Phony Targets in Makefile

[jiansoung]

Phony Targets in Makefile

.PHONY

The prerequisites of the special target .PHONY are considered to be phony targets. When it is time to consider such a target, make will run its recipe unconditionally, regardless of whether a file with that name exists or what its last-modification time is.

Phony Targets

A phony target is one that is not really the name of a file; rather it is just a name for a recipe to be executed when you make an explicit request.

There are two reasons to use a phony target: to avoid a conflict with a file of the same name, and to improve performance.

If you write a rule whose recipe will not create the target file, the recipe will be executed every time the target comes up for remaking. Here is an example:

clean:
        rm *.o temp

Because the rm command does not create a file named clean, probably no such file will ever exist. Therefore, the rm command will be executed every time you say make clean.

In this example, the clean target will not work properly if a file named clean is ever created in this directory. Since it has no prerequisites, clean would always be considered up to date and its recipe would not be executed. To avoid this problem you can explicitly declare the target to be phony by making it a prerequisite of the special target .PHONY as follows:

.PHONY: clean
clean:
        rm *.o temp

Once this is done, make clean will run the recipe regardless of whether there is a file named clean.

References

1. Phony Targets
2. What is the purpose of .PHONY in a makefile?
3. https://stackoverflow.com/a/2145605/7950607