2.6 A simple makefile

For those unfamiliar with make, this section provides a simple demonstration of its use. Make is a program in its own right and can be found on all Unix systems. To learn more about the GNU version of make you will need to consult the GNU Make manual by Richard M. Stallman and Roland McGrath (see section Further reading).

Make reads a description of a project from a makefile (by default, called 'Makefile' in the current directory). A makefile specifies a set of compilation rules in terms of targets (such as executables) and their dependencies (such as object files and source files) in the following format:

target: dependencies
        command

For each target, make checks the modification time of the corresponding dependency files to determine whether the target needs to be rebuilt using the corresponding command. Note that the command lines in a makefile must be indented with a single TAB character, not spaces.

GNU Make contains many default rules, referred to as implicit rules, to simplify the construction of makefiles. For example, these specify that '.o' files can be obtained from '.c' files by compilation, and that an executable can be made by linking together '.o' files. Implicit rules are defined in terms of make variables, such as CC (the C compiler) and CFLAGS (the compilation options for C programs), which can be set using VARIABLE=VALUE lines in the makefile. For C++ the equivalent variables are CXX and CXXFLAGS, while the make variable CPPFLAGS sets the preprocessor options. The implicit and user-defined rules are automatically chained together as necessary by GNU Make.

A simple 'Makefile' for the project above can be written as follows:

CC=gcc 
CFLAGS=-Wall
main: main.o hello_fn.o

clean:
	rm -f main main.o hello_fn.o

The file can be read like this: using the C compiler gcc, with compilation option -Wall, build the target executable main from the object files 'main.o' and 'hello_fn.o' (these, in turn, will be built via implicit rules from 'main.c' and 'hello_fn.c'). The target clean has no dependencies and simply removes all the compiled files.⁽⁴⁾ The option -f (force) on the rm command suppresses any error messages if the files do not exist.

To use the makefile, type make. When called with no arguments, the first target in the makefile is built, producing the executable 'main':

$ make
gcc -Wall   -c -o main.o main.c
gcc -Wall   -c -o hello_fn.o hello_fn.c
gcc   main.o hello_fn.o   -o main
$ ./main
Hello, world!

To rebuild the executable after modifying a source file, simply type make again. By checking the timestamps of the target and dependency files, make identifies the files which have changed and regenerates the corresponding intermediate files needed to update the targets:

$ emacs main.c  (edit the file)
$ make
gcc -Wall   -c -o main.o main.c
gcc    main.o hello_fn.o   -o main
$ ./main
Hello, everyone!

Finally, to remove the generated files, type make clean:

$ make clean
rm -f main main.o hello_fn.o

A more sophisticated makefile would usually contain additional targets for installation (make install) and testing (make check).

The examples in the rest of this book are small enough not to need makefiles, but the use of make is recommended for any larger programs.