强制目标

例子下：

.PHONY: fileB   #在makefile中.PHONY:表示fileB是一个标签，不是一个真文件。每一次更新fileB时，它的Recipe(制作方式)都会执行。

fileB:

    echo `date` >>$@

 #fileA 的 prerequisite是FORCE,FORCE是一个无prerequisite无recipe的“全无目标”，make在运行时，每次都更新它，

#这样 fileA的更新时间比FORCE的要早。所以还会执行fileA的recipe。

fileA: FORCE      

    echo `date` >>$@

FORCE:

http://www.gnu.org/software/make/manual/make.html

4.7 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 Phony Targets.