makefile pattern rules without recipes

问题

I'm observing an interesting behavior of make and I wonder if there is a reasonable explanation to it besides a bug in gmake.

Let's say we have the following in makefile:

%-animal:
        echo "$* is an animal"

%-fox: %-fox-animal

%-wolf: %-wolf-animal

The difference between the last two targets is that "%-wolf" does not have any recipe, and "%-fox" has an empty recipe (i.e. just a line with a tab at the beginning).

When we try to execute the rules, here's what happens:

[root@cv19 tmp]# make freddy-animal
echo "freddy is an animal"
freddy is an animal
[root@cv19 tmp]# make freddy-wolf
make: *** No rule to make target `freddy-wolf'.  Stop.
[root@cv19 tmp]# make freddy-fox
echo "freddy-fox is an animal"
freddy-fox is an animal

i.e.the pattern rule that has a recipe (although an empty one) works, the one that doesn't does not. Am I missing something in the way it's supposed to work?

回答1:

Pattern rules with no recipes at all are documented as meaning something quite different from those providing a recipe, even an empty one. Instead they cancel any pre-existing implicit rule:

You can cancel a built-in implicit rule by defining a pattern rule with the same target and prerequisites, but no recipe.

Thus your "%-wolf" pattern actually serves to cancel any existing implicit rule for %-wolf-animal -> %-wolf. And there wasn't one anyway.

来源：https://stackoverflow.com/questions/3728689/makefile-pattern-rules-without-recipes