Makefile

A makefile is a file (by default named "Makefile") containing a set of directives used by a make build automation tool to generate a target/goal.

Overview

Most often, the makefile directs make on how to compile and link a program. A makefile works upon the principle that files only need recreating if their dependencies are newer than the file being created/recreated. The makefile is recursively carried out (with dependency prepared before each target depending upon them) until everything has been updated (that requires updating) and the primary/ultimate target is complete. These instructions with their dependencies are specified in a makefile. If none of the files that are prerequisites have been changed since the last time the program was compiled, no actions take place. For large software projects, using Makefiles can substantially reduce build times if only a few source files have changed.

Using C/C++ as an example, when a C/C++ source file is changed, it must be recompiled. If a header file has changed, each C/C++ source file that includes the header file must be recompiled to be safe. Each compilation produces an object file corresponding to the source file. Finally, if any source file has been recompiled, all the object files, whether newly made or saved from previous compilations, must be linked together to produce the new executable program.^[1]

Operating system

Contents

Makefiles contain five kinds of things: explicit rules, implicit rules, variable definitions, directives, and comments.

• An explicit rule says when and how to remake one or more files, called the rule's targets. It lists the other files that the targets depend on, called the prerequisites of the target, and may also give a recipe to use to create or update the targets.
• An implicit rule says when and how to remake a class of files based on their names. It describes how a target may depend on a file with a name similar to the target and gives a recipe to create or update such a target.
• A variable definition is a line that specifies a text string value for a variable that can be substituted into the text later.
• A directive is an instruction for make to do something special while reading the makefile such as reading another makefile.
• ‘#’ in a line of a makefile starts a comment. It and the rest of the line is ignored.

Rules

A makefile consists of “rules” in the following form:

target: dependencies
    system command(s)

A target is usually the name of a file that is generated by a program; examples of targets are executable or object files. A target can also be the name of an action to carry out, such as "clean".

A dependency (also called prerequisite) is a file that is used as input to create the target. A target often depends on several files. However, the rule that specifies a recipe for the target need not have any prerequisites. For example, the rule containing the delete command associated with the target "clean" does not have prerequisites.

The system command(s) (also called recipe) is an action that make carries out. A recipe may have more than one command, either on the same line or each on its own line. Note the use of meaningful indentation in specifying commands; also note that the indentation must consist of a single <tab> character.

Execution

A makefile is executed with the make command, e.g. make [options] [target1 target2 ...] . By default, when make looks for the makefile, if a makefile name was not included as a parameter, it tries the following names, in order: makefile and Makefile.^[1]

Example

Here is a simple makefile that describes the way an executable file called edit depends on four object files which, in turn, depend on four C source and two header files. To be concrete, edit is a target, main.o, kbd.o, command.o and display.o are its dependencies, and cc -o edit main.o kbd.o command.o display.o is a system command.

edit: main.o kbd.o command.o display.o 
    cc -o edit main.o kbd.o command.o display.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
    cc -c display.c

clean:
     rm edit main.o kbd.o command.o display.o

To use this 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 .

See also

• Make (software)

References

Wikibooks has a book on the topic of: make