GNU Make

Syntax

The file which describes the dependency relationships between project’s sources. It should be named Makefile or makefile and has the following syntax:

target: dependency_list
<tab>command

Usually, the target’s name matches the name of the resulted file, except only those which are .PHONY targets, called virtual targets (they do not generate a specific file). List dependencies include dependencies that are required for target execution. Usually, there are files from which the target will be built. A common mistake is that spaces are used instead of TAB. This will result in an error message when running make. An example Makefile is:

exec:
	gcc foo.c bar.c main.c -o exec

This is not the best way we can use make because it doesn’t describe any dependencies, so every time we run make it will run gcc foo.c bar.c main.c -o exec, even if there are no modified sources. Better use is the following example:

exec: foo.c bar.c main.c
	gcc foo.c bar.c main.c -o exec

In this case the target exec will run only if a source has changed. Neither this case takes full advantage of the facilities make offers, because modifying a single source leads to compiling all the existing sources. An ideal Makefile describes the lowest level possible dependencies. In our case it is the object file:

exec: foo.o bar.o main.o
	gcc foo.o bar.o main.o -o exec
foo.o: foo.c
	gcc -c foo.c -o foo.o
bar.o: bar.c
	gcc -c bar.c -o bar.o
main.o: main.c
	gcc -c main.c -o main.o

How it works

A particular target is executed by running make target. If there is no argument, it will execute the first target described. To execute a target all of his dependencies must be satisfied. For our example, exec target is executed only after foo.o, bar.o, main.o, which are conditioned by foo.c, bar.c, main.c, are obtained.

Variables

In Makefile files we can declare variables to replace commonly used sequences or which are changed frequently. The variables’ values are obtained using the character $: $(variable_name). For the example above, let’s suppose that one of the source files uses functions from math.h. We will declare a variable that is meant to specify that for linking:

LDFLAGS=-lm
exec: foo.o bar.o main.o
	gcc $(LDFLAGS) foo.o bar.o main.o -o exec
foo.o: foo.c
	gcc -c foo.c -o foo.o
bar.o: bar.c
	gcc -c bar.c -o bar.o
main.o: main.c
	gcc -c main.c -o main.o

Make offers several predefined variables, of which the most important are:

• $@ - target’s name
• $^ - dependecies list
• $< - the first dependencie

The Makefile above can be written in a more simple way:

CC=gcc
LDFLAGS=-lm
exec: ana.o are.o mere.o
	$(CC) $(LDFLAGS) $^ -o $@
%.o: %.c
	$(CC) -c $< -o $@

Variables in a Makefile can also come from the environment where make is running. While running, make sees each environment variable as a local variable with the same name and the same value. Thus, assigning a value for LDFLAGS variable in the example above can cause changes to any compile command. To convert a local variable in an environment variable in order to use it in other Makefile files we use the export directive:

export variable

Inverse transformation is done using unexport:

unexport variable

.PHONY target

If we want a target to be marked permanently as out of date we will use the .PHONY target. Let’s consider that there is a pack target that creates an archive which contains the project’s sources. If there is one source named pack and it does not change, the command associated with this target will not be executed. For this we use .PHONY. Also, by convention all Makefile files contain a .PHONY target called clean used to delete the files obtained from compiling or running the program.

.PHONY: pack
pack:
	zip -r project.zip *
clean:
	rm *.o *.zip exec

Implicit Rules

Make allows us to use a simplified syntax. For example we don’t always have to write a command for some targets. This is called an implicit rule:

ana.o: ana.c

Another implicit rule is that when running the command make source.c, the file source.c will be compiled even if there is no Makefile. Implicit rules use the environment variables. Thus, the example considered by us is equivalent to:

ana.o: ana.c
	$(CC) -c $(LDFLAGS) ana.c -o ana.o

Because implicit rules use environment variables, it is easy to modify their behavior by a simple change of the variables’ values.

Final touches

In many cases, the first target in a Makefile is a target that compiles all of the sources. It is very useful because we don’t have to specify a target every time we are running make.

Adding these changes to our example we get a complete Makefile:

CC=gcc
LDFLAGS=-lm

all: exec

exec: ana.o are.o mere.o
	$(CC) $(LDFLAGS) $^ -o $@

foo.o: foo.c
	gcc -c foo.c -o foo.o
bar.o: bar.c
	gcc -c bar.c -o bar.o
main.o: main.c
	gcc -c main.c -o main.o

.PHONY: clean
	rm -rf *.o exec