Makefiles

Overview

Teaching: 30 min
Exercises: 10 min

Questions

• How do I write a simple Makefile?

Objectives

• Recognize the key parts of a Makefile, rules, targets, dependencies and actions.

• Write a simple Makefile.

• Run Make from the shell.

• Explain when and why to mark targets as .PHONY.

• Explain constraints on dependencies.

Create a file, called Makefile, with the following content:

# Count words.
isles.dat : books/isles.txt
	python countwords.py books/isles.txt isles.dat

This is a build file, which for Make is called a Makefile - a file executed by Make. Note how it resembles one of the lines from our shell script.

Let us go through each line in turn:

• # denotes a comment. Any text from # to the end of the line is ignored by Make.
• isles.dat is a target, a file to be created, or built.
• books/isles.txt is a dependency, a file that is needed to build or update the target. Targets can have zero or more dependencies.
• A colon, :, separates targets from dependencies.
• python countwords.py books/isles.txt isles.dat is an action, a command to run to build or update the target using the dependencies. Targets can have zero or more actions. These actions form a recipe to build the target from its dependencies and can be considered to be a shell script.
• Actions are indented using a single TAB character, not 8 spaces. This is a legacy of Make’s 1970’s origins. If the difference between spaces and a TAB character isn’t obvious in your editor, try moving your cursor from one side of the TAB to the other. It should jump four or more spaces.
• Together, the target, dependencies, and actions form a rule.

Our rule above describes how to build the target isles.dat using the action python countwords.py and the dependency books/isles.txt.

Information that was implicit in our shell script - that we are generating a file called isles.dat and that creating this file requires books/isles.txt - is now made explicit by Make’s syntax.

Let’s first ensure we start from scratch and delete the .dat and .png files we created earlier:

$ rm *.dat *.png

By default, Make looks for a Makefile, called Makefile, and we can run Make as follows:

$ make

By default, Make prints out the actions it executes:

python countwords.py books/isles.txt isles.dat

If we see,

Makefile:3: *** missing separator.  Stop.

then we have used a space instead of a TAB characters to indent one of our actions.

Let’s see if we got what we expected.

head -5 isles.dat

The first 5 lines of isles.dat should look exactly like before.

Makefiles Do Not Have to be Called Makefile

We don’t have to call our Makefile Makefile. However, if we call it something else we need to tell Make where to find it. This we can do using -f flag. For example, if our Makefile is named MyOtherMakefile:

$ make -f MyOtherMakefile

Sometimes, the suffix .mk will be used to identify Makefiles that are not called Makefile e.g. install.mk, common.mk etc.

When we re-run our Makefile, Make now informs us that:

make: `isles.dat' is up to date.

This is because our target, isles.dat, has now been created, and Make will not create it again. To see how this works, let’s pretend to update one of the text files. Rather than opening the file in an editor, we can use the shell touch command to update its timestamp (which would happen if we did edit the file):

$ touch books/isles.txt

If we compare the timestamps of books/isles.txt and isles.dat,

$ ls -l books/isles.txt isles.dat

then we see that isles.dat, the target, is now older than books/isles.txt, its dependency:

-rw-r--r--    1 mjj      Administ   323972 Jun 12 10:35 books/isles.txt
-rw-r--r--    1 mjj      Administ   182273 Jun 12 09:58 isles.dat

If we run Make again,

$ make

then it recreates isles.dat:

python countwords.py books/isles.txt isles.dat

When it is asked to build a target, Make checks the ‘last modification time’ of both the target and its dependencies. If any dependency has been updated since the target, then the actions are re-run to update the target. Using this approach, Make knows to only rebuild the files that, either directly or indirectly, depend on the file that changed. This is called an incremental build.

Makefiles as Documentation

By explicitly recording the inputs to and outputs from steps in our analysis and the dependencies between files, Makefiles act as a type of documentation, reducing the number of things we have to remember.

Let’s add another rule to the end of Makefile:

abyss.dat : books/abyss.txt
	python countwords.py books/abyss.txt abyss.dat

If we run Make,

$ make

then we get:

make: `isles.dat' is up to date.

Nothing happens because Make attempts to build the first target it finds in the Makefile, the default target, which is isles.dat which is already up-to-date. We need to explicitly tell Make we want to build abyss.dat:

$ make abyss.dat

Now, we get:

python countwords.py books/abyss.txt abyss.dat

“Up to Date” Versus “Nothing to be Done”

If we ask Make to build a file that already exists and is up to date, then Make informs us that:

make: `isles.dat' is up to date.

If we ask Make to build a file that exists but for which there is no rule in our Makefile, then we get message like:

$ make countwords.py

make: Nothing to be done for `countwords.py'.

up to date means that the Makefile has a rule with one or more actions whose target is the name of a file (or directory) and the file is up to date.

Nothing to be done means that the file exists but either :

• the Makefile has no rule for it, or
• the Makefile has a rule for it, but that rule has no actions

We may want to remove all our data files so we can explicitly recreate them all. We can introduce a new target, and associated rule, to do this. We will call it clean, as this is a common name for rules that delete auto-generated files, like our .dat files:

clean :
	rm -f *.dat

This is an example of a rule that has no dependencies. clean has no dependencies on any .dat file as it makes no sense to create these just to remove them. We just want to remove the data files whether or not they exist. If we run Make and specify this target,

$ make clean

then we get:

rm -f *.dat

There is no actual thing built called clean. Rather, it is a short-hand that we can use to execute a useful sequence of actions. Such targets, though very useful, can lead to problems. For example, let us recreate our data files, create a directory called clean, then run Make:

$ make isles.dat abyss.dat
$ mkdir clean
$ make clean

We get:

make: `clean' is up to date.

Make finds a file (or directory) called clean and, as its clean rule has no dependencies, assumes that clean has been built and is up-to-date and so does not execute the rule’s actions. As we are using clean as a short-hand, we need to tell Make to always execute this rule if we run make clean, by telling Make that this is a phony target, that it does not build anything. This we do by marking the target as .PHONY:

.PHONY : clean
clean :
	rm -f *.dat

If we run Make,

$ make clean

then we get:

rm -f *.dat

We can add a similar command to create all the data files. We can put this at the top of our Makefile so that it is the default target, which is executed by default if no target is given to the make command:

.PHONY : dats
dats : isles.dat abyss.dat

This is an example of a rule that has dependencies that are targets of other rules. When Make runs, it will check to see if the dependencies exist and, if not, will see if rules are available that will create these. If such rules exist it will invoke these first, otherwise Make will raise an error.

Dependencies

The order of rebuilding dependencies is arbitrary. You should not assume that they will be built in the order in which they are listed.

Dependencies must form a directed acyclic graph. A target cannot depend on a dependency which itself, or one of its dependencies, depends on that target.

This rule is also an example of a rule that has no actions. It is used purely to trigger the build of its dependencies, if needed.

If we run,

$ make dats

then Make creates the data files:

python countwords.py books/isles.txt isles.dat
python countwords.py books/abyss.txt abyss.dat

If we run dats again, then Make will see that the dependencies (isles.dat and abyss.dat) are already up to date. Given the target dats has no actions, there is nothing to be done:

$ make dats

make: Nothing to be done for `dats'.

Dry-runs

Sometimes – either when you’re debugging a Makefile of your own, or when you’re confronted by a Makefile written by someone else – it’s useful to see what make would do without actually doing it. For this you can use the -n or --dry-run option (quite a few unix commands which might have undesirable side-effects have such an option). If we deleted the *.dat files, as before, and then typed

$ make -n dats

then Make would display

python wordcount.py books/isles.txt isles.dat
python wordcount.py books/abyss.txt abyss.dat

just as before, but this time without actually creating the outputs.

Our Makefile now looks like this:

# Count words.
.PHONY : dats
dats : isles.dat abyss.dat

isles.dat : books/isles.txt
	python countwords.py books/isles.txt isles.dat

abyss.dat : books/abyss.txt
	python countwords.py books/abyss.txt abyss.dat

.PHONY : clean
clean :
	rm -f *.dat

The following figure shows a graph of the dependencies embodied within our Makefile, involved in building the dats target:

Write Two New Rules

1. Write a new rule for last.dat, created from books/last.txt.
2. Update the dats rule with this target.
3. Write a new rule for results.txt, which creates the summary table. The rule needs to:
   • Depend upon each of the three .dat files.
   • Invoke the action python testzipf.py abyss.dat isles.dat last.dat > results.txt.
4. Put this rule at the top of the Makefile so that it is the default target.
5. Update clean so that it removes results.txt.

The starting Makefile is here.

Solution

See this file for a solution.

The following figure shows the dependencies embodied within our Makefile, involved in building the results.txt target:

Key Points

• Use # for comments in Makefiles.

• Write rules as target: dependencies.

• Specify update actions in a tab-indented block under the rule.

• Use .PHONY to mark targets that don’t correspond to files.