Re: Understanding Pattern Rules

[Philip Guenther]

On Tue, May 5, 2015 at 9:25 AM, Paul Smith <address@hidden> wrote:
> On Tue, 2015-05-05 at 07:49 -0700, address@hidden wrote:
>> file.out: %.cached
>>         cat $^ > file.out
>
> This is not a pattern rule.  A pattern rule must have a pattern match
> (the "%") in the TARGET, because it matches any of a set of targets.
> Having a pattern in the prerequisite is optional, but there must be one
> in the target, or else it's an explicit rule... and thus it depends on
> the explicit prerequisite named, literally, '%.cached'.
>
> To make this a pattern rule you'd need to write:
>
>   %.out : %.cached
>           cat $^ > $@
>
> or similar.

The original poster didn't actually state what the desired behavior
was, but I'll *guess* that we wants a rule that says
   "treat as prerequisites all the files '*.cached' that make can
figure out how to build"

make has no way to express the "all the files that it figure out how
to build" part.  However, in this case, there are two other solutions.

First, based on the presence of the pattern "%.cached: %.src" in his
question, he would probably get equivalent results with a rule that
says "treat as prerequisites all the files '*.cached' for which a file
'*.src' exists", which we *can* do:
    SRCS = $(wildcard *.src)
    file.out: $(SRCS:.src=.cached)
            cat $^ >$@


The other solution is to recognize that depending on pattern matching
means that filenames become magical, which can trip things up later.
Most build systems have chosen to require an explicit action to
include a file in a build; that lets developers use whatever filenames
they want without disturbing the build.  Using pattern matching means
you can't do a build if you have new, not-yet-working code, unless you
want to develop the file under some other name and rename it into
place when it's ready.  But if you're going to require an explicit
action (renaming) to make a file for ready to include the build, why
not use an action that is simpler and clear to document by just
listing the files in the Makefile?  So, solution two:

   SRCS = foo.src bar.src baz.src quux.src..etc
   file.out: $(SRCS:.src=.cached)
           cat $^ >$@


Philip Guenther