What is the purpose of .PHONY in a Makefile?

By default, Makefile targets are "file targets" - they are used to build files from other files. Make assumes its target is a file, and this makes writing Makefiles relatively easy:

foo: bar
  create_one_from_the_other foo bar

However, sometimes you want your Makefile to run commands that do not represent physical files in the file system. Good examples for this are the common targets "clean" and "all". Chances are this isn't the case, but you may potentially have a file named clean in your main directory. In such a case Make will be confused because by default the clean target would be associated with this file and Make will only run it when the file doesn't appear to be up-to-date with regards to its dependencies.

These special targets are called phony and you can explicitly tell Make they're not associated with files, e.g.:

.PHONY: clean
clean:
  rm -rf *.o

Now make clean will run as expected even if you do have a file named clean.

In terms of Make, a phony target is simply a target that is always out-of-date, so whenever you ask make <phony_target>, it will run, independent from the state of the file system. Some common make targets that are often phony are: all, install, clean, distclean, TAGS, info, check.

It is a build target that is not a filename.

The special target .PHONY: allows to declare phony targets, so that make will not check them as actual file names: it will work all the time even if such files still exist.

You can put several .PHONY: in your Makefile :

.PHONY: all

all : prog1 prog2

...

.PHONY: clean distclean

clean :
    ...
distclean :
    ...

There is another way to declare phony targets : simply put :: without prerequisites :

all :: prog1 prog2

...

clean ::
    ...
distclean ::
    ...

The :: has other special meanings, see here, but without prerequisites it always execute the recipes, even if the target already exists, thus acting as a phony target.

I often use them to tell the default target not to fire.

superclean: clean andsomethingelse

blah: superclean

clean:
   @echo clean

%:
   @echo catcher $@

.PHONY: superclean

Without PHONY, make superclean would fire clean, andsomethingelse, and catcher superclean; but with PHONY, make superclean won't fire the catcher superclean.

We don't have to worry about telling make the clean target is PHONY, because it isn't completely phony. Though it never produces the clean file, it has commands to fire so make will think it's a final target.

However, the superclean target really is phony, so make will try to stack it up with anything else that provides deps for the superclean target — this includes other superclean targets and the % target.

Note that we don't say anything at all about andsomethingelse or blah, so they clearly go to the catcher.

The output looks something like this:

$ make clean
clean

$ make superclean
clean
catcher andsomethingelse

$ make blah 
clean
catcher andsomethingelse
catcher blah