.PHONY mean in a Makefile? I have gone through this, but it is too complicated.
Can somebody explain it to me in simple terms?
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
make clean will run as expected even if you do have a file named
In terms of Make, a phony target is simply a target that is always out-of-date, so whenever you ask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:
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
distclean, and so on.
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.
In a directory ‘test’ following files are present:
[email protected]:~/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.