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A Simple Makefile

What follows is a straightforward makefile that describes the way an executable file called edit depends on eight object files which, in turn, depend on eight C source and three header files. In the following example, all the C files include defs.h, but only those defining editing commands include command.h, and only low level files that change the editor buffer include buffer.h.

edit : main.o kbd.o command.o display.o \
       insert.o search.o files.o utils.o
         cc -o edit main.o kbd.o command.o display.o \
                            insert.o search.o files.o utils.o

main.o : main.c defs.h
          cc -c main.c
kbd.o : kbd.c defs.h command.h
          cc -c kbd.c
command.o : command.c defs.h command.h
          cc -c command.c
display.o : display.c defs.h buffer.h
          cc -c display.c
insert.o : insert.c defs.h buffer.h
          cc -c insert.c
search.o : search.c defs.h buffer.h
          cc -c search.c
files.o : files.c defs.h buffer.h command.h
          cc -c files.c
utils.o : utils.c defs.h
          cc -c utils.c
clean :
          rm edit main.o kbd.o command.o display.o \
             insert.o search.o files.o utils.o

Note:
We split each long line into two lines using backslash-newline (\); this is like using one long line, but is easier to read.

To use the previous sample makefile to create the executable file called ‘edit’, type: make.

To use this makefile to delete the executable file and all the object files from the directory, type: make clean.

In the sample makefile shown in A Simple Makefile, the targets include the executable file, edit, and the object files, main.o and kbd.o. The dependencies are files such as main.c and defs.h. In fact, each .o file is both a target and a dependency. Commands include cc -c main.c and cc -c kbd.c.

When a target is a file, it needs to be recompiled or relinked if any of its dependencies change. In addition, any dependencies that are themselves automatically generated should first be updated. In the previous example, edit depends on each of the eight object files; the object file, main.o, depends on the source file, main.c, and on the header file, defs.h.

A shell command follows each line that contains a target and dependencies. These shell commands say how to update the target file. A tab character must come at the beginning of every command line to distinguish commands lines from other lines in the makefile. (Bear in mind that make does not know anything about how the commands work. It is up to you to supply commands that will update the target file properly. All make does is execute the commands in the rule you have specified when the target file needs to be updated.)

The target clean is not a file, but merely the name of an action. Since you normally do not want to carry out the actions in this rule, clean is not a dependency of any other rule. Consequently, make never does anything with it unless you tell it specifically. This rule not only is not a dependency, it also does not have any dependencies, so the only purpose of the rule is to run the specified commands. Targets that do not refer to files but are just actions are called phony targets. See Phony Targets for information about this kind of target. See Errors in Commands to see how to cause make to ignore errors from rm or any other command.

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