问题
What does .PHONY mean in a Makefile? I have gone through this, but it is too complicated.
Can somebody explain it to me in simple terms?
回答1:
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In terms of Make, a phony target is simply a target that is always out-of-date, so whenever you ask make 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.
回答2:
Let's assume you have install target, which is a very common in makefiles. If you do not use .PHONY, and a file named install exists in the same directory as the Makefile, then make install will do nothing. This is because Make interprets the rule to mean "execute such-and-such recipe to create the file named install". Since the file is already there, and its dependencies didn't change, nothing will be done.
However if you make the install target PHONY, it will tell the make tool that the target is fictional, and that make should not expect it to create the actual file. Hence it will not check whether the install file exists, meaning: a) its behavior will not be altered if the file does exist and b) extra stat() will not be called.
Generally all targets in your Makefile which do not produce an output file with the same name as the target name should be PHONY. This typically includes all, install, clean, distclean, and so on.
回答3:
NOTE: The make tool reads the makefile and checks the modification time-stamps of the files at both the side of ':' symbol in a rule.
Example
In a directory 'test' following files are present:
prerit@vvdn105:~/test$ ls
hello hello.c makefile
In makefile a rule is defined as follows:
hello:hello.c
cc hello.c -o hello
Now assume that file 'hello' is a text file containing some data, which was created after 'hello.c' file. So the modification (or creation) time-stamp of 'hello' will be newer than that of the 'hello.c'. So when we will invoke 'make hello' from command line, it will print as:
make: `hello' is up to date.
Now access the 'hello.c' file and put some white spaces in it, which doesn't affect the code syntax or logic then save and quit. Now the modification time-stamp of hello.c is newer than that of the 'hello'. Now if you invoke 'make hello', it will execute the commands as:
cc hello.c -o hello
And the file 'hello' (text file) will be overwritten with a new binary file 'hello' (result of above compilation command).
If we use .PHONY in makefile as follow:
.PHONY:hello
hello:hello.c
cc hello.c -o hello
and then invoke 'make hello', it will ignore any file present in the pwd 'test' and execute the command every time.
Now suppose, that 'hello' target has no dependencies declared:
hello:
cc hello.c -o hello
and 'hello' file is already present in the pwd 'test', then 'make hello' will always show as:
make: `hello' is up to date.
回答4:
.PHONY: install
- means the word "install" doesn't represent a file name in this Makefile;
- means the Makefile has nothing to do with a file called "install" in the same directory.
回答5:
It is a build target that is not a filename.
回答6:
The best explanation is the GNU make manual itself: 4.6 Phony Targets section.
.PHONY is one of make's Special Built-in Target Names. There are other targets that you may be interested in, so it's worth skimming through these references.
When it is time to consider a .PHONY target, make will run its recipe unconditionally, regardless of whether a file with that name exists or what its last-modification time is.
You may also be interested in make's Standard Targets such as all and clean.
回答7:
There's also one important tricky treat of ".PHONY" - when a physical target depends on phony target that depends on another physical target:
TARGET1 -> PHONY_FORWARDER1 -> PHONY_FORWARDER2 -> TARGET2
You'd simply expect that if you updated TARGET2, then TARGET1 should be considered stale against TARGET1, so TARGET1 should be rebuild. And it really works this way.
The tricky part is when TARGET2 isn't stale against TARGET1 - in which case you should expect that TARGET1 shouldn't be rebuild.
This surprisingly doesn't work because: the phony target was run anyway (as phony targets normally do), which means that the phony target was considered updated. And because of that TARGET1 is considered stale against the phony target.
Consider:
all: fileall
fileall: file2 filefwd
echo file2 file1 >fileall
file2: file2.src
echo file2.src >file2
file1: file1.src
echo file1.src >file1
echo file1.src >>file1
.PHONY: filefwd
.PHONY: filefwd2
filefwd: filefwd2
filefwd2: file1
@echo "Produced target file1"
prepare:
echo "Some text 1" >> file1.src
echo "Some text 2" >> file2.src
You can play around with this:
- first do 'make prepare' to prepare the "source files"
- play around with that by touching particular files to see them updated
You can see that fileall depends on file1 indirectly through a phony target - but it always gets rebuilt due to this dependency. If you change the dependency in fileall from filefwd to file, now fileall does not get rebuilt every time, but only when any of dependent targets is stale against it as a file.
回答8:
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
回答9:
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 '::'
all :: prog1 prog2
...
clean ::
...
distclean ::
...
The '::' has a special meaning : targets are phony plus they can appear several times :
clean ::
rm file1
...
clean ::
rm file2
The command blocks will be called one after the other.
来源：https://stackoverflow.com/questions/2145590/what-is-the-purpose-of-phony-in-a-makefile