3 An example

Let's use a more realistic example.

We have a file called array.h that exposes the interface of an array abstract data type (ADT). We have a file called array.c that implements the array ADT. array.c includes array.h. We also have a file main.c that utilizes the array ADT. main.c also includes array.h.

In order for the make program to do its magic, it needs to have a text file that describes the dependencies amongst files. This textfile is often called the ``Makefile'', and is usually named exactly as Makefile. The following is the minimal content of this Makefile:

main:  main.o array.o
	gcc -o main main.o array.o

array.o:  array.c array.h
	gcc -c array.c

main.o:  main.c array.h
	gcc -c main.c

Each rule in a Makefile consists of two parts: a dependency and commands. The dependency of a rule specifies the target file, and the files that the target file relies on. In our example, ``array.o: array.c array.h'' means that the file array.o depends on the files array.c and array.h. This also means that if any one of array.c or array.h is modified, array.o needs to be refreshed. Technically, the filename of the lhs of the colon is called the ``target'', and all the files to the right hand side are called the prerequisite.

The command(s) of a rule specifies how to generate the target from its dependent files. In our example, the command ``gcc -c array.c'' is what we need to generate a fresh copy of array.o. Note that array.h is not supplied as a parameter because it is #included in the source file.

Also note that the each command of a rule must not begin in column 1 of a line. Each command must be preceded by at least one tab character. No, the space character doesn't do the trick! This part is difficult to convey in the sample Makefile code.

Once you have this simple Makefile constructed (and save it as Makefile), then you can invoke the make command as follows:

make main

The make program interprets the content of Makefile, and executes the necesary commands in the right order to bring the target main up-to-date.

The first target main has a special meaning. It is the ``default'' target. If you only want to bring the default target up-to-date, you can do the following instead:

make

Yep, it doesn't any easier (in terms of using make once the Makefile is constructed).

Now, you can edit array.h, and then use the make command to recompile and relink the program the way it should be done! If you do this manually, you may forget that both array.c and main.c need to be recompiled, and lead to very strange problems.