06-phony-targets.md

Phony Targets

Gaston Sanchez

Learning Objectives

• Understand Phony targets
• Declaring phony targets
• Introducing the phony target clean

---

Phony Targets

By default, Makefile targets are file targets, that is, targets are used to build files from other files—the dependencies. This implies that Make assumes the target of a rule is a file.

There are occasions, however, when a given target is not really a file, but simply the name of a recipe (i.e. an action to be performed). One example is the all target like the one in 03-various-targets/. The associate Makefile is:

# all
all: output.html output.tex output.txt

# from markdown to html
output.html: input.md
    pandoc input.md -s -o output.html

# from markdown to latex
output.tex: input.md
    pandoc input.md -s -o output.tex

# from markdown to reStructuredText
output.txt: input.md
    pandoc input.md -t rst -s -o output.txt

The first target all is not the name of a file; it is simply a convenient “label” to indicate a series of dependencies. This type of target is known in Make terminology as a phony target.

In fact, Make has several special targets. One special class of targets are Phony targets: phony meaning “not real” or “fake.”

A phony target is “not real” in the sense that it’s 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.

Declaring phony targets

To declare a phony target, you have to explicitly declare the target to be phony by making it a prerequisite of the special target .PHONY as follows:

# declaring phony target
.PHONY: all

# all
all: output.html output.tex output.txt

# from markdown to html
output.html: input.md
    pandoc input.md -s -o output.html

# from markdown to latex
output.tex: input.md
    pandoc input.md -s -o output.tex

# from markdown to reStructuredText
output.txt: input.md
    pandoc input.md -t rst -s -o output.txt

Declaration of phony targets is important because of two main reasons:

1. to avoid a conflict with a file of the same name, and

2. to improve performance

Confusion may be caused when the name of an existing file is used as the name of a phony target. In such a case Make will be confused because the 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.

The clean phony target

One very common phony target is the so-called clean target. This target appears in many makefiles to indicate a recipe for “cleaning” a directory from secondary files typically generated after some compilation process.

Likewise, the clean target is also commonly used to remove output files. A related example can be found in 06-phony-targets. It is essentially the same example as in 03-various-targets, but this time we are explicitly declaring phony targets:

# phony targets
.PHONY: all clean

# all
all: output.html output.tex output.txt


# from markdown to html
output.html: input.md
    pandoc input.md -s -o output.html

# from markdown to latex
output.tex: input.md
    pandoc input.md -s -o output.tex

# from markdown to reStructuredText
output.txt: input.md
    pandoc input.md -t rst -s -o output.txt

# remove output files
clean:
    rm -f output.*

Besides the all target, there is now another phony target: clean. Note that the rm -f command does not create a file named clean. The -f flag is used to avoid displaying a diagnostic or error message if the output.* files do not exists.

Also note that clean has no dependencies. Therefore, the target clean will be always out-of-date. So whenever you run make clean, it will run independent from the state of the file system.

---

Make Documentation

Phony Targets

---