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Chapter 4: Writing Rules 31

cleanall : cleanobj cleandiff

rm program

cleanobj :

rm *.o

cleandiff :

rm *.diff

4.6 Rules without Recipes or Prerequisites

If a rule has no prerequisites or recipe, and the target of the rule is a nonexistent ﬁle, then

make imagines this target to have been updated whenever its rule is run. This implies that

all targets depending on this one will always have their recipe run.

An example will illustrate this:

clean: FORCE

rm $(objects)

FORCE:

Here the target ‘FORCE’ satisﬁes the special conditions, so the target clean that depends

on it is forced to run its recipe. There is nothing special about the name ‘FORCE’, but that

is one name commonly used this way.

As you can see, using ‘FORCE’ this way has the same results as using ‘.PHONY: clean’.

Using ‘.PHONY’ is more explicit and more eﬃcient. However, other versions of make do

not support ‘.PHONY’; thus ‘FORCE’ appears in many makeﬁles. See Section 4.5 [Phony

Targets], page 29.

4.7 Empty Target Files to Record Events

The empty target is a variant of the phony target; it is used to hold recipes for an action

that you request explicitly from time to time. Unlike a phony target, this target ﬁle can

really exist; but the ﬁle’s contents do not matter, and usually are empty.

The purpose of the empty target ﬁle is to record, with its last-modiﬁcation time, when

the rule’s recipe was last executed. It does so because one of the commands in the recipe

is a touch command to update the target ﬁle.

The empty target ﬁle should have some prerequisites (otherwise it doesn’t make sense).

When you ask to remake the empty target, the recipe is executed if any prerequisite is more

recent than the target; in other words, if a prerequisite has changed since the last time you

remade the target. Here is an example:

print: foo.c bar.c

lpr -p $?

touch print

With this rule, ‘make print’ will execute the lpr command if either source ﬁle has changed

since the last ‘make print’. The automatic variable ‘$?’ is used to print only those ﬁles

that have changed (see Section 10.5.3 [Automatic Variables], page 120).

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Roland Ver. 4.5 Technical Information Seite 41