make-dfsg/doc/make.info-1

This is make.info, produced by makeinfo version 4.2 from make.texi.

INFO-DIR-SECTION GNU Packages
START-INFO-DIR-ENTRY
* Make: (make).            Remake files automatically.
END-INFO-DIR-ENTRY

   This file documents the GNU Make utility, which determines
automatically which pieces of a large program need to be recompiled,
and issues the commands to recompile them.

   This is Edition 0.60, last updated 08 July 2002, of `The GNU Make
Manual', for `make', Version 3.80.

   Copyright 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996,
1997, 1998, 1999, 2000, 2002 Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
Texts.  A copy of the license is included in the section entitled "GNU
Free Documentation License".


File: make.info,  Node: Top,  Next: Overview,  Prev: (dir),  Up: (dir)

Make
****

   The GNU `make' utility automatically determines which pieces of a
large program need to be recompiled, and issues the commands to
recompile them.

   This edition of the `GNU Make Manual', last updated 08 July 2002,
documents GNU `make' Version 3.80.

   This manual describes `make' and contains the following chapters:

* Menu:

* Overview::                    Overview of `make'.
* Introduction::                An introduction to `make'.
* Makefiles::                   Makefiles tell `make' what to do.
* Rules::                       Rules describe when a file must be remade.
* Commands::                    Commands say how to remake a file.
* Using Variables::             You can use variables to avoid repetition.
* Conditionals::                Use or ignore parts of the makefile based
                                  on the values of variables.
* Functions::                   Many powerful ways to manipulate text.
* Invoking make: Running.       How to invoke `make' on the command line.
* Implicit Rules::              Use implicit rules to treat many files alike,
                                  based on their file names.
* Archives::                    How `make' can update library archives.
* Features::                    Features GNU `make' has over other `make's.
* Missing::                     What GNU `make' lacks from other `make's.
* Makefile Conventions::        Conventions for writing makefiles for
                                  GNU programs.
* Quick Reference::             A quick reference for experienced users.
* Error Messages::              A list of common errors generated by `make'.
* Complex Makefile::            A real example of a straightforward,
                                  but nontrivial, makefile.

* GNU Free Documentation License::  License for copying this manual
* Concept Index::               Index of Concepts
* Name Index::                  Index of Functions, Variables, & Directives

 --- The Detailed Node Listing ---

Overview of `make'

* Preparing::                   Preparing and Running Make
* Reading::                     On Reading this Text
* Bugs::                        Problems and Bugs

An Introduction to Makefiles

* Rule Introduction::           What a rule looks like.
* Simple Makefile::             A Simple Makefile
* How Make Works::              How `make' Processes This Makefile
* Variables Simplify::          Variables Make Makefiles Simpler
* make Deduces::                Letting `make' Deduce the Commands
* Combine By Prerequisite::     Another Style of Makefile
* Cleanup::                     Rules for Cleaning the Directory

Writing Makefiles

* Makefile Contents::           What makefiles contain.
* Makefile Names::              How to name your makefile.
* Include::                     How one makefile can use another makefile.
* MAKEFILES Variable::          The environment can specify extra makefiles.
* MAKEFILE_LIST Variable::      Discover which makefiles have been read.
* Special Variables::           Other special variables.
* Remaking Makefiles::          How makefiles get remade.
* Overriding Makefiles::        How to override part of one makefile
                                  with another makefile.
* Reading Makefiles::           How makefiles are parsed.

Writing Rules

* Rule Example::                An example explained.
* Rule Syntax::                 General syntax explained.
* Prerequisite Types::          There are two types of prerequisites.
* Wildcards::                   Using wildcard characters such as `*'.
* Directory Search::            Searching other directories for source files.
* Phony Targets::               Using a target that is not a real file's name.
* Force Targets::               You can use a target without commands
                                  or prerequisites to mark other
                                  targets as phony.
* Empty Targets::               When only the date matters and the
                                  files are empty.
* Special Targets::             Targets with special built-in meanings.
* Multiple Targets::            When to make use of several targets in a rule.
* Multiple Rules::              How to use several rules with the same target.
* Static Pattern::              Static pattern rules apply to multiple targets
                                  and can vary the prerequisites according to
                                  the target name.
* Double-Colon::                How to use a special kind of rule to allow
                                  several independent rules for one target.
* Automatic Prerequisites::     How to automatically generate rules giving
                                  prerequisites from source files themselves.

Using Wildcard Characters in File Names

* Wildcard Examples::           Several examples
* Wildcard Pitfall::            Problems to avoid.
* Wildcard Function::           How to cause wildcard expansion where
                                  it does not normally take place.

Searching Directories for Prerequisites

* General Search::              Specifying a search path that applies
                                  to every prerequisite.
* Selective Search::            Specifying a search path
                                  for a specified class of names.
* Search Algorithm::            When and how search paths are applied.
* Commands/Search::             How to write shell commands that work together
                                  with search paths.
* Implicit/Search::             How search paths affect implicit rules.
* Libraries/Search::            Directory search for link libraries.

Static Pattern Rules

* Static Usage::                The syntax of static pattern rules.
* Static versus Implicit::      When are they better than implicit rules?

Writing the Commands in Rules

* Echoing::                     How to control when commands are echoed.
* Execution::                   How commands are executed.
* Parallel::                    How commands can be executed in parallel.
* Errors::                      What happens after a command execution error.
* Interrupts::                  What happens when a command is interrupted.
* Recursion::                   Invoking `make' from makefiles.
* Sequences::                   Defining canned sequences of commands.
* Empty Commands::              Defining useful, do-nothing commands.

Recursive Use of `make'

* MAKE Variable::               The special effects of using `$(MAKE)'.
* Variables/Recursion::         How to communicate variables to a sub-`make'.
* Options/Recursion::           How to communicate options to a sub-`make'.
* -w Option::                   How the `-w' or `--print-directory' option
                                  helps debug use of recursive `make' commands.

How to Use Variables

* Reference::                   How to use the value of a variable.
* Flavors::                     Variables come in two flavors.
* Advanced::                    Advanced features for referencing a variable.
* Values::                      All the ways variables get their values.
* Setting::                     How to set a variable in the makefile.
* Appending::                   How to append more text to the old value
                                  of a variable.
* Override Directive::          How to set a variable in the makefile even if
                                  the user has set it with a command argument.
* Defining::                    An alternate way to set a variable
                                  to a verbatim string.
* Environment::                 Variable values can come from the environment.
* Target-specific::             Variable values can be defined on a per-target
                                  basis.
* Pattern-specific::            Target-specific variable values can be applied
                                  to a group of targets that match a pattern.

Advanced Features for Reference to Variables

* Substitution Refs::           Referencing a variable with
                                  substitutions on the value.
* Computed Names::              Computing the name of the variable to refer to.

Conditional Parts of Makefiles

* Conditional Example::         Example of a conditional
* Conditional Syntax::          The syntax of conditionals.
* Testing Flags::               Conditionals that test flags.

Functions for Transforming Text

* Syntax of Functions::         How to write a function call.
* Text Functions::              General-purpose text manipulation functions.
* File Name Functions::         Functions for manipulating file names.
* Foreach Function::            Repeat some text with controlled variation.
* If Function::                 Conditionally expand a value.
* Call Function::               Expand a user-defined function.
* Value Function::              Return the un-expanded value of a variable.
* Eval Function::               Evaluate the arguments as makefile syntax.
* Origin Function::             Find where a variable got its value.
* Shell Function::              Substitute the output of a shell command.
* Make Control Functions::      Functions that control how make runs.

How to Run `make'

* Makefile Arguments::          How to specify which makefile to use.
* Goals::                       How to use goal arguments to specify which
                                  parts of the makefile to use.
* Instead of Execution::        How to use mode flags to specify what
                                  kind of thing to do with the commands
                                  in the makefile other than simply
                                  execute them.
* Avoiding Compilation::        How to avoid recompiling certain files.
* Overriding::                  How to override a variable to specify
                                  an alternate compiler and other things.
* Testing::                     How to proceed past some errors, to
                                  test compilation.
* Options Summary::             Summary of Options

Using Implicit Rules

* Using Implicit::              How to use an existing implicit rule
                                  to get the commands for updating a file.
* Catalogue of Rules::          A list of built-in implicit rules.
* Implicit Variables::          How to change what predefined rules do.
* Chained Rules::               How to use a chain of implicit rules.
* Pattern Rules::               How to define new implicit rules.
* Last Resort::                 How to defining commands for rules
                                  which cannot find any.
* Suffix Rules::                The old-fashioned style of implicit rule.
* Implicit Rule Search::        The precise algorithm for applying
                                  implicit rules.

Defining and Redefining Pattern Rules

* Pattern Intro::               An introduction to pattern rules.
* Pattern Examples::            Examples of pattern rules.
* Automatic::                   How to use automatic variables in the
                                  commands of implicit rules.
* Pattern Match::               How patterns match.
* Match-Anything Rules::        Precautions you should take prior to
                                  defining rules that can match any
                                  target file whatever.
* Canceling Rules::             How to override or cancel built-in rules.

Using `make' to Update Archive Files

* Archive Members::             Archive members as targets.
* Archive Update::              The implicit rule for archive member targets.
* Archive Pitfalls::            Dangers to watch out for when using archives.
* Archive Suffix Rules::        You can write a special kind of suffix rule
                                  for updating archives.

Implicit Rule for Archive Member Targets

* Archive Symbols::             How to update archive symbol directories.

Makefile Conventions

* Makefile Basics::             General Conventions for Makefiles
* Utilities in Makefiles::      Utilities in Makefiles
* Command Variables::           Variables for Specifying Commands
* Directory Variables::         Variables for Installation Directories
* Standard Targets::            Standard Targets for Users
* Install Command Categories::  Three categories of commands in the `install'

Copying This Manual


File: make.info,  Node: Overview,  Next: Introduction,  Prev: Top,  Up: Top

Overview of `make'
******************

   The `make' utility automatically determines which pieces of a large
program need to be recompiled, and issues commands to recompile them.
This manual describes GNU `make', which was implemented by Richard
Stallman and Roland McGrath.  Development since Version 3.76 has been
handled by Paul Smith.

   GNU `make' conforms to section 6.2 of `IEEE Standard 1003.2-1992'
(POSIX.2).

   Our examples show C programs, since they are most common, but you
can use `make' with any programming language whose compiler can be run
with a shell command.  Indeed, `make' is not limited to programs.  You
can use it to describe any task where some files must be updated
automatically from others whenever the others change.

* Menu:

* Preparing::                   Preparing and Running Make
* Reading::                     On Reading this Text
* Bugs::                        Problems and Bugs


File: make.info,  Node: Preparing,  Next: Reading,  Prev: Overview,  Up: Overview

Preparing and Running Make
==========================

   To prepare to use `make', you must write a file called the
"makefile" that describes the relationships among files in your program
and provides commands for updating each file.  In a program, typically,
the executable file is updated from object files, which are in turn
made by compiling source files.

   Once a suitable makefile exists, each time you change some source
files, this simple shell command:

     make

suffices to perform all necessary recompilations.  The `make' program
uses the makefile data base and the last-modification times of the
files to decide which of the files need to be updated.  For each of
those files, it issues the commands recorded in the data base.

   You can provide command line arguments to `make' to control which
files should be recompiled, or how.  *Note How to Run `make': Running.


File: make.info,  Node: Reading,  Next: Bugs,  Prev: Preparing,  Up: Overview

How to Read This Manual
=======================

   If you are new to `make', or are looking for a general introduction,
read the first few sections of each chapter, skipping the later
sections.  In each chapter, the first few sections contain introductory
or general information and the later sections contain specialized or
technical information.  The exception is the second chapter, *Note An
Introduction to Makefiles: Introduction, all of which is introductory.

   If you are familiar with other `make' programs, see *Note Features
of GNU `make': Features, which lists the enhancements GNU `make' has,
and *Note Incompatibilities and Missing Features: Missing, which
explains the few things GNU `make' lacks that others have.

   For a quick summary, see *Note Options Summary::, *Note Quick
Reference::, and *Note Special Targets::.


File: make.info,  Node: Bugs,  Prev: Reading,  Up: Overview

Problems and Bugs
=================

   If you have problems with GNU `make' or think you've found a bug,
please report it to the developers; we cannot promise to do anything but
we might well want to fix it.

   Before reporting a bug, make sure you've actually found a real bug.
Carefully reread the documentation and see if it really says you can do
what you're trying to do.  If it's not clear whether you should be able
to do something or not, report that too; it's a bug in the
documentation!

   Before reporting a bug or trying to fix it yourself, try to isolate
it to the smallest possible makefile that reproduces the problem.  Then
send us the makefile and the exact results `make' gave you, including
any error or warning messages.  Please don't paraphrase these messages:
it's best to cut and paste them into your report.  When generating this
small makefile, be sure to not use any non-free or unusual tools in
your commands: you can almost always emulate what such a tool would do
with simple shell commands.  Finally, be sure to explain what you
expected to occur; this will help us decide whether the problem was
really in the documentation.

   Once you have a precise problem you can report it in one of two ways.
Either send electronic mail to:

         bug-make@gnu.org

or use our Web-based project management tool, at:

         http://savannah.gnu.org/projects/make/

In addition to the information above, please be careful to include the
version number of `make' you are using.  You can get this information
with the command `make --version'.  Be sure also to include the type of
machine and operating system you are using.  One way to obtain this
information is by looking at the final lines of output from the command
`make --help'.


File: make.info,  Node: Introduction,  Next: Makefiles,  Prev: Overview,  Up: Top

An Introduction to Makefiles
****************************

   You need a file called a "makefile" to tell `make' what to do.  Most
often, the makefile tells `make' how to compile and link a program.

   In this chapter, we will discuss a simple makefile that describes
how to compile and link a text editor which consists of eight C source
files and three header files.  The makefile can also tell `make' how to
run miscellaneous commands when explicitly asked (for example, to remove
certain files as a clean-up operation).  To see a more complex example
of a makefile, see *Note Complex Makefile::.

   When `make' recompiles the editor, each changed C source file must
be recompiled.  If a header file has changed, each 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 editor.

* Menu:

* Rule Introduction::           What a rule looks like.
* Simple Makefile::             A Simple Makefile
* How Make Works::              How `make' Processes This Makefile
* Variables Simplify::          Variables Make Makefiles Simpler
* make Deduces::                Letting `make' Deduce the Commands
* Combine By Prerequisite::     Another Style of Makefile
* Cleanup::                     Rules for Cleaning the Directory


File: make.info,  Node: Rule Introduction,  Next: Simple Makefile,  Prev: Introduction,  Up: Introduction

What a Rule Looks Like
======================

   A simple makefile consists of "rules" with the following shape:

     TARGET ... : PREREQUISITES ...
             COMMAND
             ...
             ...

   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' (*note
Phony Targets::).

   A "prerequisite" is a file that is used as input to create the
target.  A target often depends on several files.

   A "command" is an action that `make' carries out.  A rule may have
more than one command, each on its own line.  *Please note:* you need
to put a tab character at the beginning of every command line!  This is
an obscurity that catches the unwary.

   Usually a command is in a rule with prerequisites and serves to
create a target file if any of the prerequisites change.  However, the
rule that specifies commands for the target need not have
prerequisites.  For example, the rule containing the delete command
associated with the target `clean' does not have prerequisites.

   A "rule", then, explains how and when to remake certain files which
are the targets of the particular rule.  `make' carries out the
commands on the prerequisites to create or update the target.  A rule
can also explain how and when to carry out an action.  *Note Writing
Rules: Rules.

   A makefile may contain other text besides rules, but a simple
makefile need only contain rules.  Rules may look somewhat more
complicated than shown in this template, but all fit the pattern more
or less.


File: make.info,  Node: Simple Makefile,  Next: How Make Works,  Prev: Rule Introduction,  Up: Introduction

A Simple Makefile
=================

   Here 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 this 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

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 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

   In the example makefile, the targets include the executable file
`edit', and the object files `main.o' and `kbd.o'.  The prerequisites
are files such as `main.c' and `defs.h'.  In fact, each `.o' file is
both a target and a prerequisite.  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 prerequisites change.  In addition, any prerequisites that are
themselves automatically generated should be updated first.  In this
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
prerequisites.  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 prerequisite of any other rule.  Consequently, `make'
never does anything with it unless you tell it specifically.  Note that
this rule not only is not a prerequisite, it also does not have any
prerequisites, 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".  *Note Phony Targets::, for information about
this kind of target.  *Note Errors in Commands: Errors, to see how to
cause `make' to ignore errors from `rm' or any other command.


File: make.info,  Node: How Make Works,  Next: Variables Simplify,  Prev: Simple Makefile,  Up: Introduction

How `make' Processes a Makefile
===============================

   By default, `make' starts with the first target (not targets whose
names start with `.').  This is called the "default goal".  ("Goals"
are the targets that `make' strives ultimately to update.  *Note
Arguments to Specify the Goals: Goals.)

   In the simple example of the previous section, the default goal is to
update the executable program `edit'; therefore, we put that rule first.

   Thus, when you give the command:

     make

`make' reads the makefile in the current directory and begins by
processing the first rule.  In the example, this rule is for relinking
`edit'; but before `make' can fully process this rule, it must process
the rules for the files that `edit' depends on, which in this case are
the object files.  Each of these files is processed according to its
own rule.  These rules say to update each `.o' file by compiling its
source file.  The recompilation must be done if the source file, or any
of the header files named as prerequisites, is more recent than the
object file, or if the object file does not exist.

   The other rules are processed because their targets appear as
prerequisites of the goal.  If some other rule is not depended on by the
goal (or anything it depends on, etc.), that rule is not processed,
unless you tell `make' to do so (with a command such as `make clean').

   Before recompiling an object file, `make' considers updating its
prerequisites, the source file and header files.  This makefile does not
specify anything to be done for them--the `.c' and `.h' files are not
the targets of any rules--so `make' does nothing for these files.  But
`make' would update automatically generated C programs, such as those
made by Bison or Yacc, by their own rules at this time.

   After recompiling whichever object files need it, `make' decides
whether to relink `edit'.  This must be done if the file `edit' does
not exist, or if any of the object files are newer than it.  If an
object file was just recompiled, it is now newer than `edit', so `edit'
is relinked.

   Thus, if we change the file `insert.c' and run `make', `make' will
compile that file to update `insert.o', and then link `edit'.  If we
change the file `command.h' and run `make', `make' will recompile the
object files `kbd.o', `command.o' and `files.o' and then link the file
`edit'.


File: make.info,  Node: Variables Simplify,  Next: make Deduces,  Prev: How Make Works,  Up: Introduction

Variables Make Makefiles Simpler
================================

   In our example, we had to list all the object files twice in the
rule for `edit' (repeated here):

     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

   Such duplication is error-prone; if a new object file is added to the
system, we might add it to one list and forget the other.  We can
eliminate the risk and simplify the makefile by using a variable.
"Variables" allow a text string to be defined once and substituted in
multiple places later (*note How to Use Variables: Using Variables.).

   It is standard practice for every makefile to have a variable named
`objects', `OBJECTS', `objs', `OBJS', `obj', or `OBJ' which is a list
of all object file names.  We would define such a variable `objects'
with a line like this in the makefile:

     objects = main.o kbd.o command.o display.o \
               insert.o search.o files.o utils.o

Then, each place we want to put a list of the object file names, we can
substitute the variable's value by writing `$(objects)' (*note How to
Use Variables: Using Variables.).

   Here is how the complete simple makefile looks when you use a
variable for the object files:

     objects = main.o kbd.o command.o display.o \
               insert.o search.o files.o utils.o
     
     edit : $(objects)
             cc -o edit $(objects)
     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 $(objects)


File: make.info,  Node: make Deduces,  Next: Combine By Prerequisite,  Prev: Variables Simplify,  Up: Introduction

Letting `make' Deduce the Commands
==================================

   It is not necessary to spell out the commands for compiling the
individual C source files, because `make' can figure them out: it has an
"implicit rule" for updating a `.o' file from a correspondingly named
`.c' file using a `cc -c' command.  For example, it will use the
command `cc -c main.c -o main.o' to compile `main.c' into `main.o'.  We
can therefore omit the commands from the rules for the object files.
*Note Using Implicit Rules: Implicit Rules.

   When a `.c' file is used automatically in this way, it is also
automatically added to the list of prerequisites.  We can therefore omit
the `.c' files from the prerequisites, provided we omit the commands.

   Here is the entire example, with both of these changes, and a
variable `objects' as suggested above:

     objects = main.o kbd.o command.o display.o \
               insert.o search.o files.o utils.o
     
     edit : $(objects)
             cc -o edit $(objects)
     
     main.o : defs.h
     kbd.o : defs.h command.h
     command.o : defs.h command.h
     display.o : defs.h buffer.h
     insert.o : defs.h buffer.h
     search.o : defs.h buffer.h
     files.o : defs.h buffer.h command.h
     utils.o : defs.h
     
     .PHONY : clean
     clean :
             rm edit $(objects)

This is how we would write the makefile in actual practice.  (The
complications associated with `clean' are described elsewhere.  See
*Note Phony Targets::, and *Note Errors in Commands: Errors.)

   Because implicit rules are so convenient, they are important.  You
will see them used frequently.


File: make.info,  Node: Combine By Prerequisite,  Next: Cleanup,  Prev: make Deduces,  Up: Introduction

Another Style of Makefile
=========================

   When the objects of a makefile are created only by implicit rules, an
alternative style of makefile is possible.  In this style of makefile,
you group entries by their prerequisites instead of by their targets.
Here is what one looks like:

     objects = main.o kbd.o command.o display.o \
               insert.o search.o files.o utils.o
     
     edit : $(objects)
             cc -o edit $(objects)
     
     $(objects) : defs.h
     kbd.o command.o files.o : command.h
     display.o insert.o search.o files.o : buffer.h

Here `defs.h' is given as a prerequisite of all the object files;
`command.h' and `buffer.h' are prerequisites of the specific object
files listed for them.

   Whether this is better is a matter of taste: it is more compact, but
some people dislike it because they find it clearer to put all the
information about each target in one place.


File: make.info,  Node: Cleanup,  Prev: Combine By Prerequisite,  Up: Introduction

Rules for Cleaning the Directory
================================

   Compiling a program is not the only thing you might want to write
rules for.  Makefiles commonly tell how to do a few other things besides
compiling a program: for example, how to delete all the object files
and executables so that the directory is `clean'.

   Here is how we could write a `make' rule for cleaning our example
editor:

     clean:
             rm edit $(objects)

   In practice, we might want to write the rule in a somewhat more
complicated manner to handle unanticipated situations.  We would do
this:

     .PHONY : clean
     clean :
             -rm edit $(objects)

This prevents `make' from getting confused by an actual file called
`clean' and causes it to continue in spite of errors from `rm'.  (See
*Note Phony Targets::, and *Note Errors in Commands: Errors.)

A rule such as this should not be placed at the beginning of the
makefile, because we do not want it to run by default!  Thus, in the
example makefile, we want the rule for `edit', which recompiles the
editor, to remain the default goal.

   Since `clean' is not a prerequisite of `edit', this rule will not
run at all if we give the command `make' with no arguments.  In order
to make the rule run, we have to type `make clean'.  *Note How to Run
`make': Running.


File: make.info,  Node: Makefiles,  Next: Rules,  Prev: Introduction,  Up: Top

Writing Makefiles
*****************

   The information that tells `make' how to recompile a system comes
from reading a data base called the "makefile".

* Menu:

* Makefile Contents::           What makefiles contain.
* Makefile Names::              How to name your makefile.
* Include::                     How one makefile can use another makefile.
* MAKEFILES Variable::          The environment can specify extra makefiles.
* MAKEFILE_LIST Variable::      Discover which makefiles have been read.
* Special Variables::           Other special variables.
* Remaking Makefiles::          How makefiles get remade.
* Overriding Makefiles::        How to override part of one makefile
                                  with another makefile.
* Reading Makefiles::           How makefiles are parsed.


File: make.info,  Node: Makefile Contents,  Next: Makefile Names,  Prev: Makefiles,  Up: Makefiles

What Makefiles Contain
======================

   Makefiles contain five kinds of things: "explicit rules", "implicit
rules", "variable definitions", "directives", and "comments".  Rules,
variables, and directives are described at length in later chapters.

   * 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, call the "prerequisites" of the target, and may
     also give commands to use to create or update the targets.  *Note
     Writing Rules: Rules.

   * 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 commands to
     create or update such a target.  *Note Using Implicit Rules:
     Implicit Rules.

   * A "variable definition" is a line that specifies a text string
     value for a variable that can be substituted into the text later.
     The simple makefile example shows a variable definition for
     `objects' as a list of all object files (*note Variables Make
     Makefiles Simpler: Variables Simplify.).

   * A "directive" is a command for `make' to do something special while
     reading the makefile.  These include:

        * Reading another makefile (*note Including Other Makefiles:
          Include.).

        * Deciding (based on the values of variables) whether to use or
          ignore a part of the makefile (*note Conditional Parts of
          Makefiles: Conditionals.).

        * Defining a variable from a verbatim string containing
          multiple lines (*note Defining Variables Verbatim: Defining.).

   * `#' in a line of a makefile starts a "comment".  It and the rest
     of the line are ignored, except that a trailing backslash not
     escaped by another backslash will continue the comment across
     multiple lines.  A line containing just a comment (with perhaps
     spaces before it) is effectively blank, and is ignored.  If you
     want a literal `#', escape it with a backslash (e.g., `\#').
     Comments may appear on any line in the makefile, although they are
     treated specially in certain situations.

     Within a command script (if the line begins with a TAB character)
     the entire line is passed to the shell, just as with any other
     line that begins with a TAB.  The shell decides how to interpret
     the text: whether or not this is a comment is up to the shell.

     Within a `define' directive, comments are not ignored during the
     definition of the variable, but rather kept intact in the value of
     the variable.  When the variable is expanded they will either be
     treated as `make' comments or as command script text, depending on
     the context in which the variable is evaluated.


File: make.info,  Node: Makefile Names,  Next: Include,  Prev: Makefile Contents,  Up: Makefiles

What Name to Give Your Makefile
===============================

   By default, when `make' looks for the makefile, it tries the
following names, in order: `GNUmakefile', `makefile' and `Makefile'.

   Normally you should call your makefile either `makefile' or
`Makefile'.  (We recommend `Makefile' because it appears prominently
near the beginning of a directory listing, right near other important
files such as `README'.)  The first name checked, `GNUmakefile', is not
recommended for most makefiles.  You should use this name if you have a
makefile that is specific to GNU `make', and will not be understood by
other versions of `make'.  Other `make' programs look for `makefile' and
`Makefile', but not `GNUmakefile'.

   If `make' finds none of these names, it does not use any makefile.
Then you must specify a goal with a command argument, and `make' will
attempt to figure out how to remake it using only its built-in implicit
rules.  *Note Using Implicit Rules: Implicit Rules.

   If you want to use a nonstandard name for your makefile, you can
specify the makefile name with the `-f' or `--file' option.  The
arguments `-f NAME' or `--file=NAME' tell `make' to read the file NAME
as the makefile.  If you use more than one `-f' or `--file' option, you
can specify several makefiles.  All the makefiles are effectively
concatenated in the order specified.  The default makefile names
`GNUmakefile', `makefile' and `Makefile' are not checked automatically
if you specify `-f' or `--file'.


File: make.info,  Node: Include,  Next: MAKEFILES Variable,  Prev: Makefile Names,  Up: Makefiles

Including Other Makefiles
=========================

   The `include' directive tells `make' to suspend reading the current
makefile and read one or more other makefiles before continuing.  The
directive is a line in the makefile that looks like this:

     include FILENAMES...

FILENAMES can contain shell file name patterns.

   Extra spaces are allowed and ignored at the beginning of the line,
but a tab is not allowed.  (If the line begins with a tab, it will be
considered a command line.)  Whitespace is required between `include'
and the file names, and between file names; extra whitespace is ignored
there and at the end of the directive.  A comment starting with `#' is
allowed at the end of the line.  If the file names contain any variable
or function references, they are expanded.  *Note How to Use Variables:
Using Variables.

   For example, if you have three `.mk' files, `a.mk', `b.mk', and
`c.mk', and `$(bar)' expands to `bish bash', then the following
expression

     include foo *.mk $(bar)

   is equivalent to

     include foo a.mk b.mk c.mk bish bash

   When `make' processes an `include' directive, it suspends reading of
the containing makefile and reads from each listed file in turn.  When
that is finished, `make' resumes reading the makefile in which the
directive appears.

   One occasion for using `include' directives is when several programs,
handled by individual makefiles in various directories, need to use a
common set of variable definitions (*note Setting Variables: Setting.)
or pattern rules (*note Defining and Redefining Pattern Rules: Pattern
Rules.).

   Another such occasion is when you want to generate prerequisites from
source files automatically; the prerequisites can be put in a file that
is included by the main makefile.  This practice is generally cleaner
than that of somehow appending the prerequisites to the end of the main
makefile as has been traditionally done with other versions of `make'.
*Note Automatic Prerequisites::.

   If the specified name does not start with a slash, and the file is
not found in the current directory, several other directories are
searched.  First, any directories you have specified with the `-I' or
`--include-dir' option are searched (*note Summary of Options: Options
Summary.).  Then the following directories (if they exist) are
searched, in this order: `PREFIX/include' (normally `/usr/local/include'
(1)) `/usr/gnu/include', `/usr/local/include', `/usr/include'.

   If an included makefile cannot be found in any of these directories,
a warning message is generated, but it is not an immediately fatal
error; processing of the makefile containing the `include' continues.
Once it has finished reading makefiles, `make' will try to remake any
that are out of date or don't exist.  *Note How Makefiles Are Remade:
Remaking Makefiles.  Only after it has tried to find a way to remake a
makefile and failed, will `make' diagnose the missing makefile as a
fatal error.

   If you want `make' to simply ignore a makefile which does not exist
and cannot be remade, with no error message, use the `-include'
directive instead of `include', like this:

     -include FILENAMES...

   This acts like `include' in every way except that there is no error
(not even a warning) if any of the FILENAMES do not exist.  For
compatibility with some other `make' implementations, `sinclude' is
another name for `-include'.

   ---------- Footnotes ----------

   (1) GNU Make compiled for MS-DOS and MS-Windows behaves as if PREFIX
has been defined to be the root of the DJGPP tree hierarchy.


File: make.info,  Node: MAKEFILES Variable,  Next: MAKEFILE_LIST Variable,  Prev: Include,  Up: Makefiles

The Variable `MAKEFILES'
========================

   If the environment variable `MAKEFILES' is defined, `make' considers
its value as a list of names (separated by whitespace) of additional
makefiles to be read before the others.  This works much like the
`include' directive: various directories are searched for those files
(*note Including Other Makefiles: Include.).  In addition, the default
goal is never taken from one of these makefiles and it is not an error
if the files listed in `MAKEFILES' are not found.

   The main use of `MAKEFILES' is in communication between recursive
invocations of `make' (*note Recursive Use of `make': Recursion.).  It
usually is not desirable to set the environment variable before a
top-level invocation of `make', because it is usually better not to
mess with a makefile from outside.  However, if you are running `make'
without a specific makefile, a makefile in `MAKEFILES' can do useful
things to help the built-in implicit rules work better, such as
defining search paths (*note Directory Search::).

   Some users are tempted to set `MAKEFILES' in the environment
automatically on login, and program makefiles to expect this to be done.
This is a very bad idea, because such makefiles will fail to work if
run by anyone else.  It is much better to write explicit `include'
directives in the makefiles.  *Note Including Other Makefiles: Include.


File: make.info,  Node: MAKEFILE_LIST Variable,  Next: Special Variables,  Prev: MAKEFILES Variable,  Up: Makefiles

The Variable `MAKEFILE_LIST'
============================

   As `make' reads various makefiles, including any obtained from the
`MAKEFILES' variable, the command line, the default files, or from
`include' directives, their names will be automatically appended to the
`MAKEFILE_LIST' variable.  They are added right before `make' begins to
parse them.

   This means that if the first thing a makefile does is examine the
last word in this variable, it will be the name of the current makefile.
Once the current makefile has used `include', however, the last word
will be the just-included makefile.

   If a makefile named `Makefile' has this content:

     name1 := $(word $(words $(MAKEFILE_LIST)),$(MAKEFILE_LIST))
     
     include inc.mk
     
     name2 := $(word $(words $(MAKEFILE_LIST)),$(MAKEFILE_LIST))
     
     all:
             @echo name1 = $(name1)
             @echo name2 = $(name2)

then you would expect to see this output:

     name1 = Makefile
     name2 = inc.mk

   *Note Text Functions::, for more information on the `word' and
`words' functions used above.  *Note The Two Flavors of Variables:
Flavors, for more information on simply-expanded (`:=') variable
definitions.


File: make.info,  Node: Special Variables,  Next: Remaking Makefiles,  Prev: MAKEFILE_LIST Variable,  Up: Makefiles

Other Special Variables
=======================

   GNU `make' also supports a special variable.  Note that any value
you assign to this variable will be ignored; it will always return its
special value.

   The first special variable is `.VARIABLES'.  When expanded, the
value consists of a list of the _names_ of all global variables defined
in all makefiles read up until that point.  This includes variables
which have empty values, as well as built-in variables (*note Variables
Used by Implicit Rules: Implicit Variables.), but does not include any
variables which are only defined in a target-specific context.