern rules and pattern-specific-variables problem

[Boris Kolpackov]

Shawn,

Shawn Halpenny <address@hidden> writes:

> dir/dbg/%.o : V = dir/dbg
> dir/dbg/%.o : %.cpp | $$(V)/. dbg ; echo dbg-V:$(V); touch $@
>
> dir/%.o : V = dir/nondbg
> dir/%.o : %.cpp | $$(V)/. nondbg ; echo nondbg-V:$(V); touch $@
>
>
> The value of V seems to come from the nondbg pattern rule, although
> the dbg pattern rule's commands are executed.  I think this is also
> why there is no 'mkdir -p dir/dbg' when there should be (because the
> order-only 'dbg' prerequisite was run).

GNU make "gathers" variables in the order they appear in the makefile.
In this case the second definition of patter-specific variable V
overrides the first one.

> This is a serious problem for non-recursive make scenarios that make
> extensive use of pattern rules for commands to build objects from
> source in various subdirectories, because the pattern-specific
> variable values that are used depend on the order of the rules.

Agree. You can fix this in your particular case by rewriting the makefile
like this:

dir/%.o : V = dir/nondbg
dir/dbg/%.o : V = dir/dbg

dir/dbg/%.o : %.cpp | $$(V)/. dbg ; echo dbg-V:$(V); touch $@
dir/%.o : %.cpp | $$(V)/. nondbg ; echo nondbg-V:$(V); touch $@

In other words, you need to write the most specialized variable definitions
last and the most specialized rules first.

Some time ago I proposed changing pattern rules selection algorithm from
picking the first applicable rule to picking the most specialized one
(formally, the one with the shortest stem). I guess this will also be
useful (or even more so) for pattern-specific variables.

> I found this surprising enough to file bug 14126:
> https://savannah.gnu.org/bugs/index.php?func=detailitem&item_id=14126

Thanks, I will add a comment.

hth,
-boris