A phony target is one that is not really the name of a file. It is just a name for some commands to be executed when you make an explicit request. There are two reasons to use a phony target: to avoid a conflict with a file of the same name, and to improve performance.
If you write a rule whose commands will not create the target file, the commands will be executed every time the target comes up for remaking. Here is an example:
clean:
rm *.o temp
Because the rm command does not create a file named clean,
probably no such file will ever exist. Therefore, the rm command
will be executed every time you say ‘make clean’.
The phony target will cease to work if anything ever does create a file
named clean in this directory. Since it has no prerequisites, the
file clean would inevitably be considered up to date, and its
commands would not be executed. To avoid this problem, you can explicitly
declare the target to be phony, using the special target .PHONY
(see Special Built-in Target Names) as follows:
.PHONY : clean
Once this is done, ‘make clean’ will run the commands regardless of whether there is a file named clean.
Since it knows that phony targets do not name actual files that could be
remade from other files, make skips the implicit rule search for
phony targets (see Implicit Rules). This is why declaring a target
phony is good for performance, even if you are not worried about the
actual file existing.
Thus, you first write the line that states that clean is a
phony target, then you write the rule, like this:
.PHONY: clean
clean:
rm *.o temp
Another example of the usefulness of phony targets is in conjunction
with recursive invocations of make (for more information, see
Recursive Use of make). In this case the
makefile will often contain a variable which lists a number of
subdirectories to be built. One way to handle this is with one rule
whose command is a shell loop over the subdirectories, like this:
SUBDIRS = foo bar baz
subdirs:
for dir in $(SUBDIRS); do \
$(MAKE) -C $$dir; \
done
There are a few problems with this method, however. First, any error
detected in a submake is not noted by this rule, so it will continue to
build the rest of the directories even when one fails. This can be
overcome by adding shell commands to note the error and exit, but then
it will do so even if make is invoked with the -k option,
which is unfortunate. Second, and perhaps more importantly, you cannot
take advantage of make's ability to build targets in parallel
(see Parallel Execution), since there is only one rule.
By declaring the subdirectories as phony targets (you must do this as the subdirectory obviously always exists; otherwise it won't be built) you can remove these problems:
SUBDIRS = foo bar baz
.PHONY: subdirs $(SUBDIRS)
subdirs: $(SUBDIRS)
$(SUBDIRS):
$(MAKE) -C $@
foo: baz
Here we've also declared that the foo subdirectory cannot be built until after the baz subdirectory is complete; this kind of relationship declaration is particularly important when attempting parallel builds.
A phony target should not be a prerequisite of a real target file; if it
is, its commands are run every time make goes to update that
file. As long as a phony target is never a prerequisite of a real
target, the phony target commands will be executed only when the phony
target is a specified goal (see Arguments to Specify the Goals).
Phony targets can have prerequisites. When one directory contains multiple programs, it is most convenient to describe all of the programs in one makefile ./Makefile. Since the target remade by default will be the first one in the makefile, it is common to make this a phony target named ‘all’ and give it, as prerequisites, all the individual programs. For example:
all : prog1 prog2 prog3
.PHONY : all
prog1 : prog1.o utils.o
cc -o prog1 prog1.o utils.o
prog2 : prog2.o
cc -o prog2 prog2.o
prog3 : prog3.o sort.o utils.o
cc -o prog3 prog3.o sort.o utils.o
Now you can say just ‘make’ to remake all three programs, or specify as arguments the ones to remake (as in ‘make prog1 prog3’). Phoniness is not inherited: the prerequisites of a phony target are not themselves phony, unless explicitly declared to be so.
When one phony target is a prerequisite of another, it serves as a subroutine of the other. For example, here ‘make cleanall’ will delete the object files, the difference files, and the file program:
.PHONY: cleanall cleanobj cleandiff
cleanall : cleanobj cleandiff
rm program
cleanobj :
rm *.o
cleandiff :
rm *.diff