Makefile中.PHONY的浅析
Makefile中.PHONY的浅析单词phony (即phoney)的意思是：伪造的，假的。来自collins的解释是：
If you describe something as phoney, you disapprove of it because it is false
rather than genuine.
那么，在Makefile中，.PHONY后面的target表示的也是一个伪造的target, 而不是真实存在的文件target，注意Makefile的target默认是文件。
举个例子：
$ cat -n Makefile1
1 clean:
2 rm -f foo
$ cat -n Makefile2
1 .PHONY: clean
2 clean:
3 rm -f foo
Makefile1和Makefile2的差别就是在Makefile2中定义了：
1 .PHONY: clean
直接Make看看
$ ls -l
total 8
-rw-r--r-- 1 huanli huanli 18 Jul 13 17:51 Makefile1
-rw-r--r-- 1 huanli huanli 32 Jul 13 17:51 Makefile2
$ make -f Makefile1 clean
rm -f foo
$ make -f Makefile2 clean
rm -f foo
Makefile1和Makefile2的行为没有啥子区别嘛，呵呵
创建一个文件clean, 再make看看
$ touch clean
$ ls -l
total 8
-rw-r--r-- 1 huanli huanli0 Jul 13 18:06 clean
-rw-r--r-- 1 huanli huanli 18 Jul 13 17:51 Makefile1
-rw-r--r-- 1 huanli huanli 32 Jul 13 17:51 Makefile2
$ make -f Makefile1 clean
make: 'clean' is up to date.
$ make -f Makefile2 clean
rm -f foo
区别来了，Makefile1拒绝了执行clean, 因为文件clean存在。而Makefile2却不理会文件clean的存在，总是执行clean后面的规则。由此可见，.PHONY clean发挥了作用。
小结：
.PHONY: clean
o means the word "clean" doesn't represent a file name in this Makefile;
o means the Makefile has nothing to do with a file called "clean"
in the same directory.
参考资料：
Phony Targets of GNU Make
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