What is the purpose of PHONY in a Makefile

Question

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

User · Answer

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

User · Answer

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    without prerequisites   all    prog1 prog2       clean            distclean             The    has other special meanings  see here  but without prerequisites it always execute the recipes  even if the target already exists  thus acting as a phony target

User · Answer

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

User · Answer

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

User · Answer

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  lt phony target gt   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

User · Answer

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

User · Answer

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

User · Answer

It is a build target that is not a filename

User · Answer

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  gt fileall   file2  file2 src     echo file2 src  gt file2  file1  file1 src     echo file1 src  gt file1     echo file1 src  gt  gt file1   PHONY  filefwd  PHONY  filefwd2  filefwd  filefwd2  filefwd2  file1      echo  Produced target file1    prepare      echo  Some text 1   gt  gt  file1 src     echo  Some text 2   gt  gt  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

[makefile] What is the purpose of .PHONY in a Makefile?

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