path: root/make.texinfo

\input texinfo  @c -*- Texinfo -*-
@c %**start of header 
@setfilename make.info
@settitle Make
@setchapternewpage odd
@c %**end of header

@iftex
@finalout
@end iftex

@comment Combine the variable and function indices.
@synindex vr fn

@ifinfo
This file documents the GNU Make utility.

Copyright (C) 1988, 1989, 1990, 1991 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

@ignore
Permission is granted to process this file through TeX and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).

@end ignore
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that
the section entitled ``GNU General Public License'' is included exactly as in
the original, and provided that the entire resulting derived work is
distributed under the terms of a permission notice identical to this
one.

Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the text of the translations of the section
entitled ``GNU General Public License'' must be approved for accuracy by the
Foundation.
@end ifinfo

@titlepage
@sp 10
@center @titlefont{GNU Make}
@sp 2
@center A Program for Directing Recompilation
@sp 2
@center by Richard M. Stallman and Roland McGrath
@sp 3
@center Edition 0.28 Beta,
@sp 1
@center last updated 23 September 1991,
@sp 1
@center for @code{make}, Version 3.61 Beta.
@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1988, 1989, 1990, 1991 Free Software Foundation, Inc.
@sp 2

This is Edition 0.28 Beta of the @cite{GNU Make Manual}, @*
last updated 23 September 1991, @*
for @code{make} Version 3.61 Beta.

@sp 2
Published by the Free Software Foundation @*
675 Massachusetts Avenue, @*
Cambridge, MA 02139 USA @*
Printed copies are available for $15 each.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided also that
the section entitled ``GNU General Public License'' is included exactly as in
the original, and provided that the entire resulting derived work is
distributed under the terms of a permission notice identical to this
one.

Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that the text of the translation of the section
entitled ``GNU General Public License'' must be approved for accuracy by the
Foundation.
@end titlepage
@page

@node     Top, Overview, , (dir)
@comment  node-name,  next,  previous,  up

@ifinfo
The GNU @code{make} utility determines automatically which pieces of a
large program need to be recompiled, and issues the commands to
recompile them.  This manual describes it and contains the following
chapters:
@end ifinfo

@menu
* Overview::    Introducing @code{make}.
* Copying::     Copying conditions for GNU @code{make}.
* Bugs::        If you have problems, or think you've found a bug.
* Simple::      A simple example explained.
* Makefiles::   The data base contains rules and variable definitions.
* Rules::       A rule says how and when to remake one file.
* Commands::    A rule contains shell commands that say how to remake.
* Variables::   A variable holds a text string for substitution into rules.
* Conditionals::Makefiles that do one thing or another depending on
                 variable values.
* Functions::   Functions can do text-processing within @code{make}.

* Running::     How to run @code{make}; how you can adjust the way
                 @code{make} uses the makefile.

* Implicit::    Implicit rules take over if the makefile doesn't say
                 how a file is to be remade.
* Archives::    How to use @code{make} to update archive files.

* Features::    GNU @code{make}'s advanced features and how GNU @code{make}
                 relates to other versions of @code{make}.
* Missing::     Features of other @code{make}s not supported
                 by GNU @code{make}.

* Concept Index::Index of cross-references to where concepts are discussed.
* Name Index::  Index of cross-references for names of @code{make}'s
                 variables, functions, special targets and directives.
@end menu


@node Overview, Copying, Top, Top
@chapter Overview of @code{make}

The purpose of the @code{make} utility is to determine automatically which
pieces of a large program need to be recompiled, and issue the commands to
recompile them.  This manual describes the GNU implementation of
@code{make}, which was implemented by Richard Stallman and Roland McGrath.

Our examples show C programs, since they are most common, but you can use
@code{make} with any programming language whose compiler can be run with a
shell command.  In fact, @code{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.

To prepare to use @code{make}, you must write a file called
the @dfn{makefile} that describes the relationships among files
in your program, and the states the 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.@refill

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

@example
make
@end example

@noindent
suffices to perform all necessary recompilations.  The @code{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.

@iftex
Command arguments to @code{make} can be used to control which files should
be recompiled, or how.  @xref{Running}.
@end iftex

@node Copying, Bugs, Overview, Top

@include gpl.texinfo

@node Bugs, Simple, Copying, Top
@chapter Problems and Bugs

If you have problems with GNU @code{make} or think you've found a bug,
please report it to Roland McGrath; he doesn't promise to do anything
but he 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 @code{make} gave you.  Also say what
you expected to occur; this will help us decide whether the problem
was really in the documentation.

Once you've got a precise problem, send email to (Internet)
@samp{bug-gnu-utils@@prep.ai.mit.edu} or (UUCP)
@samp{mit-eddie!prep.ai.mit.edu!bug-gnu-utils}.  Please include the version
number of @code{make} you are using.  You can get this information with the
command @samp{make -v -f /dev/null}.@refill

Non-bug suggestions are always welcome as well.
If you have questions about things that are unclear in the documentation
or are just obscure features, ask Roland McGrath; he'll be happy to help
you out (but no promises).  You can send him electronic mail at Internet
address @samp{roland@@prep.ai.mit.edu} or UUCP path
@samp{mit-eddie!prep.ai.mit.edu!roland}.@refill

@node Simple, Makefiles, Bugs, Top
@chapter Simple Example of @code{make}

Suppose we have a text editor consisting of eight C source files and three
header files.  We need a makefile to tell @code{make} how to compile and
link the editor.  Assume that all the C files include @file{defs.h}, but
only those defining editing commands include @file{commands.h} and only low
level files that change the editor buffer include @file{buffer.h}.

To recompile the editor, each changed C source file must be recompiled.  If
a header file has changed, to be safe each C source file that
includes the header file must be recompiled.  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.

Here is a straightforward makefile that describes these criteria and says
how to compile and link when the time comes:

@example
edit : main.o kbd.o commands.o display.o \
       insert.o search.o files.o utils.o
        cc -o edit main.o kbd.o commands.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
commands.o : command.c defs.h command.h
        cc -c commands.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
@end example

We split each long line into two lines using a backslash-newline; this is
like using one long line, but is easier to read.

Each file that is generated by a program---that is to say, each file except
for source files---is the @dfn{target} of a @dfn{rule} (@pxref{Rules}).
(In this example, these are the object files such as @file{main.o},
@file{kbd.o}, etc., and the executable file @file{edit}.)  The target
appears at the beginning of a line, followed by a colon.

After the colon come the target's @dfn{dependencies}: all the files that
are used as input when the target file is updated.  A target file needs to
be recompiled or relinked if any of its dependencies changes.  In addition,
any dependencies that are themselves automatically generated should be
updated first.  In this example, @file{edit} depends on each of the eight
object files; the object file @file{main.o} depends on the source file
@file{main.c} and on the header file @file{defs.h}.

By default, @code{make} starts with the first rule (not counting rules
whose target names start with @samp{.}).  This is called the @dfn{default
goal}.  Therefore, we put the rule for the executable program @file{edit}
first.  The other rules are processed because their targets appear as
dependencies of the goal.

After each line containing a target and dependencies come one or more lines
of shell commands that say how to update the target file.  These lines
start with a tab to tell @code{make} that they are command lines.  But
@code{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 @code{make} does is execute the commands you have specified when the
target file needs to be updated.

@section How @code{make} Processes This Makefile

After reading the makefile, @code{make} begins its real work by processing
the first rule, the one for relinking @file{edit}; but before it can fully
process this rule, it must process the rules for the files @file{edit}
depends on: all the object files.  Each of these files is processed
according to its own rule.  These rules say to update the @samp{.o} file by
compiling its source file.  The recompilation must be done if the source
file, or any of the header files named as dependencies, is more recent than
the object file, or if the object file does not exist.

Before recompiling an object file, @code{make} considers updating its
dependencies, the source file and header files.  This makefile does not
specify anything to be done for them---the @samp{.c} and @samp{.h} files
are not the targets of any rules---so nothing needs to be done.  But
automatically generated C programs, such as made by Bison or Yacc, would
be updated by their own rules at this time.

After recompiling whichever object files need it, @code{make} can now
decide whether to relink @file{edit}.  This must be done if the file
@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
@file{edit}, so @file{edit} will be relinked.

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

@section Variables Make Makefiles Simpler

In our example, we had to list all the object files twice in the rule for
@file{edit} (repeated here):

@example
edit : main.o kbd.o commands.o display.o \
              insert.o search.o files.o utils.o
        cc -o edit main.o kbd.o commands.o display.o \
                   insert.o search.o files.o utils.o
@end example

@vindex objects
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 @dfn{variable}.  Variables
allow a text string to be defined once and substituted in multiple places
later (@pxref{Variables}).

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

@example
objects = main.o kbd.o commands.o display.o \
              insert.o search.o files.o utils.o
@end example

@noindent
Then, each place we want to put a list of the object file names, we can
substitute the variable's value by writing @samp{$(objects)}
(@pxref{Variables}).  Here is how the rule for @code{edit} looks as a
result:

@example
edit : $(objects)
        cc -o edit $(objects)
@end example

@section Letting @code{make} Deduce the Commands

It is not necessary to spell out the commands for compiling the individual
C source files, because @code{make} can figure them out: it has an
@dfn{implicit rule} for updating a @samp{.o} file from a correspondingly
named @samp{.c} file using a @samp{cc -c} command.  For example, it will
use the command @samp{cc -c main.c -o main.o} to compile @file{main.c} into
@file{main.o}.  We can therefore omit the commands from the rules for the
object files.  @xref{Implicit}.@refill

When a @samp{.c} file is used automatically in this way, it is also
automatically added to the list of dependencies.  We can therefore omit
the @samp{.c} files from the dependencies, provided we omit the commands.

Here is the entire example, with both of these changes, and a variable
@code{objects} as suggested above:

@example
objects =  main.o kbd.o commands.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
commands.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
@end example

@noindent
This is how we would write the makefile in actual practice.

@section Another Style of Makefile

Since the rules for the object files specify only dependencies, no
commands, one can alternatively combine them by dependency instead of by
target.  Here is what it looks like:

@example
objects =  main.o kbd.o commands.o display.o \
 insert.o search.o files.o utils.o

edit : $(objects)
        cc -o edit $(objects)

$(objects) : defs.h
kbd.o commands.o files.o : command.h
display.o insert.o search.o files.o : buffer.h
@end example

@noindent
Here @file{defs.h} is given as a dependency of all the object files;
@file{commands.h} and @file{buffer.h} are dependencies 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.

@section Rules for Cleaning the Directory

Compiling a program isn't the only thing you might want to write rules
for.  Makefiles commonly tell how to do a few other things besides
compiling the program: for example, how to delete all the object files
and executables so that the directory is ``clean''.  Here is how we
would write a @code{make} rule for cleaning our example editor:

@example
clean:
        rm edit $(objects)
@end example

@noindent
This rule would be added at the end of the makefile, because we don't
want it to run by default!  We want the rule for @code{edit}, which
recompiles the editor, to remain the default goal.

Since @code{clean} is not a dependency of @code{edit}, this rule won't
run at all if we give the command @samp{make} with no arguments.  In
order to make the rule run, we have to type @samp{make clean}.

@node Makefiles, Rules, Simple, Top
@chapter Writing Makefiles

@cindex makefile
The information that tells @code{make} how to recompile a system comes from
reading a data base called the @dfn{makefile}.

@menu
* Contents: Makefile Contents.   Overview of what you put in a makefile.
* Names: Makefile Names.         Where @code{make} finds the makefile.
* Include::                      How one makefile can use another makefile.
* MAKEFILES Variable::           The environment can specify extra makefiles.
* Remaking Makefiles::           How makefiles get remade.
* Overriding Makefiles::         How to override part of one makefile
                                 with another makefile.
@end menu

@node Makefile Contents, Makefile Names, Makefiles, Makefiles
@section What Makefiles Contain

Makefiles contain four kinds of things: @dfn{rules}, @dfn{variable
definitions}, @dfn{directives} and @dfn{comments}.  Rules, variables and
directives are described at length in later chapters.@refill

@itemize @bullet
@item
A rule says when and how to remake one or more files, called the rule's
@dfn{targets}.  It lists the other files that the targets @dfn{depend on},
and may also give commands to use to create or update the targets.
@xref{Rules}.

@item
A variable definition is a line that specifies a text string value
for a @dfn{variable} that can be substituted into the text later.
The simple makefile example (@pxref{Simple}) shows a variable definition
for @code{objects} as a list of all object files.
@xref{Variables}, for full details.

@item
A directive is a command for @code{make} to do something special while
reading the makefile.  These include:

@itemize @bullet
@item
Reading another makefile (@pxref{Include}).

@item
Deciding (based on the values of variables) whether to use or
ignore a part of the makefile (@pxref{Conditionals}).

@item
Defining a variable from a verbatim string containing multiple lines
(@pxref{Defining}).
@end itemize

@item
@cindex comments
@samp{#} 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.
Comments may appear on any of the lines in the makefile, except within a
@code{define} directive, and perhaps within commands (where the shell
decides what is a comment).  A line containing just a comment (with
perhaps spaces before it) is effectively blank, and is ignored.@refill
@end itemize

@node Makefile Names, Include, Makefile Contents, Makefiles
@section What Name to Give Your Makefile
@cindex makefile names
@cindex names of makefiles
@cindex default makefile names

By default, when @code{make} looks for the makefile, it tries the names
@file{GNUmakefile}, @file{makefile} and @file{Makefile}, in that order.

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

If @code{make} finds none of these names, it does not use any makefile.
Then you must specify a goal with a command argument, and @code{make}
will attempt to figure out how to remake it using only its built-in
implicit rules.  @xref{Implicit}.

@cindex @code{-f}
If you want to use a nonstandard name for your makefile, you can specify
the makefile name with the @samp{-f} option.  The arguments @samp{-f
@var{name}} tell @code{make} to read the file @var{name} as the
makefile.  If you use more than one @samp{-f} option, you can specify
several makefiles.  All the makefiles are effectively concatenated in
the order specified.  The default makefile names @file{GNUmakefile},
@file{makefile} and @file{Makefile} are not checked automatically if you
specify @samp{-f}.@refill

@node Include, MAKEFILES Variable, Makefile Names, Makefiles
@section Including Other Makefiles

@findex include
The @code{include} directive tells @code{make} to suspend reading the
current makefile and read another makefile before continuing.  The
directive is a line in the makefile that looks like this:

@example
include @var{filename}
@end example

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 @code{include} and
@var{filename}; extra whitespace is ignored there and at the end of the
directive.  A comment starting with @samp{#} is allowed at the end of the
line.  If @var{filename} contains any variable or function references, they
are expanded.  (@xref{Variables}.)

When @code{make} processes an @code{include} directive, it suspends
reading of the containing makefile and reads from @var{filename}
instead.  When that is finished, @code{make} resumes reading the
makefile in which the directive appears.

One occasion for using @code{include} directives is when several programs,
handled by individual makefiles in various directories, need to use a
common set of variable definitions (@pxref{Setting}) or pattern rules
(@pxref{Pattern Rules}).

Another such occasion is when you want to automatically generate
dependencies from source files; the dependencies can be put in a file that
is included by the main makefile.  This practice is generally cleaner than
that of somehow appending the dependencies to the end of the main makefile
as has been traditionally done with other versions of @code{make}.

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 @samp{-I} option are
searched (@pxref{Options}).  Then the following directories (if they
exist) are searched, in this order: @file{/usr/gnu/include},
@file{/usr/local/include}, @file{/usr/include}.
If an included makefile cannot be found in any of these directories, a
warning message is generated, but it is not a fatal error; processing
of the makefile containing the @code{include} continues.@refill

@node MAKEFILES Variable, Remaking Makefiles, Include, Makefiles
@section The Variable @code{MAKEFILES}

@vindex MAKEFILES
If the environment variable @code{MAKEFILES} is defined, @code{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 @code{include} directive: various directories are searched for those
files (@pxref{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 @code{MAKEFILES} are not found.@refill

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

Some users are tempted to set @code{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 @code{include} directives
in the makefiles.

@node Remaking Makefiles, Overriding Makefiles, MAKEFILES Variable, Makefiles
@section How Makefiles Are Remade

@cindex updating makefiles
@cindex remaking makefiles
@cindex makefiles, remaking of
Sometimes makefiles can be remade from other files, such as RCS or SCCS
files.  If a makefile can be remade from other files, you probably want
@code{make} to get an up-to-date version of the makefile to read in.

To this end, after reading in all makefiles, @code{make} will consider
each as a goal target and attempt to update it.  If a makefile has a
rule which says how to update it (found either in that very makefile or
in another one) or if an implicit rule applies to it (@pxref{Implicit}),
it will be updated if necessary.  After all makefiles have been checked,
if any have actually been changed, @code{make} starts with a clean slate
and reads all the makefiles over again.  (It will also attempt to update
each of them over again, but normally this will not change them again,
since they are already up to date.)@refill

If the makefiles specify commands to remake a file but no dependencies,
the file will always be remade.  In the case of makefiles, a makefile
that has commands but no dependencies will be remade every time
@code{make} is run, and then again after @code{make} starts over and
reads the makefiles in again.  This would cause an infinite loop;
@code{make} would constantly remake the makefile, and never do anything
else.  So, to avoid this, @code{make} will @emph{not} attempt to remake
makefiles which are specified as targets but have no dependencies.@refill

If you do not specify any makefiles to be read with @samp{-f} options,
@code{make} will try the default makefile names; @pxref{Makefile Names}.
Unlike makefiles explicitly requested with @samp{-f} options,
@code{make} is not certain that these makefiles should exist.  However,
if a default makefile does not exist but can be created by running
@code{make} rules, you probably want the rules to be run so that the
makefile can be used.

Therefore, if none of the default makefiles exists, @code{make} will try
to make each of them in the same order in which they are searched for
(@pxref{Makefile Names}) until it succeeds in making one, or it runs out
of names to try.  Note that it is not an error if @code{make} cannot
find or make any makefile; a makefile is not always necessary.@refill

When you use the @samp{-t} option (touch targets), you would not want to
use an out-of-date makefile to decide which targets to touch.  So the
@samp{-t} option has no effect on updating makefiles; they are really
updated even if @samp{-t} is specified.  Likewise, @samp{-q} and
@samp{-n} do not prevent updating of makefiles, because an out-of-date
makefile would result in the wrong output for other targets.  Thus,
@samp{make -f mfile -n foo} will update @file{mfile}, read it in, and
then print the commands to update @file{foo} and its dependencies
without running them.  The commands printed for @file{foo} will be those
specified in the updated contents of @file{mfile}.

However, on occasion you might actually wish to prevent updating of even
the makefiles.  You can do this by specifying the makefiles as goals in
the command line as well as specifying them as makefiles.  When the
makefile name is specified explicitly as a goal, the options @samp{-t}
and so on do apply to them.

Thus, @samp{make -f mfile -n mfile foo} would read the makefile
@file{mfile}, print the commands needed to update it without actually
running them, and then print the commands needed to update @file{foo}
without running them.  The commands for @file{foo} will be those
specified by the existing contents of @file{mfile}.

@node Overriding Makefiles,  , Remaking Makefiles, Makefiles
@section Overriding Part of One Makefile with Another Makefile

@cindex overriding makefiles
Sometimes it is useful to have a makefile that is mostly just like
another makefile.  You can often use the @samp{include} directive to
include one in the other, and add more targets or variable definitions.
However, if the two makefiles give different commands for the same
target, @code{make} will not let you just do this.  But there is another way.

In the containing makefile (the one that wants to include the other),
you can use the @code{.DEFAULT} special target to say that to remake any
target that cannot be made from the information in the containing
makefile, @code{make} should look in another makefile.  @xref{Last
Resort}, for more information on @code{.DEFAULT}.

For example, if you have a makefile called @file{Makefile} that says how
to make the target @samp{foo} (and other targets), you can write a
makefile called @file{GNUmakefile} that contains:

@example
foo:
        frobnicate > foo

.DEFAULT:
        @@$(MAKE) -f Makefile $@@
@end example

If you say @samp{make foo}, @code{make} will find @file{GNUmakefile},
read it, and see that to make @file{foo}, it needs to run the command
@samp{frobnicate > foo}.  If you say @samp{make bar}, @code{make} will
find no way to make @file{bar} in @file{GNUmakefile}, so it will use the
commands from @code{.DEFAULT}: @samp{make -f Makefile bar}.  If
@file{Makefile} provides a rule for updating @file{bar}, @code{make}
will apply the rule.  And likewise for any other target that
@file{GNUmakefile} does not say how to make.@refill

@node Rules, Commands, Makefiles, Top
@chapter Writing Rules

@cindex rule
@cindex target
@cindex dependency
A @dfn{rule} appears in the makefile and says when and how to remake
certain files, called the rule's @dfn{targets} (usually only one per rule).
It lists the other files that are the @dfn{dependencies} of the target, and
@dfn{commands} to use to create or update the target.

The order of rules is not significant, except for determining the
@dfn{default goal}: the target for @code{make} to consider, if you do
not otherwise specify one.  The default goal is the target of the first
rule in the first makefile, except that targets starting with a period
do not count unless they contain slashes as well; also, a target that
defines a pattern rule (@pxref{Pattern Rules}) or a suffix rule
(@pxref{Suffix Rules}) has no effect on the default goal.

Therefore, we usually write the makefile so that the first rule is the
one for compiling the entire program or all the programs described by
the makefile.  @xref{Goals}.

@menu
* Rule Example::        An explained example of a rule.
* Rule Syntax::         General syntax of rules, with explanation.

* Wildcards::           Using wildcard characters like `*' in file names.
* Directory Search::    Searching other directories for source files.

* Phony Targets::       Using a target that isn't a real file's name.
* Force Targets::       A target without commands or dependencies can
                         be used to mark other targets as phony.
* Special Targets::     Targets with special built-in meanings.
* Empty Targets::       Real files that are empty--only the date matters.
* Multiple Targets::    When it is useful to have several targets in a rule.
* Static Pattern::      Static pattern rules apply to multiple targets
                         and can vary the dependencies according to the
                         target name.
* Multiple Rules::      Using several rules with the same target.
* Double-Colon::        Special kind of rule allowing
                          several independent rules for one target.
* Commands::            Special features and details of how commands
                         in a rule are executed.
@end menu

@ifinfo
@node Rule Example, Rule Syntax, Rules, Rules
@isection Rule Example

Here is an example of a rule:

@example
foo.o : foo.c defs.h       # module for twiddling the frobs
        cc -c -g foo.c
@end example

Its target is @file{foo.o} and its dependencies are @file{foo.c} and
@file{defs.h}.  It has one command, which is @samp{cc -c -g foo.c}.
The command line starts with a tab to identify it as a command.

This rule says two things:

@itemize @bullet
@item
How to decide whether @file{foo.o} is out of date: it is out of date
if it does not exist, or if either @file{foo.c} or @file{defs.h} is
more recent than it.

@item
How to update the file @file{foo.o}: by running @code{cc} as stated.
The command does not explicitly mention @file{defs.h}, but we presume
that @file{foo.c} includes it, and that that is why @file{defs.h} was
added to the dependencies.
@end itemize
@end ifinfo

@node Rule Syntax, Wildcards, Rule Example, Rules
@section Rule Syntax

In general, a rule looks like this:

@example
@var{targets} : @var{dependencies}
        @var{command}
        @var{command}
        ...
@end example

@noindent
or like this:

@example
@var{targets} : @var{dependencies} ; @var{command}
        @var{command}
        @var{command}
        ...
@end example

The @var{targets} are file names, separated by spaces.  Wild card
characters may be used (@pxref{Wildcards}) and a name of the form
@file{@var{a}(@var{m})} represents member @var{m} in archive file
@var{a} (@pxref{Archive Members}).  Usually there is only one target
per rule, but occasionally there is a reason to have more; @xref{Multiple
Targets}.@refill

The @var{command} lines start with a tab character.  The first command may
appear on the line after the dependencies, with a tab character, or may
appear on the same line, with a semicolon.  Either way, the effect is the
same.  @xref{Commands}.

Because dollar signs are used to start variable references, if you really
want a dollar sign in the rule you must write two of them (@samp{$$}).
@xref{Variables}.  You may split a long line by inserting a backslash
followed by a newline, but this is not required, as @code{make} places no
limit on the length of a line in a makefile.

A rule tells @code{make} two things: when the targets are out of date,
and how to update them when necessary.

The criterion for being out of date is specified in terms of the
@var{dependencies}, which consist of file names separated by spaces.
(Wildcards and archive members are allowed here too.)  A target is out of
date if it does not exist or if it is older than any of the dependencies
(by comparison of last-modification times).  The idea is that the contents
of the target file are computed based on information in the dependencies,
so if any of the dependencies changes, the contents of the existing target
file are no longer necessarily valid.

How to update is specified by @var{commands}.  These are lines to be
executed by the shell (normally @samp{sh}), but with some extra features
(@pxref{Commands}).

@node Wildcards, Directory Search, Rule Syntax, Rules
@section Using Wildcards Characters in File Names
@cindex wildcard
@cindex file name

A single file name can specify many files using @dfn{wildcard characters}.
The wildcard characters in @code{make} are @samp{*}, @samp{?} and
@samp{[@dots{}]}, the same as in the Bourne shell.  For example, @file{*.c}
specifies a list of all the files (in the working directory) whose names
end in @samp{.c}.@refill

@cindex ~
The character @samp{~} at the beginning of a file name also has special
significance.  If alone, or followed by a slash, it represents your home
directory.  For example @file{~/bin} expands to @file{/home/you/bin}.
If the @samp{~} is followed by a word, the string represents the home
directory of the user named by that word.  For example @file{~me/bin}
expands to @file{/home/me/bin}.@refill

Wildcard expansion happens automatically in targets, in dependencies, and
in commands.  In other contexts, wildcard expansion happens only if you
request it explicitly with the @code{wildcard} function.

The special significance of a wildcard character can be turned off by
preceding it with a backslash.  Thus, @file{foo\*bar} would refer to a
specific file whose name consists of @samp{foo}, an asterisk, and
@samp{bar}.@refill

@menu
* Examples: Wildcard Examples.    Some simple examples.
* Pitfall: Wildcard Pitfall.      @code{*.o} won't do what you want!
* Function: Wildcard Function.
       How to do wildcard expansion when defining a variable
       using the function @code{wildcard}.
@end menu

@node Wildcard Examples, Wildcard Pitfall, Wildcards, Wildcards
@subsection Wildcard Examples

Wildcards can be used in the commands of a rule.  For example, here is a
rule to delete all the object files:

@example
clean:
        rm -f *.o
@end example

Wildcards are also useful in the dependencies of a rule.  With the
following rule in the makefile, @samp{make print} will print all the
@samp{.c} files that have changed since the last time you printed them:

@example
print: *.c
        lpr -p $?
        touch print
@end example

@noindent
This rule uses @file{print} as an empty target file; @pxref{Empty Targets}.

Wildcard expansion does not happen when you define a variable.  Thus, if
you write this:

@example
objects=*.o
@end example

@noindent
then the value of the variable @code{objects} is the actual string
@samp{*.o}.  However, if you use the value of @code{objects} in a target,
dependency or command, wildcard expansion will take place at that time.

@node Wildcard Pitfall, Wildcard Function, Wildcard Examples, Wildcards
@subsection Pitfalls of Using Wildcards

Now here is an example of a naive way of using wildcard expansion, that
does not do what you would intend.  Suppose you would like to say that the
executable file @file{foo} is made from all the object files in the
directory, and you write this:

@example
objects=*.o

foo : $(objects)
        cc -o foo $(CFLAGS) $(objects)
@end example

@noindent
The value of @code{objects} is the actual string @samp{*.o}.  Wildcard
expansion happens in the rule for @file{foo}, so that each @emph{existing}
@samp{.o} file becomes a dependency of @file{foo} and will be recompiled if
necessary.

But what if you delete all the @samp{.o} files?  Then @samp{*.o} will
expand into @emph{nothing}.  The target @file{foo} will have no
dependencies and would be remade by linking no object files.  This is not
what you want!

Actually it is possible to obtain the desired result with wildcard
expansion, but you need more sophisticated techniques, including the
@code{wildcard} function and string substitution.
@ifinfo
@xref{Wildcard Function}.
@end ifinfo
@iftex
These are described in the following section.
@end iftex

@node Wildcard Function,  , Wildcard Pitfall, Wildcards
@subsection The Function @code{wildcard}
@findex wildcard

Wildcard expansion happens automatically in rules.  But wildcard expansion
does not normally take place when a variable is set, or inside the
arguments of a function.  If you want to do wildcard expansion in such
places, you need to use the @code{wildcard} function, like this:

@example
$(wildcard @var{pattern})
@end example

@noindent
This string, used anywhere in a makefile, is replaced by a space-separated
list of names of existing files that match the pattern @var{pattern}.

One use of the @code{wildcard} function is to get a list of all the C source
files in a directory, like this:

@example
$(wildcard *.c)
@end example

We can change the list of C source files into a list of object files by
substituting @samp{.o} for @samp{.c} in the result, like this:

@example
$(subst .c,.o,$(wildcard *.c))
@end example

@noindent
(Here we have used another function, @code{subst}.
@xref{Text Functions}.)@refill

Thus, a makefile to compile all C source files in the directory and then
link them together could be written as follows:

@example
objects:=$(subst .c,.o,$(wildcard *.c))

foo : $(objects)
        cc -o foo $(LDFLAGS) $(objects)
@end example

@noindent
(This takes advantage of the implicit rule for compiling C programs, so
there is no need to write explicit rules for compiling the files.
@xref{Flavors}, for an explanation of @samp{:=}, which is a variant of
@samp{=}.)

@node Directory Search, Phony Targets, Wildcards, Rules
@section Searching Directories for Dependencies
@vindex VPATH
@findex vpath
@cindex vpath
@cindex search path for dependencies
@cindex directory search

For large systems, it is often desirable to put sources in a separate
directory from the binaries.  The @dfn{directory search} features of
@code{make} facilitate this by searching several directories automatically
to find a dependency.  When you redistribute the files among directories,
you do not need to change the individual rules, just the search paths.

@menu
* General Search::    The @code{VPATH} variable specifies a search path
                        that applies to every dependency.
* Selective Search::  The @code{vpath} directive specifies a search path
                        for a specified class of names.
* 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.
@end menu

@node General Search, Selective Search, Directory Search, Directory Search
@subsection @code{VPATH}: Search Path for All Dependencies

The value of the @code{make} variable @code{VPATH} specifies a list of
directories which @code{make} should search (in the order specified) for
dependency files.  The directory names are separated by colons.  For
example:

@example
VPATH = src:../headers
@end example

@noindent
specifies a path containing two directories, @file{src} and @file{../headers}.

Whenever a file listed as a dependency does not exist in the current
directory, the directories listed in @code{VPATH} are searched for a file
with that name.  If a file is found in one of them, that file becomes the
dependency.  Rules may then specify the names of source files as if they
all existed in the current directory.

Using the value of @code{VPATH} set in the previous example, a rule like this:

@example
foo.o : foo.c
@end example

@noindent
is interpreted as if it were written like this:

@example
foo.o : src/foo.c
@end example

@noindent
assuming the file @file{foo.c} does not exist in the current directory but
is found in the directory @file{src}.

@node Selective Search, Commands/Search, General Search, Directory Search
@subsection The @code{vpath} Directive

Similar to the @code{VPATH} variable but more selective is the @code{vpath}
directive, which allows you to specify a search path for a particular class
of filenames, those that match a particular pattern.  Thus you can supply
certain search directories for one class of filenames and other directories
(or none) for other filenames.

There are three forms of the @code{vpath} directive:

@table @code
@item vpath @var{pattern} @var{directories}
Specify the search path @var{directories} for filenames that match
@code{pattern}.  If another path was previously specified for the same
pattern, the new path is effectively appended to the old path.@refill

The search path, @var{directories}, is a colon-separated list of
directories to be searched, just like the search path used in the
@code{VPATH} variable.

@item vpath @var{pattern}
Clear out the search path associated with @var{pattern}.

@item vpath
Clear all search paths previously specified with @code{vpath} directives.
@end table

A @code{vpath} pattern is a string containing a @samp{%} character.  The
string must match the filename of a dependency that is being searched for,
the @samp{%} character matching any sequence of zero or more characters (as
in pattern rules; @pxref{Pattern Rules}).  (If there is no @samp{%}, the
pattern must match the dependency, which is not useful very often.)

@samp{%} characters in a @code{vpath} directive's pattern can be quoted
with preceding backslashes (@samp{\}).  Backslashes that would otherwise
quote @samp{%} characters can be quoted with more backslashes.
Backslashes that quote @samp{%} characters or other backslashes are
removed from the pattern before it is compared to file names.  Backslashes
that are not in danger of quoting @samp{%} characters go unmolested.@refill

When a dependency fails to exist in the current directory, if the
@var{pattern} in a @code{vpath} directive matches the name of the
dependency file, then the @var{directories} in that directive are searched
just like (and before) the directories in the @code{VPATH} variable.  For
example,

@example
vpath %.h ../headers
@end example

@noindent
tells @code{make} to look for any dependency whose name ends in @file{.h}
in the directory @file{../headers} if the file is not found in the current
directory.

If several @code{vpath} patterns match the dependency file's name, then
@code{make} processes each matching @code{vpath} directive one by one,
searching all the directories mentioned in each directive.  The @code{vpath}
directives are processed in the order in which they appear in the makefiles.

@node Commands/Search, Implicit/Search, Selective Search, Directory Search
@subsection Writing Shell Commands with Directory Search

When a dependency is found in another directory through directory search,
this cannot change the commands of the rule; they will execute as written.
Therefore, you must write the commands with care so that they will look for
the dependency in the directory where @code{make} finds it.

This is done with the @dfn{automatic variables} such as @samp{$^}
(@pxref{Automatic}).  For instance, the value of @samp{$^} is a
list of all the dependencies of the rule, including the names of
the directories in which they were found, and the value of
@samp{$@@} is the target.  Thus:@refill

@example
foo.o : foo.c
        cc -c $(CFLAGS) $^ -o $@@
@end example

@noindent
The variable @code{CFLAGS} exists so you can specify flags for C
compilation by implicit rule; we use it here for consistency so it will
affect all C compilations uniformly (@pxref{Implicit Variables}).

Often the dependencies include header files as well, which you don't want
to mention in the commands.  The function @code{firstword} can be used to
extract just the first dependency from the entire list, as shown here
(@pxref{Filename Functions}):

@example
VPATH = src:../headers
foo.o : foo.c defs.h hack.h
        cc -c $(CFLAGS) $(firstword $^) -o $@@
@end example

@noindent
Here the value of @samp{$^} would be something like @samp{src/foo.c
../headers/defs.h hack.h}, from which @samp{$(firstword $^)} extracts just
@samp{src/foo.c}.@refill

@node Implicit/Search, Libraries/Search, Commands/Search, Directory Search
@subsection Directory Search and Implicit Rules

The search through the directories specified in @code{VPATH} or with
@code{vpath} happens also during consideration of implicit rules
(@pxref{Implicit}).

For example, when a file @file{foo.o} has no explicit rule, @code{make}
considers implicit rules, such as to compile @file{foo.c} if that file
exists.  If such a file is lacking in the current directory, the
appropriate directories are searched for it.  If @file{foo.c} exists (or is
mentioned in the makefile) in any of the directories, the implicit rule for
C compilation is applicable.

The commands of all the built-in implicit rules normally use automatic
variables as a matter of necessity; consequently they will use the file
names found by directory search with no extra effort.

@node Libraries/Search,  , Implicit/Search, Directory Search
@subsection Directory Search for Link Libraries

Directory search applies in a special way to libraries used with the
linker.  This special feature comes into play when you write a dependency
whose name is of the form @samp{-l@var{name}}.  (You can tell something
funny is going on here because the dependency is normally the name of a
file, and the @emph{file name} of the library looks like
@file{lib@var{name}.a}, not like @samp{-l@var{name}}.)@refill

When a dependency's name has the form @samp{-l@var{name}}, @code{make}
handles it specially by searching for the file @samp{lib@var{name}.a} in
the directories @samp{/lib} and @samp{/usr/lib}, and then using matching
@code{vpath} search paths and the @code{VPATH} search path.@refill

For example,

@example
foo : foo.c -lcurses
        cc $^ -o $@@
@end example

@noindent
would cause the command @samp{cc foo.c -lcurses -o foo} to be executed when
@file{foo} is older than @file{foo.c} or than @file{libcurses.a} (which has
probably been found by directory search in the file
@file{/usr/lib/libcurses.a}).@refill

As shown by the example above, the file name found by directory search is
used only for comparing the file time with the target file's time.  It
does not replace the file's name in later usage (such as in automatic
variables like @code{$^}); the name remains unchanged, still starting
with @samp{-l}.  This leads to the correct results because the linker
will repeat the appropriate search when it processes this argument.@refill

@node Phony Targets, Force Targets, Directory Search, Rules
@section Phony Targets
@cindex phony targets

A phony target is one that is not really the name of a file.  It is just a
name for some commands to be executed when you make an explicit request.

If you write a rule whose commands will not create the target file, the
commands will be executed every time the target comes up for remaking.
Here is an example:

@example
clean:
        rm *.o temp
@end example

@noindent
Because the @code{rm} command does not create a file named @file{clean},
probably no such file will ever exist.  Therefore, the @code{rm} command
will be executed every time you say @samp{make clean}.

@findex .PHONY
The phony target will cease to work if anything ever does create a file
named @file{clean} in this directory.  Since it has no dependencies, the
file @file{clean} would inevitably be considered up to date, and its
commands would not be executed.  To avoid this problem, you can explicitly
declare the target to be phony, using the special target @code{.PHONY}
(@pxref{Special Targets}) as follows:

@example
.PHONY : clean
@end example

@noindent
Once this is done, @samp{make clean} will run the commands regardless of
whether there is a file named @file{clean}.

A phony target should not be a dependency of a real target file; strange
things can result from that.  As long as you don't do that, the phony
target commands will be executed only when the phony target is a specified
goal (@pxref{Goals}).

Phony targets can have dependencies.  When one directory contains multiple
programs, it is most convenient to describe all of the programs in one
makefile @file{./Makefile}.  Since the target remade by default will be the
first one in the makefile, it is common to make this a phony target named
@samp{all} and give it, as dependencies, all the individual programs.  For
example:

@example
all : prog1 prog2 prog3
.PHONY : all

prog1 : prog1.o utils.o
        cc -o prog1 prog1.o utils.o

prog2 : prog2.o
        cc -o prog2 prog2.o

prog3 : prog3.o sort.o utils.o
        cc -o prog3 prog3.o sort.o utils.o
@end example

@noindent
Now you can say just @samp{make} to remake all three programs, or specify
as arguments the ones to remake (as in @samp{make prog1 prog3}).

When one phony target is a dependency of another, it serves as a subroutine
of the other.  For example, here @samp{make cleanall} will delete the
object files, the difference files, and the file @file{program}:

@example
cleanall : cleanobj cleandiff
        rm program

cleanobj :
        rm *.o

cleandiff :
        rm *.diff
@end example

@node Force Targets, Empty Targets, Phony Targets, Rules
@section Rules without Commands or Dependencies

If a rule has no dependencies or commands, and the target of the rule
is a nonexistent file, then @code{make} imagines this target to have
been updated whenever its rule is run.  This implies that all targets
depending on this one will always have their commands run.

An example will illustrate this:

@example
clean: FORCE
        rm $(objects)
FORCE:
@end example

Here the target @samp{FORCE} satisfies the special conditions, so the
target @file{clean} that depends on it is forced to run its commands.
There is nothing special about the name @samp{FORCE}, but that is one name
commonly used this way.

As you can see, using @samp{FORCE} this way has the same results as using
@samp{.PHONY: clean}.  The latter is more explicit, but other versions of
@code{make} do not support it; thus @samp{FORCE} appears in many makefiles.

@node Empty Targets, Special Targets, Force Targets, Rules
@section Empty Target Files to Record Events
@cindex empty targets

The @dfn{empty target} is a variant of the phony target; it is used to hold
commands for an action that you request explicitly from time to time.
Unlike a phony target, this target file can really exist; but the file's
contents do not matter, and usually are empty.

The purpose of the empty target file is to record, with its
last-modification time, when the rule's commands were last executed.  It
does so because one of the commands is a @code{touch} command to update the
target file.

The empty target file must have some dependencies.  When you ask to remake
the empty target, the commands are executed if any dependency is more
recent than the target; in other words, if a dependency has changed since
the last time you remade the target.  Here is an example:

@example
print: foo.c bar.c
        lpr -p $?
        touch print
@end example

@noindent
With this rule, @samp{make print} will execute the @code{lpr} command if
either source file has changed since the last @samp{make print}.  The
automatic variable @samp{$?} is used to print only those files that have
changed (@pxref{Automatic}).

@node Special Targets, Multiple Targets, Empty Targets, Rules
@section Special Built-in Target Names
@cindex special targets

Certain names have special meanings if they appear as targets.

@table @code
@item .PHONY
The dependencies of the special target @code{.PHONY} are considered to
be phony targets.  When it is time to consider such a target,
@code{make} will run its commands unconditionally, regardless of
whether a file with that name exists or what its last-modification
time is.  @xref{Phony Targets}.

@item .SUFFIXES
The dependencies of the special target @code{.SUFFIXES} are the list
of suffixes to be used in checking for suffix rules.  @xref{Suffix
Rules}.

@item .DEFAULT
The commands specified for @code{.DEFAULT} are used for any target for
which no other commands are known (either explicitly or through an
implicit rule).  If @code{.DEFAULT} commands are specified, every
nonexistent file mentioned as a dependency will have these commands
executed on its behalf.  @xref{Search Algorithm}.

@item .PRECIOUS
@cindex precious targets
The targets which @code{.PRECIOUS} depends on are given this special
treatment: if @code{make} is killed or interrupted during the
execution of their commands, the target is not deleted.
@xref{Interrupts}.

@item .IGNORE
Simply by being mentioned as a target, @code{.IGNORE} says to ignore
errors in execution of commands.  The dependencies and commands for
@code{.IGNORE} are not meaningful.

@samp{.IGNORE} exists for historical compatibility.  Since
@code{.IGNORE} affects every command in the makefile, it is not very
useful; we recommend you use the more selective ways to ignore errors
in specific commands.  @xref{Errors}.

@item .SILENT
Simply by being mentioned as a target, @code{.SILENT} says not to
print commands before executing them.  The dependencies and commands
for @code{.SILENT} are not meaningful.

@samp{.SILENT} exists for historical compatibility.  We recommend you
use the more selective ways to silence specific commands.
@xref{Echoing}.
@end table

Any defined implicit rule suffix also counts as a special target if it
appears as a target, and so does the concatenation of two suffixes, such
as @samp{.c.o}.  These targets are suffix rules, an obsolete way of
defining implicit rules (but a way still widely used).  In principle, any
target name could be special in this way if you break it in two and add
both pieces to the suffix list.  In practice, suffixes normally begin with
@samp{.}, so these special target names also begin with @samp{.}.
@xref{Suffix Rules}.

@node Multiple Targets, Static Pattern, Special Targets, Rules
@section Multiple Targets in a Rule

A rule with multiple targets is equivalent to writing many rules, each with
one target, and all identical aside from that.  The same commands apply to
all the targets, but their effects may vary because you can substitute the
actual target name into the command using @samp{$@@}.  The rule contributes
the same dependencies to all the targets also.

This is useful in two cases.

@itemize @bullet
@item
You want just dependencies, no commands.  For example:

@example
kbd.o commands.o files.o: command.h
@end example

@noindent
gives an additional dependency to each of the three object files
mentioned.

@item
Similar commands work for all the targets.  The commands do not need
to be absolutely identical, since the automatic variable @samp{$@@}
can be used to substitute the particular target to be remade into the
commands (@pxref{Automatic}).  For example:

@example
@group
bigoutput littleoutput : text.g
        generate text.g -$(subst output,,$@@) > $@@
@end group
@end example

@noindent
is equivalent to

@example
bigoutput : text.g
        generate text.g -big > bigoutput
littleoutput : text.g
        generate text.g -little > littleoutput
@end example

@noindent
Here we assume the hypothetical program @code{generate} makes two
types of output, one if given @samp{-big} and one if given
@samp{-little}.@refill
@end itemize

@ifinfo
Suppose you would like to vary the dependencies according to the target,
much as the variable @samp{$@@} allows you to vary the commands.
You cannot do this with multiple targets in an ordinary rule, but you can
do it with a @dfn{static pattern rule}.  @xref{Static Pattern}.
@end ifinfo

@node Static Pattern, Multiple Rules, Multiple Targets, Rules
@section Static Pattern Rules
@cindex static pattern rules
@cindex varying dependencies

@dfn{Static pattern rules} are rules which specify multiple targets and
construct the dependency names for each target based on the target name.
They are more general than ordinary rules with multiple targets because the
targets don't have to have identical dependencies.  Their dependencies must
be @emph{analogous}, but not necessarily @emph{identical}.

@menu
* Usage: Static Usage.  How to use static pattern rules.
* Static vs Implicit::  When are they better than implicit rules?
@end menu

@node Static Usage, Static vs Implicit, Static Pattern, Static Pattern
@subsection Syntax of Static Pattern Rules

Here is the syntax of a static pattern rule:

@example
@var{targets}: @var{target-pattern}: @var{dep-patterns} @dots{}
        @var{commands}
        @dots{}
@end example

@noindent
The @var{targets} gives the list of targets that the rule applies to.  The
targets can contain wildcard characters, just like the targets of ordinary
rules (@pxref{Wildcards}).

The @var{target-pattern} and @var{dep-patterns} say how to compute the
dependencies of each target.  Each target is matched against the
@var{target-pattern} to extract a part of the target name, called the
@dfn{stem}.  This stem is substituted into each of the @var{dep-patterns}
to make the dependency names (one from each @var{dep-pattern}).

Each pattern normally contains the character @samp{%} just once.  When the
@var{target-pattern} matches a target, the @samp{%} can match any part of
the target name; this part is called the @dfn{stem}.  The rest of the
pattern must match exactly.  For example, the target @file{foo.o} matches
the pattern @samp{%.o}, with @samp{foo} as the stem.  The targets
@file{foo.c} and @file{foo.out} don't match that pattern.@refill

The dependency names for each target are made by substituting the stem
for the @samp{%} in each dependency pattern.  For example, if one
dependency pattern is @file{%.c}, then substitution of the stem
@samp{foo} gives the dependency name @file{foo.c}.  It is legitimate
to write a dependency pattern that doesn't contain @samp{%}; then this
dependency is the same for all targets.

@samp{%} characters in pattern rules can be quoted with preceding
backslashes (@samp{\}).  Backslashes that would otherwise quote @samp{%}
characters can be quoted with more backslashes.  Backslashes that quote
@samp{%} characters or other backslashes are removed from the pattern
before it is compared file names or has a stem substituted into it.
Backslashes that are not in danger of quoting @samp{%} characters go
unmolested.  For example, the pattern @file{the\%weird\\%pattern\\} has
@samp{the%weird\} preceding the operative @samp{%} character, and
@samp{pattern\\} following it.  The final two backslashes are left alone
because they can't affect any @samp{%} character.@refill

Here is an example, which compiles each of @file{foo.o} and @file{bar.o}
from the corresponding @file{.c} file:

@example
objects = foo.o bar.o

$(objects): %.o: %.c
        $(CC) -c $(CFLAGS) $< -o $@@
@end example

Each target specified must match the target pattern; a warning is issued
for each target that does not.  If you have a list of files, only some of
which will match the pattern, you can use the @code{filter} function to
remove nonmatching filenames (@pxref{Text Functions}):

@example
files = foo.elc bar.o lose.o

$(filter %.o,$(files)): %.o: %.c
        $(CC) -c $(CFLAGS) $< -o $@@
$(filter %.elc,$(files)): %.elc: %.el
        emacs -f batch-byte-compile $<
@end example

@noindent
Here the result of @samp{$(filter %.o,$(files))} is @file{bar.o lose.o},
and the first static pattern rule causes each of these object files to be
updated by compiling the corresponding C source file.  The result of
@samp{$(filter %.elc,$(files))} is @file{foo.elc}, so that file is made
from @file{foo.el}.@refill

@node Static vs Implicit,  , Static Usage, Static Pattern
@subsection Static Pattern Rules versus Implicit Rules

A static pattern rule has much in common with an implicit rule defined as a
pattern rule (@pxref{Pattern Rules}).  Both have a pattern for the target
and patterns for constructing the names of dependencies.  The difference is
in how @code{make} decides @emph{when} the rule applies.

An implicit rule @emph{can} apply to any target that matches its pattern,
but it @emph{does} apply only when the target has no commands otherwise
specified, and only when the dependencies can be found.  If more than one
implicit rule appears applicable, only one applies; the choice depends on
the order of rules.

By contrast, a static pattern rule applies to the precise list of targets
that you specify in the rule.  It cannot apply to any other target and it
invariably does apply to each of the targets specified.  If two conflicting
rules apply, and both have commands, that's an error.

The static pattern rule can be better than an implicit rule for these
reasons:

@itemize @bullet
@item
You may wish to override the usual implicit rule for a few
files whose names cannot be categorized syntactically but
can be given in an explicit list.

@item
If you cannot be sure of the precise contents of the directories
you are using, you may not be sure which other irrelevant files
might lead @code{make} to use the wrong implicit rule.  The choice
might depend on the order in which the implicit rule search is done.
With static pattern rules, there is no uncertainty: each rule applies
to precisely the targets specified.
@end itemize

@node Multiple Rules, Double-Colon, Static Pattern, Rules
@section Multiple Rules for One Target

One file can be the target of several rules.  All the dependencies
mentioned in all the rules are merged into one list of dependencies for
the target.  If the target is older than any dependency from any rule,
the commands are executed.

There can only be one set of commands to be executed for a file.
If more than one rule gives commands for the same file, the last
@code{make} uses the last set given and prints an error message.
(As a special case, if the file's name begins with a dot, no
error message is printed.  This odd behavior is only for
compatibility with other @code{make}s.)  There is no reason to
write your makefiles this way; that is why @code{make} gives you
an error message.@refill

An extra rule with just dependencies can be used to give a few extra
dependencies to many files at once.  For example, one usually has a
variable named @code{objects} containing a list of all the compiler output
files in the system being made.  An easy way to say that all of them must
be recompiled if @file{config.h} changes is to write

@example
objects = foo.o bar.o
foo.o : defs.h
bar.o : defs.h test.h
$(objects) : config.h
@end example

This could be inserted or taken out without changing the rules that really
say how to make the object files, making it a convenient form to use if
you wish to add the additional dependency intermittently.

Another wrinkle is that the additional dependencies could be specified with
a variable that you could set with a command argument to @code{make}
(@pxref{Overriding}).  For example,

@example
@group
extradeps=
$(objects) : $(extradeps)
@end group
@end example

@noindent
means that the command @samp{make extradeps=foo.h} will consider
@file{foo.h} as a dependency of each object file, but plain @samp{make}
will not.

If none of the explicit rules for a target has commands, then @code{make}
searches for an applicable implicit rule to find some commands.
@xref{Implicit}.

@node Double-Colon,  , Multiple Rules, Rules
@section Double-Colon Rules
@cindex double-colon rule

@dfn{Double-colon} rules are rules written with @samp{::} instead of
@samp{:} after the target names.  They are handled differently from
ordinary rules when the same target appears in more than one rule.

When a target appears in multiple rules, all the rules must be the same
type: all ordinary, or all double-colon.  If they are double-colon, each of
them is independent of the others.  Each double-colon rule's commands are
executed if the target is older than any dependencies of that rule.  This
can result in executing none, any or all of the double-colon rules.

Double-colon rules with the same target are in fact completely separate
from one another.  Each double-colon rule is processed individually, just
as rules with different targets are processed.

The double-colon rules for a target are executed in the order they appear
in the makefile.  However, the cases where double-colon rules really make
sense are those where the order of executing the commands would not matter.

Double-colon rules are somewhat obscure and not often very useful; they
provide a mechanism for cases in which the method used to update a target
differs depending on which dependency files caused the update, and such
cases are rare.

Each double-colon rule should specify commands; if it does not, an
implicit rule will be used if one applies.  @xref{Implicit}.

@node Commands, Variables, Rules, Top
@chapter Writing the Commands in Rules
@cindex commands

The commands of a rule consist of shell command lines to be executed one by
one.  Each command line must start with a tab, except that the first
command line may be attached to the target-and-dependencies line with a
semicolon in between.  Blank lines and lines of just comments may appear
among the command lines; they are ignored.

Users use many different shell programs, but commands in makefiles are
always interpreted by @file{/bin/sh} unless the makefile specifies otherwise.

Whether comments can be written on command lines, and what syntax they use,
is under the control of the shell that is in use.  If it is @file{/bin/sh},
a @samp{#} at the start of a word starts a comment.

@menu
* Echoing::       Normally commands are echoed before execution,
                    but you can control this in several ways.
* Execution::     How commands are executed.
* Parallel::      Commands of several rules can be executed in parallel,
                    to reduce total time.
* Errors::        What happens after an error in command execution.
                   How to ignore errors in certain commands.
* Interrupts::    If a command is interrupted or killed,
                   the target may be deleted.
* Recursion::     Invoking @code{make} from commands in makefiles.
* Sequences::     Defining canned sequences of commands.
* Empty Commands::Defining commands that do nothing (but are useful).
@end menu

@node Echoing, Execution, Commands, Commands
@section Command Echoing

@cindex echoing (of commands)
@cindex silent operation
@cindex @@ (in commands)
Normally @code{make} prints each command line before it is executed.
We call this @dfn{echoing} because it gives the appearance that you
are typing the commands yourself.

When a line starts with @samp{@@}, the echoing of that line is suppressed.
The @samp{@@} is discarded before the command is passed to the shell.
Typically you would use this for a command whose only effect is to print
something, such as an @code{echo} command to indicate progress through
the makefile:

@example
@@echo About to make distribution files
@end example

When @code{make} is given the flag @samp{-n}, echoing is all that happens,
no execution.  @xref{Options}.  In this case and only this case, even the
commands starting with @samp{@@} are printed.  This flag is useful for
finding out which commands @code{make} thinks are necessary without
actually doing them.

@cindex @code{-s}
@findex .SILENT
The @samp{-s} flag to @code{make} prevents all echoing, as if all commands
started with @samp{@@}.  A rule in the makefile for the special target
@code{.SILENT} has the same effect (@pxref{Special Targets}).
@code{.SILENT} is essentially obsolete since @samp{@@} is more flexible.@refill

@node Execution, Parallel, Echoing, Commands
@section Command Execution
@cindex execution
@cindex shell

When it is time to execute commands to update a target, they are executed
by making a new subshell for each line.  (In practice, @code{make} may
take shortcuts that do not affect the results.)

This implies that shell commands such as @code{cd} that set variables local
to each process will not affect the following command lines.  If you want
to use @code{cd} to affect the next command, put the two on a single line
with a semicolon between them.  Then @code{make} will consider them a
single command and pass them, together, to a shell which will execute them
in sequence.  For example:

@example
foo : bar/lose
        cd bar; gobble lose > ../foo
@end example

If you would like to split a single shell command into multiple lines of
text, you must use a backslash at the end of all but the last subline.
Such a sequence of lines is combined into a single line, by deleting the
backslash-newline sequences, before passing it to the shell.  Thus, the
following is equivalent to the preceding example:

@example
@group
foo : bar/lose
        cd bar;  \
        gobble lose > ../foo
@end group
@end example

@vindex SHELL
The program used as the shell is taken from the variable @code{SHELL}.
By default, the program @file{/bin/sh} is used.

Unlike most variables, the variable @code{SHELL} will not be set from the
environment, except in a recursive @code{make}.  This is because the
environment variable @code{SHELL} is used to specify your personal choice
of shell program for interactive use.  It would be very bad for personal
choices like this to affect the functioning of makefiles.
@xref{Environment}.

@node Parallel, Errors, Execution, Commands
@section Parallel Execution

@cindex parallel execution
@cindex execution in parallel
@cindex job slots
GNU @code{make} knows how to execute several commands at once.
Normally, @code{make} will execute only one command at a time, waiting
for it to finish before executing the next.  However, the @samp{-j}
option tells @code{make} to execute many commands simultaneously.@refill

If the @samp{-j} option is followed by an integer, this is the number of
commands to execute at once; this is called the number of @dfn{job slots}.
If there is nothing looking like an integer after the @samp{-j} option,
there is no limit on the number of job slots.  The default number of job
slots is one, which means serial execution (one thing at a time).

One unpleasant consequence of running several commands simultaneously is
that output from all of the commands comes when the commands send it, so
messages from different commands may be interspersed.

Another problem is that two processes cannot both take input from the same
device; so to make sure that only one command tries to take input from the
terminal at once, @code{make} will invalidate the standard input streams of
all but one running command.  This means that attempting to read from
standard input, for most child processes if there are several, will usually
be a fatal error (a @samp{Broken pipe} signal).

It is unpredictable which command will have a valid standard input stream
(which will come from the terminal, or wherever you redirect the standard
input of @code{make}).  The first command run will always get it first, and
the first command started after that one finishes will get it next, and so
on.

We will change how this aspect of @code{make} works if we find a better
alternative.  In the mean time, you should not rely on any command using
standard input at all if you are using the parallel execution feature; but
if you are not using this feature, then standard input works normally in
all commands.

If a command fails (is killed by a signal or exits with a nonzero
status), and errors are not ignored for that command (@pxref{Errors}),
the remaining command lines to remake the same target will not be run.
If a command fails and the @samp{-k} option was not given
(@pxref{Options}), @code{make} aborts execution.  If make terminates for
any reason (including a signal) with child processes running, it waits
for them to finish before actually exiting.@refill

When the system is heavily loaded, you will probably want to run fewer jobs
than when it is lightly loaded.  You can use the @samp{-l} option to tell
@code{make} to limit the number of jobs to run at once, based on the load
average.  The @samp{-l} option is followed by a floating-point number.  For
example,

@example
-l 2.5
@end example

@noindent
will not let @code{make} start more than one job if the load average is
above 2.5.  The @samp{-l} option with no following number removes the
load limit, if one was given with a previous @samp{-l} option.@refill

More precisely, when @code{make} goes to start up a job, and it already has
at least one job running, it checks the current load average; if it is not
lower than the limit given with @samp{-l}, @code{make} waits until the load
average goes below that limit, or until all the other jobs finish.

By default, there is no load limit.

@node Errors, Interrupts, Parallel, Commands
@section Errors in Commands

@cindex error (in commands)
After each shell command returns, @code{make} looks at its exit status.
If the command completed successfully, the next command line is executed in
a new shell, or after the last command line is executed, the rule is finished.

If there is an error (the exit status is nonzero), @code{make} gives up on
the current rule, and perhaps on all rules.

Sometimes the failure of a certain command does not indicate a problem.
For example, you may use the @code{mkdir} command to insure that a
directory exists.  If the directory already exists, @code{mkdir} will
report an error, but you probably want @code{make} to continue regardless.

@cindex - (in commands)
To ignore errors in a command line, write a @samp{-} at the beginning of
the line's text (after the initial tab).  The @samp{-} is discarded before
the command is passed to the shell for execution.  For example,

@example
clean:
        -rm -f *.o
@end example

@cindex @code{-i}
@findex .IGNORE
When @code{make} is run with the @samp{-i} flag, errors are ignored in
all commands of all rules.  A rule in the makefile for the special target
@code{.IGNORE} has the same effect.  These ways of ignoring errors are
obsolete because @samp{-} is more flexible.

When errors are to be ignored, because of either a @samp{-} or the
@samp{-i} flag, @code{make} treats an error return just like success,
except that it prints out a message telling you the status code the
command exited with and saying that the error has been ignored.

When an error happens that @code{make} has not been told to ignore,
it implies that the current target cannot be correctly remade, and neither
can any other that depends on it either directly or indirectly.  No further
commands will be executed for these targets, since their preconditions
have not been achieved.

Normally @code{make} gives up immediately in this circumstance, returning a
nonzero status.  However, if the @samp{-k} flag is specified, @code{make}
continues to consider the other dependencies of the pending targets,
remaking them if necessary, before it gives up and returns nonzero status.
For example, after an error in compiling one object file, @samp{make -k}
will continue compiling other object files even though it already knows
that linking them will be impossible.  @xref{Options}.

The usual behavior assumes that your purpose is to get the specified
targets up to date; once @code{make} learns that this is impossible, it
might as well report the failure immediately.  The @samp{-k} option says
that the real purpose is to test as much as possible of the changes made in
the program, perhaps to find several independent problems so that you can
correct them all before the next attempt to compile.  This is why Emacs's
@code{compile} command passes the @samp{-k} flag by default.

@node Interrupts, Recursion, Errors, Commands
@section Interrupting or Killing @code{make}
@cindex interrupt
@cindex signal
@cindex deletion of target files

If @code{make} gets a fatal signal while a command is executing, it may
delete the target file that the command was supposed to update.  This is
done if the target file's last-modification time has changed since
@code{make} first checked it.

The purpose of deleting the target is to make sure that it is remade from
scratch when @code{make} is next run.  Why is this?  Suppose you type
@kbd{Ctrl-c} while a compiler is running, and it has begun to write an
object file @file{foo.o}.  The @kbd{Ctrl-c} kills the compiler, resulting
in an incomplete file whose last-modification time is newer than the source
file @file{foo.c}.  But @code{make} also receives the @kbd{Ctrl-c} signal
and deletes this incomplete file.  If @code{make} did not do this, the next
invocation of @code{make} would think that @file{foo.o} did not require
updating---resulting in a strange error message from the linker when it
tries to link an object file half of which is missing.

@findex .PRECIOUS
You can prevent the deletion of a target file in this way by making the
special target @code{.PRECIOUS} depend on it.  Before remaking a target,
@code{make} checks to see whether it appears on the dependencies of
@code{.PRECIOUS}, and thereby decides whether the target should be deleted
if a signal happens.  Some reasons why you might do this are that the
target is updated in some atomic fashion, or exists only to record a
modification-time (its contents do not matter), or must exist at all
times to prevent other sorts of trouble.

@node Recursion, Sequences, Interrupts, Commands
@section Recursive Use of @code{make}
@cindex recursion

Recursive use of @code{make} means using @code{make} as a command in a
makefile.  This technique is useful when you want separate makefiles for
various subsystems that compose a larger system.  For example, suppose you
have a subdirectory @file{subdir} which has its own makefile, and you would
like the containing directory's makefile to run @code{make} on the
subdirectory.  You can do it by writing this:

@example
subsystem:
        cd subdir; $(MAKE)
@end example

@noindent
or, equivalently, this (@pxref{Options}):

@example
subsystem:
        $(MAKE) -C subdir
@end example

You can write recursive @code{make} commands just by copying this example,
but there are many things to know about how they work and why, and about
how the sub-@code{make} relates to the top-level @code{make}.

@menu
* MAKE Variable::        Special effects of using @samp{$(MAKE)}.
* Variables/Recursion::  How variables are communicated to a sub-@code{make}.
* Options/Recursion::    How options are communicated to a sub-@code{make}.
* -w Option::            The @samp{-w} option facilitates debugging
                           makefiles with recursive @code{make} commands.
@end menu

@node MAKE Variable, Variables/Recursion, Recursion, Recursion
@subsection How the @code{MAKE} Variable Works
@vindex MAKE

Recursive @code{make} commands should always use the variable @code{MAKE},
not the explicit command name @samp{make}, as shown here:

@example
subsystem:
        cd subdir; $(MAKE)
@end example

The value of this variable is the file name with which @code{make} was
invoked.  If this file name was @file{/bin/make}, then the command executed
is @samp{cd subdir; /bin/make}.  If you use a special version of
@code{make} to run the top-level makefile, the same special version will be
executed for recursive invocations.

Also, any arguments that define variable values are added to @code{MAKE},
so the sub-@code{make} gets them too.  Thus, if you do @samp{make
CFLAGS=-O}, so that all C compilations will be optimized, the
sub-@code{make} is run with @samp{cd subdir; /bin/make CFLAGS=-O}.@refill

As a special feature, using the variable @code{MAKE} in the commands of a
rule alters the effects of the @samp{-t}, @samp{-n} or @samp{-q} option.
(@xref{Instead of Execution}.)@refill

Consider the command @samp{make -t} in the above example.  Following the
usual definition of @samp{-t}, this would create a file named
@file{subsystem} and do nothing else.  What you really want it to do is run
@samp{cd subdir; make -t}; but that would require executing the command,
and @samp{-t} says not to execute commands.@refill

The special feature makes this do what you want: whenever a rule's commands
use the variable @code{MAKE}, the flags @samp{-t}, @samp{-n} or @samp{-q}
do not apply to that rule.  The commands of that rule are executed normally
despite the presence of a flag that causes most commands not to be run.
The usual @code{MAKEFLAGS} mechanism passes the flags to the
sub-@code{make} (@pxref{Options/Recursion}), so your request to touch the
files, or print the commands, is propagated to the subsystem.@refill

@node Variables/Recursion, Options/Recursion, MAKE Variable, Recursion
@subsection Communicating Variables to a Sub-@code{make}
@cindex environment and recursion

Most variable values of the top-level @code{make} are passed to the
sub-@code{make} through the environment.  These variables are defined in
the sub-@code{make} as defaults, but do not override what is specified
in the sub-@code{make}'s makefile.

Variables are passed down if their names consist only of letters,
numbers and underscores.  Some shells cannot cope with environment
variable names consisting of characters other than letters, numbers,
and underscores.

Variable are @emph{not} passed down if they were created by default by
@code{make} (@pxref{Implicit Variables}).  The sub-@code{make} will
define these for itself.@refill

The way this works is that @code{make} adds each variable and its value
to the environment for running each command.  The sub-@code{make}, in
turn, uses the environment to initialize its table of variable values.
@xref{Environment}.

@vindex MAKELEVEL
As a special feature, the variable @code{MAKELEVEL} is changed when it is
passed down from level to level.  This variable's value is a string which
is the depth of the level as a decimal number.  The value is @samp{0} for
the top-level @code{make}; @samp{1} for a sub-@code{make}, @samp{2} for a
sub-sub-@code{make}, and so on.  The incrementation happens when
@code{make} sets up the environment for a command.@refill

The main use of @code{MAKELEVEL} is to test it in a conditional directive
(@pxref{Conditionals}); this way you can write a makefile that behaves one
way if run recursively and another way if run directly by you.

@vindex MAKEFILES
You can use the variable @code{MAKEFILES} to cause all sub-@code{make}
commands to use additional makefiles.  The value of @code{MAKEFILES} is a
whitespace-separated list of filenames.  This variable, if defined in the
outer-level makefile, is passed down through the environment as usual; then
it serves as a list of extra makefiles for the sub-@code{make} to read
before the usual or specified ones.  @xref{MAKEFILES Variable}.

@node Options/Recursion, -w Option, Variables/Recursion, Recursion
@subsection Communicating Options to a Sub-@code{make}
@cindex options and recursion

@vindex MAKEFLAGS
Flags such as @samp{-s} and @samp{-k} are passed automatically to the
sub-@code{make} through the variable @code{MAKEFLAGS}.  This variable is
set up automatically by @code{make} to contain the flag letters that
@code{make} received.  Thus, if you do @samp{make -ks} then
@code{MAKEFLAGS} gets the value @samp{ks}.@refill

As a consequence, every sub-@code{make} gets a value for @code{MAKEFLAGS}
in its environment.  In response, it takes the flags from that value and
processes them as if they had been given as arguments.  @xref{Options}.

The options @samp{-C}, @samp{-f}, @samp{-I}, @samp{-o}, and @samp{-W}
are not put into @code{MAKEFLAGS}; these options are not passed down.@refill

The @samp{-j} (@pxref{Parallel}) option is a special case.  If you set
it to some numeric value, @samp{-j 1} is always put into
@code{MAKEFLAGS} instead of the value you specified.  This is because if
the @samp{-j} option were passed down to sub-@code{make}s, you would get
many more jobs running in parallel than you asked for.  If you give
@samp{-j} with no numeric argument, meaning to run as many jobs as
possible in parallel, this is passed down, since multiple infinities are
no more than one.@refill

If you don't want to pass the other flags down, you must change the
value of @code{MAKEFLAGS}, like this:

@example
MAKEFLAGS=
subsystem:
        cd subdir; $(MAKE)
@end example

or like this:

@example
subsystem:
        cd subdir; $(MAKE) MAKEFLAGS=
@end example

@vindex MFLAGS
A similar variable @code{MFLAGS} exists also, for historical compatibility.
It has the same value as @code{MAKEFLAGS} except that a hyphen is added at
the beginning if it is not empty.  @code{MFLAGS} was traditionally used
explicitly in the recursive @code{make} command, like this:

@example
subsystem:
        cd subdir; $(MAKE) $(MFLAGS)
@end example

@noindent
but now @code{MAKEFLAGS} makes this usage redundant.

@cindex setting options from the environment
@cindex options, setting from the environment
@cindex setting options in makefiles
@cindex options, setting in makefiles
The @code{MAKEFLAGS} and @code{MFLAGS} variables can also be useful if you
want to have certain options, such as @samp{-k} (@pxref{Options}) set each
time you run @code{make}.  Just put @samp{MAKEFLAGS=k} or @samp{MFLAGS=-k}
in your environment.  These variables may also be set in makefiles, so a
makefile can specify additional flags that should also be in effect for
that makefile.@refill

If you do put @code{MAKEFLAGS} or @code{MFLAGS} in your environment, you
should be sure not to include any options that will drastically affect
the actions of @code{make} and undermine the purpose of makefiles and of
@code{make} itself.  For instance, the @samp{-t}, @samp{-n}, and
@samp{-q} options, if put in one of these variables, could have
disastrous consequences and would certainly have at least surprising and
probably annoying effects.@refill

@node -w Option,  , Options/Recursion, Recursion
@subsection The @samp{-w} Option

If you use several levels of recursive @code{make} invocations, the
@samp{-w} option can make the output a lot easier to understand by showing
each directory as @code{make} starts processing it and as @code{make}
finishes processing it.  For example, if @samp{make -w} is run in the
directory @file{/u/gnu/make}, @code{make} will print a line of the
form:@refill

@example
make: Entering directory `/u/gnu/make'.
@end example

@noindent
before doing anything else, and a line of the form:

@example
make: Leaving directory `/u/gnu/make'.
@end example

@noindent
when processing is completed.

@node Sequences, Empty Commands, Recursion, Commands
@section Defining Canned Command Sequences
@cindex sequences of commands

When the same sequence of commands is useful in making various targets, you
can define it as a canned sequence with the @code{define} directive, and
refer to the canned sequence from the rules for those targets.  The canned
sequence is actually a variable, so the name must not conflict with other
variable names.

Here is an example of defining a canned sequence of commands:

@example
define run-yacc
yacc $(firstword $^)
mv y.tab.c $@@
endef
@end example

@noindent
Here @code{run-yacc} is the name of the variable being defined;
@code{endef} marks the end of the definition; the lines in between are the
commands.  The @code{define} directive does not expand variable references
and function calls in the canned sequence; the @samp{$} characters,
parentheses, variable names, and so on, all become part of the value of the
variable you are defining.  @xref{Defining}, for a complete explanation of
@code{define}.

The first command in this example runs Yacc on the first dependency (of
whichever rule uses the canned sequence).  The output file from Yacc is
always named @file{y.tab.c}.  The second command moves the output to the
rule's target file name.

To use the canned sequence, substitute the variable into the commands of a
rule.  You can substitute it like any other variable (@pxref{Reference}).
Because variables defined by @code{define} are recursively expanded
variables, all the variable references you wrote inside the @code{define}
are expanded now.  For example:

@example
foo.c : foo.y
        $(run-yacc)
@end example

@noindent
@samp{foo.y} will substituted for the variable @samp{$^} when it occurs in
@code{run-yacc}'s value, and @samp{foo.c} for @samp{$@@}.@refill

This is a realistic example, but this particular one is not needed in
practice because @code{make} has an implicit rule to figure out these
commands based on the file names involved.  @xref{Implicit}.

@node Empty Commands,  , Sequences, Commands
@section Defining Empty Commands
@cindex empty commands

It is sometimes useful to define commands which do nothing.  This is done
simply by giving a command that consists of nothing but whitespace.  For
example:

@example
target:;
@end example

@noindent
defines an empty command string for @file{target}.  You could also use a
line beginning with a tab character to define an empty command string,
but this would be confusing because such a line looks empty.

You may be wondering why you would want to define a command string that
does nothing.  The only reason this is useful is to prevent a target
from getting implicit commands (from implicit rules or the
@code{.DEFAULT} special target; @pxref{Implicit} and @pxref{Last Resort}).

You may be inclined to define empty command strings for targets that are
not actual files, but only exist so that their dependencies can be
remade.  However, this is not the best way to do that, because if the
target file actually does exist, its dependencies may not be remade.
@xref{Phony Targets}, for a better way to do this.

@node Variables, Conditionals, Commands, Top
@chapter How to Use Variables
@cindex variable
@cindex value
@cindex recursive variable expansion
@cindex simple variable expansion

A @dfn{variable} is a name defined within @code{make} to represent a string
of text, called the variable's @dfn{value}.  These values can be
substituted by explicit request into targets, dependencies, commands and
other parts of the makefile.

Variables can represent lists of file names, options to pass to compilers,
programs to run, directories to look in for source files, directories to
write output in, or anything else you can imagine.

A variable name may be any sequence characters not containing @samp{:},
@samp{#}, @samp{=}, or leading or trailing whitespace.  However,
variable names containing characters other than letters, numbers and
underscores should be avoided, as they may be given special meanings in the
future, and they are not passed through the environment to a
sub-@code{make} (@pxref{Variables/Recursion}).

It is traditional to use upper case letters in variable names, but we
recommend using lower case letters for variable names that serve internal
purposes in the makefile, and reserving upper case for parameters that
control implicit rules or for parameters that the user should override with
command options (@pxref{Overriding}).

@menu
* 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.
* Override Directive:: Setting 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.
@end menu

@node Reference, Flavors, Variables, Variables
@section Basics of Variable References

To substitute a variable's value, write a dollar sign followed by the name
of the variable in parentheses or braces: either @samp{$(foo)} or
@samp{$@{foo@}} is a valid reference to the variable @code{foo}.  This
special significance of @samp{$} is why you must write @samp{$$} to have
the effect of a single dollar sign in a file name or command.

Variable references can be used in any context: targets, dependencies,
commands, most directives, and new variable values.  Here is a common kind
of example, where a variable holds the names of all the object files in a
program:

@example
objects = program.o foo.o utils.o
program : $(objects)
        cc -o program $(objects)

$(objects) : defs.h
@end example

Variable references work by strict textual substitution.  Thus, the rule

@example
foo = c
prog.o : prog.c
        $(foo)$(foo) prog.c
@end example

@noindent
could be used to compile a C program @file{prog.c}.  Since spaces around
the variable value are ignored in variable assignments, the value of
@code{foo} is precisely @samp{c}.  (Don't actually write your makefiles
this way!)

A dollar sign followed by a character other than a dollar sign,
open-parenthesis or open-brace treats that single character as the
variable name.  Thus, you could reference the variable @code{x} with
@samp{$x}.  However, this practice is strongly discouraged, except in
the case of the automatic variables (@pxref{Automatic}).

@node Flavors, Advanced, Reference, Variables
@section The Two Flavors of Variables
@cindex flavors (of variables)
@cindex recursive variable expansion

There are two ways that a variables in GNU @code{make} can have a value;
we call them two @dfn{flavors} of variables.  The two flavors are
distinguished in how they are defined and in what they do when expanded.

The first flavor of variable is a @dfn{recursively expanded} variable.
Variables of this sort are defined by lines using @samp{=}
(@pxref{Setting}).  The value you specify is installed verbatim; if it
contains references to other variables, these references are expanded
whenever this variable is substituted (in the course of expanding some
other string).  When this happens, it is called @dfn{recursive
expansion}.

For example,

@example
foo = $(bar)
bar = $(ugh)
ugh = Huh?

all:;echo $(foo)
@end example

@noindent
will echo @samp{Huh?}: @samp{$(foo)} expands to @samp{$(bar)} which
expands to @samp{$(ugh)} which finally expands to @samp{Huh?}.@refill

This flavor of variable is the only sort supported by other versions of
@code{make}.  It has its advantages and its disadvantages.  An advantage
(most would say) is that:

@example
CFLAGS = $(include_dirs) -O
include_dirs = -Ifoo -Ibar
@end example

@noindent
will do what was intended: when @samp{CFLAGS} is expanded in a command,
it will expand to @samp{-Ifoo -Ibar -O}.  A major disadvantage is that you
can't append something on the end of a variable, as in

@example
CFLAGS = $(CFLAGS) -O
@end example

@noindent
because it will cause an infinite loop in the variable expansion.
(Actually @code{make} detects the infinite loop and reports an error.)

Another disadvantage is that any functions (@pxref{Functions})
referenced in the definition will be executed every time the variable is
expanded.  This makes @code{make} run slower; worse, it causes the
@code{wildcard} and @code{shell} functions to give unpredictable results
because you cannot easily control when they are called, or even how many
times.

To avoid all the problems and inconveniences of recursively expanded
variables, there is another flavor: @dfn{simply expanded} variables.

Simply expanded variables are defined by lines using @samp{:=}
(@pxref{Setting}).  The value of a simply expanded variable is scanned
once and for all, expanding any references to other variables and
functions, when the variable is defined.  The actual value of the simply
expanded variable is the result of expanding the text that you write.
It does not contain any references to other variables; it contains their
values @emph{as of the time this variable was defined}.  Therefore,

@example
x := foo
y := $(x) bar
x := later
@end example

@noindent
is equivalent to

@example
y := foo bar
x := later
@end example

When a simply expanded variable is referenced, its value is substituted
verbatim.

Simply expanded variables generally make complicated makefile programming
more predictable because they work like variables in most programming
languages.  They allow you to redefine a variable using its own value (or
its value processed in some way by one of the expansion functions) and to
use the expansion functions much more efficiently (@pxref{Functions}).

You can also use them to introduce controlled leading or trailing spaces
into variable values.  Such spaces are discarded from your input before
substitution of variable references and function calls; this means you can
include leading or trailing spaces in a variable value by protecting them
with variable references, like this:

@example
nullstring :=
space := $(nullstring) $(nullstring)
@end example

@noindent
Here the value of the variable @code{space} is precisely one space.

@node Advanced, Values, Flavors, Variables
@section Advanced Features for Reference to Variables
@cindex reference to variables

This section describes some advanced features you can use to reference
variables in more flexible ways.

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

@node Substitution Refs, Computed Names, Advanced, Advanced
@subsection Substitution References
@cindex modified variable reference
@cindex substitution variable reference

A @dfn{substitution reference} substitutes the value of a variable with
alterations that you specify.  It has the form
@samp{$(@var{var}:@var{a}=@var{b})} (or
@samp{$@{@var{var}:@var{a}=@var{b}@}}) and its meaning is to take the value
of the variable @var{var}, replace every @var{a} at the end of a word with
@var{b} in that value, and substitute the resulting string.

When we say ``at the end of a word'', we mean that @var{a} must appear
either followed by whitespace or at the end of the value in order to be
replaced; other occurrences of @var{a} in the value are unaltered.  For
example:@refill

@example
foo := a.o b.o c.o
bar := $(foo:.o=.c)
@end example

@noindent
sets @samp{bar} to @samp{a.c b.c c.c}.  @xref{Setting}.

A substitution reference is actually an abbreviation for use of the
@code{patsubst} expansion function (@pxref{Text Functions}).  We provide
substitution references as well as @code{patsubst} for compatibility with
other implementations of @code{make}.

Another type of substitution reference lets you use the full power of the
@code{patsubst} function.  It has the same form
@samp{$(@var{var}:@var{a}=@var{b})} described above, except that now
@var{a} must contain a single @samp{%} character.  This case is equivalent
to @samp{$(patsubst @var{a},@var{b},$(@var{var}))}.
@xref{Text Functions}, for a description of the @code{patsubst} function.
For example:@refill

@example
foo := a.o b.o c.o
bar := $(foo:%.o=%.c)
@end example

@noindent
sets @samp{bar} to @samp{a.c b.c c.c}.

@node Computed Names,  , Substitution Refs, Advanced
@subsection Computed Variable Names
@cindex nested variable reference
@cindex computed variable name
@cindex variable reference, nested

Computed variable names are a complicated concept needed only for
sophisticated makefile programming.  For most purposes you need not
consider about them, except to know that making a variable with a dollar
sign in its name might have strange results.  However, if you are the
type that wants to understand everything, or you are actually interested
in what they do, read on.

Variables may be referenced inside the name of a variable.  This is
called a @dfn{computed variable name} or a @dfn{nested variable
reference}.  For example,

@example
x = y
y = z
a := $($(x))
@end example

@noindent
defines @code{a} as @samp{z}: the @samp{$(x)} inside @samp{$($(x))} expands
to @samp{y}, so @samp{$($(x))} expands to @samp{$(y)} which in turn expands
to @samp{z}.  Here the name of the variable to reference is not stated
explicitly; it is computed by expansion of @samp{$(x)}.  The reference
@samp{$(x)} here is nested within the outer variable reference.

The previous example shows two levels of nesting, but any number of levels
is possible.  For example, here are three levels:

@example
x = y
y = z
z = u
a := $($($(x)))
@end example

@noindent
Here the innermost @samp{$(x)} expands to @samp{y}, so @samp{$($(x))}
expands to @samp{$(y)} which in turn expands to @samp{z}; now we have
@samp{$(z)}, which becomes @samp{u}.

References to recursively-expanded variables within a variable name are
reexpanded in the usual fashion.  For example:

@example
x = $(y)
y = z
z = Hello
a := $($(x))
@end example

@noindent
defines @code{a} as @samp{Hello}: @samp{$($(x))} becomes @samp{$($(y))}
which becomes @samp{$(z)} which becomes @samp{Hello}.

Nested variable references can also contain modified references and
function invocations (@pxref{Functions}), just like any other reference.
For example, using the @code{subst} function (@pxref{Text Functions}):

@example
x = variable1
variable2 := Hello
y = $(subst 1,2,$(x))
z = y
a := $($($(z)))
@end example

@noindent
eventually defines @code{a} as @samp{Hello}.  It is doubtful that anyone
would ever want to write a nested reference as convoluted as this one, but
it works: @samp{$($($(z)))} expands to @samp{$($(y))} which becomes
@samp{$($(subst 1,2,$(x)))}.  This gets the value @samp{variable1} from
@code{x} and changes it by substitution to @samp{variable2}, so that the
entire string becomes @samp{$(variable2)}, a simple variable reference
whose value is @samp{Hello}.@refill

A computed variable name need not consist entirely of a single variable
reference.  It can contain several variable references, as well as some
invariant text.  For example,

@example
a_dirs := dira dirb
1_dirs := dir1 dir2

a_files := filea fileb
1_files := file1 file2

ifeq "$(use_a)" "yes"
a1 := a
else
a1 := 1
endif

ifeq "$(use_dirs)" "yes"
df := dirs
else
df := files
endif

dirs := $($(a1)_$(df))
@end example

@noindent
will give @code{dirs} the same value as @code{a_dirs}, @code{1_dirs},
@code{a_files} or @code{1_files} depending on the settings of @code{use_a}
and @code{use_dirs}.@refill

Computed variable names can also be used in substitution references:

@example
a_objects := a.o b.o c.o
1_objects := 1.o 2.o 3.o

sources := $($(a1)_object:.o=.c)
@end example

@noindent
defines @code{sources} as either @samp{a.c b.c c.c} or @samp{1.c 2.c 3.c},
depending on the value of @code{a1}.

The only restriction on this sort of use of nested variable references
is that they cannot specify part of the name of a function to be called.
This is because the test for a recognized function name is done before
the expansion of nested references.  For example,

@example
ifdef do_sort
func := sort
else
func := strip
endif

bar := a d b g q c

foo := $($(func) $(bar))
@end example

@noindent
attempts to give @samp{foo} the value of the variable @samp{sort a d b g
q c} or @samp{strip a d b g q c}, rather than giving @samp{a d b g q c}
as the argument to either the @code{sort} or the @code{strip} function.
This restriction could be removed in the future if that change is shown
to be a good idea.

Note that @dfn{nested variable references} are quite different from
@dfn{recursively expanded variables} (@pxref{Flavors}), though both are
used together in complex ways when doing makefile programming.@refill

@node Values, Setting, Advanced, Variables
@section How Variables Get Their Values

Variables can get values in several different ways:

@itemize @bullet
@item
You can specify an overriding value when you run @code{make}.
@xref{Overriding}.

@item
You can specify a value in the makefile, either
with an assignment (@pxref{Setting}) or with a
verbatim definition (@pxref{Defining}).@refill

@item
Values are inherited from the environment.  @xref{Environment}.

@item
Several @dfn{automatic} variables are given new values for each rule.
Each of these has a single conventional use.  @xref{Automatic}.

@item
Several variables have constant initial values.
@xref{Implicit Variables}.
@end itemize

@node Setting, Override Directive, Values, Variables
@section Setting Variables
@cindex setting variables
@cindex =
@cindex :=

To set a variable from the makefile, write a line starting with the
variable name followed by @samp{=} or @samp{:=}.  Whatever follows the
@samp{=} or @samp{:=} on the line becomes the value.  For example,

@example
objects = main.o foo.o bar.o utils.o
@end example

@noindent
defines a variable named @code{objects}.  Whitespace around the variable
name and immediately after the @samp{=} is ignored.

Variables defined with @samp{=} are @dfn{recursively expanded} variables.
Variables defined with @samp{:=} are @dfn{simply expanded} variables; these
definitions can contain variable references which will be expanded before
the definition is made.  @xref{Flavors}.

There is no limit on the length of the value of a variable except the
amount of swapping space on the computer.  When a variable definition is
long, it is a good idea to break it into several lines by inserting
backslash-newline at convenient places in the definition.  This will not
affect the functioning of @code{make}, but it will make the makefile easier
to read.

Most variable names are considered to have the empty string as a value if
you have never set them.  Several variables have built-in initial values
that are not empty, but can be set by you in the usual ways
(@pxref{Implicit Variables}).  Several special variables are set
automatically to a new value for each rule; these are called the
@dfn{automatic} variables (@pxref{Automatic}).

@node Override Directive, Defining, Setting, Variables
@section The @code{override} Directive
@findex override
@cindex overriding with @code{override}

If a variable has been set with a command argument (@pxref{Overriding}),
then ordinary assignments in the makefile are ignored.  If you want to set
the variable in the makefile even though it was set with a command
argument, you can use an @code{override} directive, which is a line that
looks like this:@refill

@example
override @var{variable} = @var{value}
@end example

or

@example
override @var{variable} := @var{value}
@end example

The @code{override} directive was not invented for escalation in the war
between makefiles and command arguments.  It was invented so you can alter
and add to values that the user specifies with command arguments.

For example, suppose you always want the @samp{-g} switch when you run the
C compiler, but you would like to allow the user to specify the other
switches with a command argument just as usual.  You could use this
@code{override} directive:

@example
override CFLAGS := $(CFLAGS) -g
@end example

You can also use @code{override} directives with @code{define} directives.
This is done as you might expect:

@example
override define foo
bar
endef
@end example

@noindent
@iftex
See the next section.
@end iftex
@ifinfo
@xref{Defining}.
@end ifinfo

@node Defining, Environment, Override Directive, Variables
@section Defining Variables Verbatim
@findex define
@findex endef

Another way to set the value of a variable is to use the @code{define}
directive.  This directive has a different syntax which allows newline
characters to be included in the value, which is convenient for defining
canned sequences of commands (@pxref{Sequences}).

The @code{define} directive is followed on the same line by the name of the
variable and nothing more.  The value to give the variable appears on the
following lines.  The end of the value is marked by a line containing just
the word @code{endef}.  Aside from this difference in syntax, @code{define}
works just like @samp{=}; it creates a recursively-expanded variable
(@pxref{Flavors}).

@example
define two-lines
echo foo
echo $(bar)
endef
@end example

The value in an ordinary assignment cannot contain a newline; but the
newlines that separate the lines of the value in a @code{define} become
part of the variable's value (except for the final newline which precedes
the @code{endef} and is not considered part of the value).@refill

The previous example is functionally equivalent to this:

@example
two-lines = echo foo; echo $(bar)
@end example

@noindent
since the shell will interpret the semicolon and the newline identically.

If you want variable definitions made with @code{define} to take precedence
over command-line variable definitions, the @code{override} directive can
be used together with @code{define}:

@example
override define two-lines
foo
$(bar)
endef
@end example

@noindent
@xref{Override Directive}.

@node Environment,  , Defining, Variables
@section Variables from the Environment

@cindex environment
Variables in @code{make} can come from the environment with which
@code{make} is run.  Every environment variable that @code{make} sees when
it starts up is transformed into a @code{make} variable with the same name
and value.  But an explicit assignment in the makefile, or with a command
argument, overrides the environment.  (If the @samp{-e} flag is specified,
then values from the environment override assignments in the makefile.
@xref{Options}.  But this is not recommended practice.)

Thus, by setting the variable @code{CFLAGS} in your environment, you can
cause all C compilations in most makefiles to use the compiler switches you
prefer.  This is safe for variables with standard or conventional meanings
because you know that no makefile will use them for other things.  (But
this is not totally reliable; some makefiles set @code{CFLAGS} explicitly
and therefore are not affected by the value in the environment.)

When @code{make} is invoked recursively, variables defined in the outer
invocation are automatically passed to inner invocations through the
environment (@pxref{Recursion}).  This is the main purpose of turning
environment variables into @code{make} variables, and it requires no
attention from you.@refill

Other use of variables from the environment is not recommended.  It is not
wise for makefiles to depend for their functioning on environment variables
set up outside their control, since this would cause different users to get
different results from the same makefile.  This is against the whole
purpose of most makefiles.

Such problems would be especially likely with the variable @code{SHELL},
which is normally present in the environment to specify the user's choice
of interactive shell.  It would be very undesirable for this choice to
affect @code{make}.  So @code{make} ignores the environment value of
@code{SHELL} if the value of @code{MAKELEVEL} is zero (which is normally
true except in recursive invocations of @code{make}).@refill

@node Conditionals, Functions, Variables, Top
@chapter Conditional Parts of Makefiles

@cindex conditionals
A @dfn{conditional} causes part of a makefile to be obeyed or ignored
depending on the values of variables.  Conditionals can compare the value
of one variable with another, or the value of a variable with a constant
string.  Conditionals control what @code{make} actually ``sees'' in the
makefile, so they @emph{cannot} be used to control shell commands at the
time of execution.@refill

@menu
* Example: Conditional Example.   An annotated example.
* Syntax: Conditional Syntax.     Precise rules for syntax of conditionals.
* Flags: Testing Flags.           Conditionals testing flags such as @samp{-t}.
@end menu

@node Conditional Example, Conditional Syntax, Conditionals, Conditionals
@section Example of a Conditional

This conditional tells @code{make} to use one set of libraries if the
@code{CC} variable is @samp{gcc}, and a different set of libraries
otherwise.  It works by controlling which of two command lines will be used
as the command for a rule.  The result is that @samp{CC=gcc} as an argument
to @code{make} changes not only which compiler is used but also which
libraries are linked.

@example
libs_for_gcc = -lgnu
normal_libs =

foo: $(objects)
ifeq ($(CC),gcc)
        $(CC) -o foo $(objects) $(libs_for_gcc)
else
        $(CC) -o foo $(objects) $(normal_libs)
endif
@end example

This conditional uses three directives: one @code{ifeq}, one @code{else}
and one @code{endif}.

The @code{ifeq} directive begins the conditional, and specifies the
condition.  It contains two arguments, separated by a comma and surrounded
by parentheses.  Variable substitution is performed on both arguments and
then they are compared.  The lines of the makefile following the
@code{ifeq} are obeyed if the two arguments match; otherwise they are
ignored.

The @code{else} directive causes the following lines to be obeyed if the
previous conditional failed.  In the example above, this means that the
second alternative linking command is used whenever the first alternative
is not used.  It is optional to have an @code{else} in a conditional.

The @code{endif} directive ends the conditional.  Every conditional must
end with an @code{endif}.  Unconditional makefile text follows.

Conditionals work at the textual level: the lines of the conditional are
treated as part of the makefile, or ignored, according to the condition.
This is why the larger syntactic units of the makefile, such as rules, may
cross the beginning or the end of the conditional.

When the variable @code{CC} has the value @samp{gcc}, the above example has
this effect:

@example
foo: $(objects)
        $(CC) -o foo $(objects) $(libs_for_gcc)
@end example

@noindent
When the variable @code{CC} has any other value, the effect is this:

@example
foo: $(objects)
        $(CC) -o foo $(objects) $(normal_libs)
@end example

Equivalent results can be obtained in another way by conditionalizing a
variable assignment and then using the variable unconditionally:

@example
libs_for_gcc = -lgnu
normal_libs =

ifeq ($(CC),gcc)
  libs=$(libs_for_gcc)
else
  libs=$(normal_libs)
endif

foo: $(objects)
        $(CC) -o foo $(objects) $(libs)
@end example

@node Conditional Syntax, Testing Flags, Conditional Example, Conditionals
@section Syntax of Conditionals
@findex ifdef
@findex ifeq
@findex ifndef
@findex ifneq
@findex else
@findex endif

The syntax of a simple conditional with no @code{else} is as follows:

@example
@var{conditional-directive}
@var{text-if-true}
endif
@end example

@noindent
The @var{text-if-true} may be any lines of text, to be considered as part
of the makefile if the condition is true.  If the condition is false, no
text is used instead.

The syntax of a complex conditional is as follows:

@example
@var{conditional-directive}
@var{text-if-true}
else
@var{text-if-false}
endif
@end example

@noindent
If the condition is true, @var{text-if-true} is used; otherwise,
@var{text-if-false} is used instead.  The @var{text-if-false} can be any
number of lines of text.

The syntax of the @var{conditional-directive} is the same whether the
conditional is simple or complex.  There are four different directives that
test different conditions.  Here is a table of them:

@table @code
@item ifeq (@var{arg1}, @var{arg2})
@itemx ifeq '@var{arg1}' '@var{arg2}'
@itemx ifeq "@var{arg1}" "@var{arg2}"
@itemx ifeq "@var{arg1}" '@var{arg2}'
@itemx ifeq '@var{arg1}' "@var{arg2}"
Expand all variable references in @var{arg1} and @var{arg2} and
compare them.  If they are identical, the @var{text-if-true} is
effective; otherwise, the @var{text-if-false}, if any, is effective.

@item ifneq (@var{arg1}, @var{arg2})
@itemx ifneq '@var{arg1}' '@var{arg2}'
@itemx ifneq "@var{arg1}" "@var{arg2}"
@itemx ifneq "@var{arg1}" '@var{arg2}'
@itemx ifneq '@var{arg1}' "@var{arg2}"
Expand all variable references in @var{arg1} and @var{arg2} and
compare them.  If they are different, the @var{text-if-true} is
effective; otherwise, the @var{text-if-false}, if any, is effective.

@item ifdef @var{variable-name}
If the variable @var{variable-name} has a non-empty value, the
@var{text-if-true} is effective; otherwise, the @var{text-if-false},
if any, is effective.  Variables that have never been defined have an
empty value.

@item ifndef @var{variable-name}
If the variable @var{variable-name} has an empty value, the
@var{text-if-true} is effective; otherwise, the @var{text-if-false},
if any, is effective.
@end table

Extra spaces are allowed and ignored at the beginning of the conditional
directive line, but a tab is not allowed.  (If the line begins with a tab,
it will be considered a command for a rule.)  Aside from this, extra spaces
or tabs may be inserted with no effect anywhere except within the directive
name or within an argument.  A comment starting with @samp{#} may appear at
the end of the line.

The other two directives that play a part in a conditional are @code{else}
and @code{endif}.  Each of these directives is written as one word, with no
arguments.  Extra spaces are allowed and ignored at the beginning of the
line, and spaces or tabs at the end.  A comment starting with @samp{#} may
appear at the end of the line.

Conditionals work at the textual level.  The lines of the
@var{text-if-true} are read as part of the makefile if the condition is
true; if the condition is false, those lines are ignored completely.  It
follows that syntactic units of the makefile, such as rules, may safely be
split across the beginning or the end of the conditional.@refill

To prevent intolerable confusion, it is not permitted to start a
conditional in one makefile and end it in another.  However, you may
write an @code{include} directive within a conditional, provided you do
not attempt to terminate the conditional inside the included file.

@node Testing Flags,  , Conditional Syntax, Conditionals
@section Conditionals that Test Flags

You can write a conditional that tests @code{make} command flags such as
@samp{-t} by using the variable @code{MAKEFLAGS} together with the
@code{findstring} function.  This is useful when @code{touch} is not
enough to make a file appear up to date.

The @code{findstring} function determines whether one string appears as a
substring of another.  If you want to test for the @samp{-t} flag,
use @samp{t} as the first string and the value of @code{MAKEFLAGS} as
the other.

For example, here is how to arrange to use @samp{ranlib -t} to finish
marking an archive file up to date:

@example
archive.a: @dots{}
ifneq (,$(findstring t,$(MAKEFLAGS)))
        @@echo $(MAKE) > /dev/null
        touch archive.a
        ranlib -t archive.a
else
        ranlib archive.a
endif
@end example

@noindent
The @code{echo} command does nothing when executed; but its presence, with
a reference to the variable @code{MAKE}, marks the rule as ``recursive'' so
that its commands will be executed despite use of the @samp{-t} flag.
@xref{Recursion}.

@node Functions, Running, Conditionals, Top
@chapter Functions for Transforming Text
@cindex function

@dfn{Functions} allow you to do text processing in the makefile to compute
the files to operate on or the commands to use.  You use a function in a
@dfn{function call}, where you give the name of the function and some text
(the @dfn{arguments}) for the function to operate on.  The result of the
function's processing is substituted into the makefile at the point of the
call, just as a variable might be substituted.

@menu
* Syntax of Functions:: How to write a function call.
* Text Functions::      General-purpose text manipulation functions.
* Filename Functions::  Functions for manipulating file names.
* Foreach Function::    Repeat some text with controlled variation.
* Origin Function::     Find where a variable got its value.
* Shell Function::      Substitute the output of a shell command.
@end menu

@node Syntax of Functions, Text Functions, Functions, Functions
@section Function Call Syntax
@cindex $ (function call)
@cindex arguments

A function call resembles a variable reference.  It looks like this:

@example
$(@var{function} @var{arguments})
@end example

@noindent
or like this:

@example
$@{@var{function} @var{arguments}@}
@end example

Here @var{function} is a function name; one of a short list of names that
are part of @code{make}.  There is no provision for defining new functions.

The @var{arguments} are the arguments of the function.  They are separated
from the function name by one or more spaces and/or tabs, and if there is
more than one argument they are separated by commas.  Such whitespace and
commas are not part of any argument's value.  The delimiters which you use
to surround the function call, whether parentheses or braces, can appear in
an argument only in matching pairs; the other kind of delimiters may appear
singly.  If the arguments themselves contain other function calls or
variable references, it is wisest to use the same kind of delimiters for
all the references; in other words, write @w{@samp{$(subst a,b,$(x))}}, not
@w{@samp{$(subst a,b,$@{x@})}}.  This is both because it is clearer, and
because only one type of delimiters is matched to find the end of the
reference.  Thus in @w{@samp{$(subst a,b,$@{subst c,d,$@{x@}@})}}
doesn't work because the second @code{subst} function invocation ends at
the first @samp{@}}, not the second.

The text written for each argument is processed by substitution of
variables and function calls to produce the argument value, which
is the text on which the function acts.  The substitution is done in the
order in which the arguments appear.

Commas and unmatched parentheses or braces cannot appear in the text of an
argument as written; leading spaces cannot appear in the text of the first
argument as written.  These characters can be put into the argument value
by variable substitution.  First define variables @code{comma} and
@code{space} whose values are isolated comma and space characters, then
substitute those variables where such characters are wanted, like this:

@example
comma:= ,
space:= $(empty) $(empty)
foo:= a b c
bar:= $(subst $(space),$(comma),$(foo))
# @r{bar is now `a,b,c'.}
@end example

@noindent
Here the @code{subst} function replaces each space with a comma, through
the value of @code{foo}, and substitutes the result.

@node Text Functions, Filename Functions, Syntax of Functions, Functions
@section Functions for String Substitution and Analysis

Here are some functions that operate on strings:

@table @code
@item $(subst @var{from},@var{to},@var{text})
@findex subst
Performs a textual replacement on the text @var{text}: each occurrence
of @var{from} is replaced by @var{to}.  The result is substituted for
the function call.  For example,

@example
$(subst ee,EE,feet on the street)
@end example

substitutes the string @samp{fEEt on the strEEt}.

@item $(patsubst @var{pattern},@var{replacement},@var{text})
@findex patsubst
Finds whitespace-separated words in @var{text} that match
@var{pattern} and replaces them with @var{replacement}.  Here
@var{pattern} may contain a @samp{%} which acts as a wildcard,
matching any number of any characters within a word.  If
@var{replacement} also contains a @samp{%}, the @samp{%} is replaced
by the text that matched the @samp{%} in @var{pattern}.@refill

@samp{%} characters in @code{patsubst} function invocations can be
quoted with preceding backslashes (@samp{\}).  Backslashes that would
otherwise quote @samp{%} characters can be quoted with more backslashes.
Backslashes that quote @samp{%} characters or other backslashes are
removed from the pattern before it is compared file names or has a stem
substituted into it.  Backslashes that are not in danger of quoting
@samp{%} characters go unmolested.  For example, the pattern
@file{the\%weird\\%pattern\\} has @samp{the%weird\} preceding the
operative @samp{%} character, and @samp{pattern\\} following it.  The
final two backslashes are left alone because they can't affect any
@samp{%} character.@refill


Whitespace between words is folded into single space characters;
leading and trailing whitespace is discarded.

For example,

@example
$(patsubst %.c,%.o,x.c.c bar.c)
@end example

@noindent
produces the value @samp{x.c.o bar.o}.

@item $(strip @var{string})
@findex strip
Removes leading and trailing whitespace from @var{string} and replaces
each internal sequence of one or more whitespace characters with a
single space.  Thus, @samp{$(strip a b  c )} results in @samp{a b c}.

@item $(findstring @var{find},@var{in})
@findex findstring
Searches @var{in} for an occurrence of @var{find}.  If it occurs, the
value is @var{find}; otherwise, the value is empty.  You can use this
function in a conditional to test for the presence of a specific
substring in a given string.  Thus, the two examples,

@example
$(findstring a,a b c)
$(findstring a,b c)
@end example

@noindent
produce the values @samp{a} and @samp{}, respectively.  @xref{Testing
Flags}, for a practical application of @code{findstring}.@refill

@item $(filter @var{pattern},@var{text})
@findex filter
Removes all whitespace-separated words in @var{text} that do
@emph{not} match @var{pattern}, returning only matching words.  The
pattern is written using @samp{%}, just like the patterns used in
@code{patsubst} function above.@refill

The @code{filter} function can be used to separate out different types
of strings (such as filenames) in a variable.  For example:

@example
sources := foo.c bar.c ugh.h
foo: $(sources)
        cc $(filter %.c,$(sources)) -o foo
@end example

@noindent
says that @file{foo} depends of @file{foo.c}, @file{bar.c} and
@file{ugh.h} but only @file{foo.c} and @file{bar.c} should be specified
in the command to the compiler.@refill

@item $(filter-out @var{pattern},@var{text})
@findex filter-out
Removes all whitespace-separated words in @var{text} that @emph{do}
match @var{pattern}, returning only the words that match.  This is the
exact opposite of the @code{filter} function.@refill

@item $(sort @var{list})
@findex sort
Sorts the words of @var{list} in lexical order, removing duplicate
words.  The output is a list of words separated by single spaces.
Thus,

@example
$(sort foo bar lose)
@end example

@noindent
returns the value @samp{bar foo lose}.
@end table

Here is a realistic example of the use of @code{subst} and @code{patsubst}.
Suppose that a makefile uses the @code{VPATH} variable to specify a list of
directories that @code{make} should search for dependency files.  This
example shows how to tell the C compiler to search for header files in the
same list of directories.

The value of @code{VPATH} is a list of directories separated by colons,
such as @samp{src:../headers}.  First, the @code{subst} function is used to
change the colons to spaces:

@example
$(subst :, ,$(VPATH))
@end example

@noindent
This produces @samp{src ../headers}.  Then @code{patsubst} is used to turn
each directory name into a @samp{-I} flag.  These can be added to the
value of the variable @code{CFLAGS}, which is passed automatically to the C
compiler, like this:

@example
override CFLAGS:= $(CFLAGS) $(patsubst %,-I%,$(subst :, ,$(VPATH)))
@end example

@noindent
The effect is to append the text @samp{-Isrc -I../headers} to the
previously given value of @code{CFLAGS}.  The @code{override} directive is
used so that the new value is assigned even if the previous value of
@code{CFLAGS} was specified with a command argument (@pxref{Override
Directive}).

The function @code{strip} can be very useful when used in conjunction
with conditionals.  When comparing something with the null string
@samp{""} using @code{ifeq} or @code{ifneq}, you usually want a string
of just whitespace to match the null string.  Thus,

@example
.PHONY: all
ifneq   "$(needs_made)" ""
all: $(needs_made)
else
all:;@@echo 'Nothing to make!'
endif
@end example

@noindent
might fail to have the desired results.  Replacing the variable reference
@samp{"$(needs_made)"} with the function call @samp{"$(strip
$(needs_made))"} in the @code{ifneq} directive would make it more robust.

@node Filename Functions, Foreach Function, Text Functions, Functions
@section Functions for File Names

Several of the built-in expansion functions relate specifically to
taking apart file names or lists of file names.

Each of the following functions performs a specific transformation on a
file name.  The argument of the function is regarded as a series of file
names, separated by whitespace.  (Leading and trailing whitespace is
ignored.)  Each file name in the series is transformed in the same way and
the results are concatenated with single spaces between them.

@table @code
@item $(dir @var{names})
@findex dir
Extracts the directory-part of each file name in @var{names}.  The
directory-part of the file name is everything up through (and
including) the last slash in it.  If the file name contains no slash,
the directory part is the string @samp{./}.  For example,

@example
$(dir src/foo.c hacks)
@end example

@noindent
produces the result @samp{src/ ./}.

@item $(notdir @var{names})
@findex notdir
Extracts all but the directory-part of each file name in @var{names}.
If the file name contains no slash, it is left unchanged.  Otherwise,
everything through the last slash is removed from it.

A file name that ends with a slash becomes an empty string.  This is
unfortunate, because it means that the result does not always have the
same number of whitespace-separated file names as the argument had;
but we do not see any other valid alternative.

For example,

@example
$(notdir src/foo.c hacks)
@end example

@noindent
produces the result @samp{foo.c hacks}.

@item $(suffix @var{names})
@findex suffix
Extracts the suffix of each file name in @var{names}.  If the file name
contains a period, the suffix is everything starting with the last
period.  Otherwise, the suffix is the empty string.  This frequently
means that the result will be empty when @var{names} is not, and if
@var{names} contains multiple file names, the result may contain fewer
file names.

For example,

@example
$(suffix src/foo.c hacks)
@end example

@noindent
produces the result @samp{.c}.

@item $(basename @var{names})
@findex basename
Extracts all but the suffix of each file name in @var{names}.  If the
file name contains a period, the basename is everything starting up to
(and not including) the last period.  Otherwise, the basename is the
entire file name.  For example,

@example
$(basename src/foo.c hacks)
@end example

@noindent
produces the result @samp{src/foo hacks}.

@item $(addsuffix @var{suffix},@var{names})
@findex addsuffix
The argument @var{names} is regarded as a series of names, separated
by whitespace; @var{suffix} is used as a unit.  The value of
@var{suffix} is appended to the end of each individual name and the
resulting larger names are concatenated with single spaces between
them.  For example,

@example
$(addsuffix .c,foo bar)
@end example

@noindent
produces the result @samp{foo.c bar.c}.

@item $(addprefix @var{prefix},@var{names})
@findex addprefix
The argument @var{names} is regarded as a series of names, separated
by whitespace; @var{prefix} is used as a unit.  The value of
@var{prefix} is appended to the front of each individual name and the
resulting larger names are concatenated with single spaces between
them.  For example,

@example
$(addprefix src/,foo bar)
@end example

@noindent
produces the result @samp{src/foo src/bar}.

@item $(join @var{list1},@var{list2})
@findex join
Concatenates the two arguments word by word: the two first words (one
from each argument) concatenated form the first word of the result, the
two second words form the second word of the result, and so on.  So the
@var{n}th word of the result comes from the @var{n}th word of each
argument.  If one argument has more words that the other, the extra
words are copied unchanged into the result.

For example, @samp{$(join a b,.c .o)} produces @samp{a.c b.o}.

Whitespace between the words in the lists is not preserved; it is
replaced with a single space.

This function can merge the results of the @code{dir} and
@code{notdir} functions, to produce the original list of files which
was given to those two functions.@refill

@item $(word @var{n},@var{text})
@findex word
Returns the @var{n}th word of @var{text}.  The legitimate values of
@var{n} start from 1.  If @var{n} is bigger than the number of words
in @var{text}, the value is empty.  For example,

@example
$(word 2, foo bar baz)
@end example

@noindent
returns @samp{bar}.

@item $(words @var{text})
@findex words
Returns the number of words in @var{text}.  Thus, @code{$(word
$(words @var{text}),@var{text})} is the last word of @var{text}.@refill

@item $(firstword @var{names})
@findex firstword
The argument @var{names} is regarded as a series of names, separated
by whitespace.  The value is the first name in the series.  The rest
of the names are ignored.  For example,

@example
$(firstword foo bar)
@end example

@noindent
produces the result @samp{foo}.  Although @code{$(firstword
@var{text})} is the same as @code{$(word 1,@var{text})}, the
@code{firstword} function is retained for its simplicity.@refill

@item $(wildcard @var{pattern})
@findex wildcard
The argument @var{pattern} is a file name pattern, typically containing
wildcard characters.  The result of @code{wildcard} is a space-separated
list of the names of existing files that match the pattern.

Wildcards are expanded automatically in rules.  @xref{Wildcards}.
But they are not normally expanded when a variable is set, or inside the
arguments of other functions.  Those occasions are when the @code{wildcard}
function is useful.@refill
@end table

@node Foreach Function, Origin Function, Filename Functions, Functions
@section The @code{foreach} Function
@findex foreach

The @code{foreach} function is very different from other functions.  It
causes one piece of text to be used repeatedly, each time with a different
substitution performed on it.  It resembles the @code{for} command in the
shell @code{sh} and the @code{foreach} command in the C-shell @code{csh}.

The syntax of the @code{foreach} function is:

@example
$(foreach @var{var},@var{list},@var{text})
@end example

@noindent
The first two arguments, @var{var} and @var{list}, are expanded before
anything else is done; note that the last argument, @var{text}, is
@emph{not} expanded at the same time.  Then for each word of the expanded
value of @var{list}, the variable named by the expanded value of @var{var}
is set to that word, and @var{text} is expanded.  Presumably @var{text}
contains references to that variable, so its expansion will be different
each time.

The result is that @var{text} is expanded as many times as there are
whitespace-separated words in @var{list}.  The multiple expansions of
@var{text} are concatenated, with spaces between them, to make the result
of @code{foreach}.

This simple example sets the variable @samp{files} to the list of all files
in the directories in the list @samp{dirs}:

@example
dirs := a b c d
files := $(foreach dir,$(dirs),$(wildcard $(dir)/*))
@end example

Here @var{text} is @samp{$(wildcard $(dir)/*)}.  The first repetition
finds the value @samp{a} for @code{dir}, so it produces the same result
as @samp{$(wildcard a/*)}; the second repetition produces the result
of @samp{$(wildcard b/*)}; and the third, that of @samp{$(wildcard c/*)}.

This example has the same result (except for setting @samp{find_files},
@samp{dirs} and @samp{dir}) as the following example:

@example
files := $(wildcard a/* b/* c/* d/*)
@end example

When @var{text} is complicated, you can improve readability by giving it
a name, with an additional variable:

@example
find_files = $(wildcard $(dir)/*)
dirs := a b c d
files := $(foreach dir,$(dirs),$(find_files))
@end example

@noindent
Here we use the variable @code{find_files} this way.  We use plain @samp{=}
to define a recursively-expanding variable, so that its value contains an
actual function call to be reexpanded under the control of @code{foreach};
a simply-expanded variable would not do, since @code{wildcard} would be
called only once at the time of defining @code{find_files}.

The @code{foreach} function has no permanent effect on the variable
@var{var}; its value and flavor after the @code{foreach} function call are
the same as they were beforehand.  The other values which are taken from
@var{list} are in effect only temporarily, during the execution of
@code{foreach}.  The variable @var{var} is a simply-expanded variable
during the execution of @code{foreach}.  If @var{var} was undefined
before the @code{foreach} function call, it is undefined after the call.
@xref{Flavors}.@refill

You must take care when using complex variable expressions that result in
variable names because many strange things are valid variable names, but
are probably not what you intended.  For example,

@example
files := $(foreach Es escrito en espanol!,b c ch,$(find_files))
@end example

@noindent
might be useful if the value of @code{find_files} references the variable
whose name is @samp{Es escrito en espanol!} (es un nombre bastante largo,
que no?), but it is more likely to be a mistake.

@node Origin Function, Shell Function, Foreach Function, Functions
@section The @code{origin} Function
@findex origin

The @code{origin} function is unlike most other functions in that it does
not operate on the values of variables; it tells you something @emph{about}
a variable.  Specifically, it tells you where it came from.

The syntax of the @code{origin} function is:

@example
$(origin @var{variable})
@end example

Note that @var{variable} is the @emph{name} of a variable to inquire about;
not a @emph{reference} to that variable.  Therefore you would not normally
use a @samp{$} or parentheses when writing it.  (You can, however, use a
variable reference in the name if you want the name not to be a constant.)

The result of this function is a string telling you how the variable
@var{variable} was defined:

@table @samp
@item undefined
if @var{variable} was never defined.

@item default
if @var{variable} has a default definition, as is usual with @code{CC}
and so on.  @xref{Implicit Variables}.  Note that if you have
redefined a default variable, the @code{origin} function will return
the origin of the later definition.

@item environment
if @var{variable} was defined as an environment variable and the
@samp{-e} option is @emph{not} turned on (@pxref{Options}).

@item environment override
if @var{variable} was defined as an environment variable and the
@samp{-e} option @emph{is} turned on (@pxref{Options}).

@item file
if @var{variable} was defined in a makefile.

@item command line
if @var{variable} was defined on the command line.

@item override
if @var{variable} was defined with an @code{override} directive in a
makefile (@pxref{Override Directive}).

@item automatic
if @var{variable} is an automatic variable defined for the
execution of the commands for each rule.
@end table

This information is primarily useful (other than for your curiosity) to
determine if you want to believe the value of a variable.  For example,
suppose you have a makefile @file{foo} that includes another makefile
@file{bar}.  You want a variable @code{bletch} to be defined in @file{bar}
if you run the command @samp{make -f bar}, even if the environment contains
a definition of @code{bletch}.  However, if @file{foo} defined
@code{bletch} before including @file{bar}, you don't want to override that
definition.  This could be done by using an @code{override} directive in
@file{foo}, giving that definition precedence over the later definition in
@file{bar}; unfortunately, the @code{override} directive would also
override any command line definitions.  So, @file{bar} could
include:@refill

@example
ifdef bletch
ifeq "$(origin bletch)" "environment"
bletch = barf, gag, etc.
endif
endif
@end example

@noindent
If @code{bletch} has been defined from the environment, this will redefine
it.

If you want to override a previous definition of @code{bletch} if it came
from the environment, even under @samp{-e}, you could instead write:

@example
ifneq "$(findstring environment,$(origin bletch))" ""
bletch = barf, gag, etc.
endif
@end example

Here the redefinition takes place if @samp{$(origin bletch)} returns either
@samp{environment} or @samp{environment override}.

@node Shell Function,  , Origin Function, Functions
@section The @code{shell} Function
@findex shell
@cindex command expansion
@cindex backquotes

The @code{shell} function is unlike any other function except the
@code{wildcard} function (@pxref{Wildcard Function}) in that it
communicates with the world outside of @code{make}.

The @code{shell} function performs the same function that backquotes
(@samp{`}) perform in most shells: it does @dfn{command expansion}.  This
means that it takes an argument that is a shell command and returns the
output of the command.  The only processing @code{make} does on the result,
before substituting it into the surrounding text, is to convert newlines to
spaces.@refill

The commands run by calls to the @code{shell} function are run when the
function calls are expanded.  In most cases, this is when the makefile is
read in.  The exception is that function calls in the commands of the rules
are expanded when the commands are run, and this applies to @code{shell}
function calls like all others.

Here are some examples of the use of the @code{shell} function:

@example
contents := $(shell cat foo)
@end example

@noindent
sets @code{contents} to the contents of the file @file{foo}, with a space
(rather than a newline) separating each line.

@example
files := $(shell echo *.c)
@end example

@noindent
sets @code{files} to the expansion of @samp{*.c}.  Unless @code{make} is
using a very strange shell, this has the same result as @samp{$(wildcard
*.c)}.@refill

@node Running, Implicit, Functions, Top
@chapter How to Run @code{make}

A makefile that says how to recompile a program can be used in more than
one way.  The simplest use is to recompile every file that is out of date.
This is what @code{make} will do if run with no arguments.

But you might want to update only some of the files; you might want to use
a different compiler or different compiler options; you might want just to
find out which files are out of date without changing them.

By specifying arguments when you run @code{make}, you can do any of these
things or many others.

@menu
* Makefile Arguments::    Arguments to specify which makefile to use.

* Goals::                 Goal arguments specify which parts of the makefile
                           should be used.

* Instead of Execution::  Mode flags 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::            Overriding a variable can specify an alternate
                           compiler, or alternate flags for the compiler,
                           or whatever else you program into the makefile.

* Testing::               How to proceed past some errors, to test compilation.

* Options::               Summary of all options @code{make} accepts.
@end menu

@node Makefile Arguments, Goals, Running, Running
@section Arguments to Specify the Makefile

The way to specify the name of the makefile is with the @samp{-f} option.
For example, @samp{-f altmake} says to use the file @file{altmake} as
the makefile.

If you use the @samp{-f} flag several times (each time with a
following argument), all the specified files are used jointly as
makefiles.

If you do not use the @samp{-f} flag, the default is to try
@file{GNUmakefile}, @file{makefile}, or @file{Makefile}, in that order, and
use the first of these three which exists.  @xref{Makefiles}.@refill

@node Goals, Instead of Execution, Makefile Arguments, Running
@section Arguments to Specify the Goals
@cindex goal

The @dfn{goals} are the targets that @code{make} should strive ultimately
to update.  Other targets are updated as well if they appear as
dependencies of goals, or dependencies of dependencies of goals, etc.

By default, the goal is the first target in the makefile (not counting
targets that start with a period).  Therefore, makefiles are usually
written so that the first target is for compiling the entire program or
programs they describe.

You can specify a different goal or goals with arguments to @code{make}.
Use the name of the goal as an argument.  If you specify several goals,
@code{make} processes each of them in turn, in the order you name them.

Any target in the makefile may be specified as a goal (unless it starts
with @samp{-} or contains an @samp{=}).  Even targets not in the makefile
may be specified, if @code{make} can find implicit rules that say how to
make them.

One use of specifying a goal is if you want to compile only a part of
the program, or only one of several programs.  Specify as a goal each
file that you wish to remake.  For example, consider a directory containing
several programs, with a makefile that starts like this:

@example
.PHONY: all
all: size nm ld ar as
@end example

If you are working on the program @code{size}, you might want to say
@samp{make size} so that only the files of that program are recompiled.

Another use of specifying a goal is to make files that aren't normally
made.  For example, there may be a file of debugging output, or a version
of the program that is compiled specially for testing, which has a rule
in the makefile but isn't a dependency of the default goal.

Another use of specifying a goal is to run the commands associated with a
phony target (@pxref{Phony Targets}) or empty target (@pxref{Empty Targets}).
Many makefiles contain a phony target named @file{clean} which deletes
everything except source files.  Naturally, this is done only if you
request it explicitly with @samp{make clean}.  Here is a list of typical
phony and empty target names:

@table @file
@item all
Make all the top-level targets the makefile knows about.

@item clean
Delete all files that are normally created by running @code{make}.

@item distclean
@itemx realclean
@itemx clobber
Any of these three might be defined to delete everything that would
not be part of a standard distribution.  For example, this would
delete configuration files or links that you would normally create as
preparation for compilation, even if the makefile itself cannot create
these files.

@item install
Copy the executable file into a directory that users typically search
for commands; copy any auxiliary files that the executable uses into
the directories where it will look for them.

@item print
Print listings of the source files that have changed.

@item tar
Create a tar file of the source files.

@item shar
Create a shell archive (shar file) of the source files.

@item dist
Create a distribution file of the source files.  This might
be a tar file, or a shar file, or a compressed version of one of the
above, or even more than one of the above.
@end table

@node Instead of Execution, Avoiding Compilation, Goals, Running
@section Instead of Executing the Commands

The makefile tells @code{make} how to tell whether a target is up to date,
and how to update each target.  But updating the targets is not always
what you want.  Certain options specify other activities for @code{make}.

@table @samp
@item -t
@cindex touching files
@cindex @code{-t}
``Touch''.  The activity is to mark the targets as up to date without
actually changing them.  In other words, @code{make} pretends to compile
the targets but does not really change their contents.

@item -n
@cindex @code{-n}
``No-op''.  The activity is to print what commands would be used to make
the targets up to date, but not actually execute them.

@item -q
@cindex @code{-q}
@cindex question mode
``Question''.  The activity is to find out silently whether the targets
are up to date already; but execute no commands in either case.  In other
words, neither compilation nor output will occur.

@item -W
@cindex @code{-W}
@cindex what if
``What if''.  Each @samp{-W} flag is followed by a file name.  The given
files' modification times are recorded by @code{make} as being the present
time, although the actual modification times remain the same.  When used in
conjunction with the @samp{-n} flag, the @samp{-W} flag provides a way to
see what would happen if you were to modify specific files.@refill
@end table

With the @samp{-n} flag, @code{make} prints without execution the commands
that it would normally execute.

With the @samp{-t} flag, @code{make} ignores the commands in the rules
and uses (in effect) the command @code{touch} for each target that needs to
be remade.  The @code{touch} command is also printed, unless @samp{-s} or
@code{.SILENT} is used.  For speed, @code{make} does not actually invoke
the program @code{touch}.  It does the work directly.

With the @samp{-q} flag, @code{make} prints nothing and executes no
commands, but the exit status code it returns is zero if and only if the
targets to be considered are already up to date.

It is an error to use more than one of these three flags in the same
invocation of @code{make}.

The @samp{-n}, @samp{-t}, and @samp{-q} options do not affect command
lines that begin with @samp{+} characters or contain the strings
@samp{$(MAKE)} or @samp{$@{MAKE@}}.  Note that only the line containing
the @samp{+} character or the strings @samp{$(MAKE)} or @samp{$@{MAKE@}}
is run regardless of these options.  Other lines in the same rule are
not run unless they too begin with @samp{+} or contain @samp{$(MAKE)} or
@samp{$@{MAKE@}}.@refill

The @samp{-W} flag provides two features:

@itemize @bullet
@item
If you also use the @samp{-n} or @samp{-q} flag, you can see what
@code{make} would do if you were to modify some files.

@item
Without the @samp{-n} or @samp{-q} flag, when @code{make} is actually
executing commands, the @samp{-W} flag can direct @code{make} to act
as if some files had been modified, without actually modifying the
files.@refill
@end itemize

Note that the options @samp{-p} and @samp{-v} allow you to obtain other
information about @code{make} or about the makefiles in use.
@xref{Options}.@refill

@node Avoiding Compilation, Overriding, Instead of Execution, Running
@section Avoiding Recompilation of Some Files
@cindex @code{-o}

Sometimes you may have changed a source file but you don't want to
recompile all the files that depend on it.  For example, suppose you add a
macro or a declaration to a header file that many other files depend on.
Being conservative, @code{make} assumes that any change in the header file
requires recompilation of all dependent files, but you know that they don't
need to be recompiled and you would rather not waste the time waiting for
them to compile.

If you anticipate the problem before changing the header file, you can
use the @samp{-t} flag.  This flag tells @code{make} not to run the
commands in the rules, but rather to mark the target up to date by
changing its last-modification date.  You would follow this procedure:

@enumerate
@item
Use the command @samp{make} to recompile the source files that really
need recompilation.

@item
Make the changes in the header files.

@item
Use the command @samp{make -t} to mark all the object files as
up to date.  The next time you run @code{make}, the changes in the
header files will not cause any recompilation.
@end enumerate

If you have already changed the header file at a time when some files do
need recompilation, it is too late to do this.  Instead, you can use the
@samp{-o @var{file}} flag, which marks a specified file as ``old''
(@pxref{Options}).  This means that the file itself won't be remade,
and nothing else will be remade on its account.  Follow this procedure:

@enumerate
@item
Recompile the source files that need compilation for reasons independent
of the particular header file, with @samp{make -o @var{headerfile}}.
If several header files are involved, use a separate @samp{-o} option
for each header file.

@item
Touch all the object files with @samp{make -t}.
@end enumerate

@node Overriding, Testing, Avoiding Compilation, Running
@section Overriding Variables
@cindex overriding variables with arguments

An argument that contains @samp{=} specifies the value of a variable:
@samp{@var{v}=@var{x}} sets the value of the variable @var{v} to @var{x}.
If you specify a value in this way, all ordinary assignments of the same
variable in the makefile are ignored; we say they have been
@dfn{overridden} by the command line argument.

The most common way to use this facility is to pass extra flags to
compilers.  For example, in a properly written makefile, the variable
@code{CFLAGS} is included in each command that runs the C compiler, so a
file @file{foo.c} would be compiled something like this:

@example
cc -c $(CFLAGS) foo.c
@end example

Thus, whatever value you set for @code{CFLAGS} affects each compilation
that occurs.  The makefile probably specifies the usual value for
@code{CFLAGS}, like this:

@example
CFLAGS=-g
@end example

Each time you run @code{make}, you can override this value if you wish.
For example, if you say @samp{make CFLAGS='-g -O'}, each C compilation will
be done with @samp{cc -c -g -O}.  (This illustrates how you can enclose
spaces and other special characters in the value of a variable when you
override it.)

The variable @code{CFLAGS} is only one of many standard variables that
exist just so that you can change them this way.  @xref{Implicit
Variables}, for a complete list.

You can also program the makefile to look at additional variables of your
own, giving the user the ability to control other aspects of how the
makefile works by changing the variables.

When you override a variable with a command argument, you can define either
a recursively-expanded variable or a simply-expanded variable.  The
examples shown above make a recursively-expanded variable; to make a
simply-expanded variable, write @samp{:=} instead of @samp{=}.  But, unless
you want to include a variable reference or function call in the
@emph{value} that you specify, it makes no difference which kind of
variable you create.

There is one way that the makefile can change a variable that you have
overridden.  This is to use the @code{override} directive, which is a line
that looks like this: @samp{override @var{variable} = @var{value}}.
@xref{Override Directive}.

@node Testing, Options, Overriding, Running
@section Testing the Compilation of a Program
@cindex testing compilation

Normally, when an error happens in executing a shell command, @code{make}
gives up immediately, returning a nonzero status.  No further commands are
executed for any target.  The error implies that the goal cannot be
correctly remade, and @code{make} reports this as soon as it knows.

When you are compiling a program that you have just changed, this is not
what you want.  Instead, you would rather that @code{make} try compiling
every file that can be tried, to show you as many compilation errors
as possible.

@cindex @code{-k}
On these occasions, you should use the @samp{-k} flag.  This tells
@code{make} to continue to consider the other dependencies of the pending
targets, remaking them if necessary, before it gives up and returns nonzero
status.  For example, after an error in compiling one object file,
@samp{make -k} will continue compiling other object files even though it
already knows that linking them will be impossible.  In addition to
continuing after failed shell commands, @samp{make -k} will continue as much
as possible after discovering that it doesn't know how to make a target or
dependency file.  This will always cause an error message, but without
@samp{-k}, it is a fatal error.  @xref{Options}.

The usual behavior of @code{make} assumes that your purpose is to get the
goals up to date; once @code{make} learns that this is impossible, it might
as well report the failure immediately.  The @samp{-k} flag says that the
real purpose is to test as much as possible of the changes made in the
program, perhaps to find several independent problems so that you can
correct them all before the next attempt to compile.  This is why Emacs's
@kbd{M-x compile} command passes the @samp{-k} flag by default.

@node Options,  , Testing, Running
@section Summary of Options
@cindex options
@cindex flags

Here is a table of all the options @code{make} understands:

@table @samp
@item -b
@itemx -m
These options are ignored for compatibility with other versions of @code{make}.

@item -C @var{dir}
Change to directory @var{dir} before reading the makefiles.  If multiple
@samp{-C} options are specified, each is interpreted relative to the
previous one: @samp{-C / -C etc} is equivalent to @samp{-C /etc}.
This is typically used with recursive invocations of @code{make}
(@pxref{Recursion}).

@item -d
Print debugging information in addition to normal processing.  The
debugging information says which files are being considered for
remaking, which file-times are being compared and with what results,
which files actually need to be remade, which implicit rules are
considered and which are applied---everything interesting about how
@code{make} decides what to do.

@item -e
Give variables taken from the environment precedence
over variables from makefiles.  @xref{Environment}.

@item -f @var{file}
Use file @var{file} as a makefile.  @xref{Makefiles}.

@item -i
Ignore all errors in commands executed to remake files.
@xref{Errors}.

@item -I @var{dir}
Specifies a directory @var{dir} to search for included makefiles.
@xref{Include}.  If several @samp{-I} options are used to specify several
directories, the directories are searched in the order specified.  Unlike
the arguments to other flags of @code{make}, directories given with
@samp{-I} flags may come directly after the flag: @samp{-I@var{dir}} is
allowed, as well as @samp{-I @var{dir}}.  This syntax is allowed for
compatibility with the C preprocessor's @samp{-I} flag.@refill

@item -j @var{jobs}
Specifies the number of jobs (commands) to run simultaneously.  If
there is more than one @samp{-j} option, the last one is effective.
@xref{Execution}, for more information on how commands are run.

@item -k
Continue as much as possible after an error.  While the target that
failed, and those that depend on it, cannot be remade, the other
dependencies of these targets can be processed all the same.
@xref{Testing}.

@item -l @var{load}
@item -l
Specifies that no new jobs (commands) should be started if there are
others jobs running and the load average is at least @var{load} (a
floating-point number).  With no argument, removes a previous load
limit.  @xref{Parallel}.

@item -n
Print the commands that would be executed, but do not execute them.
@xref{Instead of Execution}.

@item -o @var{file}
Do not remake the file @var{file} even if it is older than its
dependencies, and do not remake anything on account of changes in
@var{file}.  Essentially the file is treated as very old and its rules
are ignored.  @xref{Avoiding Compilation}.

@item -p
Print the data base (rules and variable values) that results from
reading the makefiles; then execute as usual or as otherwise
specified.  This also prints the version information given by
the @samp{-v} switch (see below).  To print the data base without
trying to remake any files, use @samp{make -p -f /dev/null}.

@item -q
``Question mode''.  Do not run any commands, or print anything; just
return an exit status that is zero if the specified targets are
already up to date, nonzero otherwise.  @xref{Instead of Execution}.

@item -r
Eliminate use of the built-in implicit rules (@pxref{Implicit}).
Also clear out the default list of suffixes for suffix rules
(@pxref{Suffix Rules}).

@item -s
Silent operation; do not print the commands as they are executed.
@xref{Echoing}.

@item -S
Cancel the effect of the @samp{-k} option.  This is never necessary
except in a recursive @code{make} where @samp{-k} might be inherited
from the top-level @code{make} via @code{MAKEFLAGS} (@pxref{Recursion})
or if you set @samp{-k} in @code{MAKEFLAGS} in your environment.@refill

@item -t
Touch files (mark them up to date without really changing them)
instead of running their commands.  This is used to pretend that the
commands were done, in order to fool future invocations of
@code{make}.  @xref{Instead of Execution}.

@item -v
Print the version of the @code{make} program plus a copyright, a list
of authors and a notice that there is no warranty.  After this
information is printed, processing continues normally.  To get this
information without doing anything else, use @samp{make -v -f
/dev/null}.

@item -w
Print a message containing the working directory both before and after
executing the makefile.  This may be useful for tracking down errors
from complicated nests of recursive @code{make} commands.
@xref{Recursion}.

@item -W @var{file}
Pretend that the target @var{file} has just been modified.  When used
with the @samp{-n} flag, this shows you what would happen if you were
to modify that file.  Without @samp{-n}, it is almost the same as
running a @code{touch} command on the given file before running
@code{make}, except that the modification time is changed only in the
imagination of @code{make}.@refill
@end table

@node Implicit, Archives, Running, Top
@chapter Using Implicit Rules
@cindex implicit rule

Certain standard ways of remaking target files are used very often.  For
example, one customary way to make an object file is from a C source file
using the C compiler, @code{cc}.

@dfn{Implicit rules} tell @code{make} how to use customary techniques so
that you don't have to specify them in detail when you want to use them.
For example, there is an implicit rule for C compilation.  Implicit rules
work based on file names.  For example, C compilation typically takes a
@file{.c} file and makes a @file{.o} file.  So @code{make} applies the
implicit rule for C compilation when it sees this combination of file-name
endings.

A chain of implicit rules can apply in sequence; for example, @code{make}
will remake a @file{.o} file from a @file{.y} file by way of a @file{.c} file.
@iftex
@xref{Chained Rules}.
@end iftex

The built-in implicit rules use several variables in their commands so
that, by changing the values of the variables, you can change the way the
implicit rule works.  For example, the variable @code{CFLAGS} controls the
flags given to the C compiler by the implicit rule for C compilation.
@iftex
@xref{Implicit Variables}.
@end iftex

You can define your own implicit rules by writing @dfn{pattern rules}.
@iftex
@xref{Pattern Rules}.
@end iftex

@menu
* Using Implicit::       How to use an existing implicit rule
                          to get the commands for updating a file.

* Catalogue of Rules::   Catalogue of built-in implicit rules.

* Implicit Variables::   By changing certain variables, you can
                          change what the predefined implicit rules do.

* Chained Rules::        Using a chain of implicit rules.

* Pattern Rules::        Defining new implicit rules.

* Last Resort::          Defining commands for rules which can't find any.

* Suffix Rules::         The old-fashioned style of implicit rule.

* Search Algorithm::     Precise algorithm for applying implicit rules.
@end menu

@node Using Implicit, Catalogue of Rules, Implicit, Implicit
@section Using Implicit Rules

To allow @code{make} to find a customary method for updating a target file,
all you have to do is refrain from specifying commands yourself.  Either
write a rule with no command lines, or don't write a rule at all.  Then
@code{make} will figure out which implicit rule to use based on which
kind of source file exists.

For example, suppose the makefile looks like this:

@example
foo : foo.o bar.o
        cc -o foo foo.o bar.o $(CFLAGS) $(LDFLAGS)
@end example

@noindent
Because you mention @file{foo.o} but do not give a rule for it, @code{make}
will automatically look for an implicit rule that tells how to update it.
This happens whether or not the file @file{foo.o} currently exists.

If an implicit rule is found, it supplies both commands and one or more
dependencies (the source files).  You would want to write a rule for
@file{foo.o} with no command lines if you need to specify additional
dependencies, such as header files, that the implicit rule cannot supply.

Each implicit rule has a target pattern and dependency patterns.  There may
be many implicit rules with the same target pattern.  For example, numerous
rules make @samp{.o} files: one, from a @samp{.c} file with the C compiler;
another, from a @samp{.p} file with the Pascal compiler; and so on.  The rule
that actually applies is the one whose dependencies exist or can be made.
So, if you have a file @file{foo.c}, @code{make} will run the C compiler;
otherwise, if you have a file @file{foo.p}, @code{make} will run the Pascal
compiler; and so on.

Of course, when you write the makefile, you know which implicit rule you
want @code{make} to use, and you know it will choose that one because you
know which possible dependency files are supposed to exist.
@xref{Catalogue of Rules}, for a catalogue of all the predefined implicit
rules.

Above, we said an implicit rule applies if the required dependencies ``exist
or can be made''.  A file ``can be made'' if it is mentioned explicitly in
the makefile as a target or a dependency, or if an implicit rule can be
recursively found for how to make it.  When an implicit dependency is the
result of another implicit rule, we say that @dfn{chaining} is occurring.
@xref{Chained Rules}.

In general, @code{make} searches for an implicit rule for each target, and
for each double-colon rule, that has no commands.  A file that is mentioned
only as a dependency is considered a target whose rule specifies nothing,
so implicit rule search happens for it.  @xref{Search Algorithm}, for the
details of how the search is done.

If you don't want an implicit rule to be used for a target that has no
commands, you can give that target empty commands by writing a semicolon.
@xref{Empty Commands}.

@node Catalogue of Rules, Implicit Variables, Using Implicit, Implicit
@section Catalogue of Implicit Rules

Here is a catalogue of predefined implicit rules which are always available
unless the makefile explicitly overrides or cancels them.
@xref{Canceling Rules}, for information on canceling or overriding an
implicit rule.  The @samp{-r} option cancels all predefined rules.

Not all of these rules will always be defined, even when the @samp{-r}
option is not given.  Many of the predefined implicit rules are
implemented in @code{make} as suffix rules, so which ones will be
defined depends on the @dfn{suffix list} (the list of dependencies of
the special target @code{.SUFFIXES}).  @xref{Suffix Rules}.
The default suffix list is: @samp{.out}, @samp{.a}, @samp{.o},
@samp{.c}, @samp{.cc}, @samp{.C}, @samp{.p}, @samp{.f}, @samp{.F}, @samp{.r},
@samp{.e}, @samp{.y}, @samp{.ye}, @samp{.yr}, @samp{.l}, @samp{.s},
@samp{.S}, @samp{.h}, @samp{.info}, @samp{.dvi}, @samp{.tex},
@samp{.texinfo}, @samp{.cweb}, @samp{.web}, @samp{.sh}, @samp{.elc},
@samp{.el}. All of the implicit rules described below whose dependencies
have one of these suffixes are actually suffix rules.  If you modify
the suffix list, the only predefined suffix rules in effect will be those
named by one or two of the suffixes that are on the list you specify;
rules whose suffixes fail to be on the list are disabled.@refill

@table @asis
@item Compiling C programs
@file{@var{n}.o} will be made automatically from @file{@var{n}.c} with
a command of the form @samp{$(CC) -c $(CPPFLAGS) $(CFLAGS)}.@refill

@item Compiling C++ programs
@file{@var{n}.o} will be made automatically from @file{@var{n}.cc} or
@file{@var{n}.C} with a command of the form @samp{$(C++) -c $(CPPFLAGS)
$(C++FLAGS)}.  We encourage you to use the suffix @samp{.cc} for C++
source files instead of @samp{.C}.@refill

@item Compiling Pascal programs
@file{@var{n}.o} will be made automatically from @file{@var{n}.p}
with the command @samp{$(PC) -c $(PFLAGS)}.@refill

@item Compiling Fortran and Ratfor programs
@file{@var{n}.o} will be made automatically from @file{@var{n}.r},
@file{@var{n}.F} or @file{@var{n}.f} by running the
Fortran compiler.  The precise command used is as follows:@refill

@table @samp
@item .f
@samp{$(FC) -c $(FFLAGS)}.
@item .F
@samp{$(FC) -c $(FFLAGS) $(CPPFLAGS)}.
@item .r
@samp{$(FC) -c $(FFLAGS) $(RFLAGS)}.
@end table

@item Preprocessing Fortran and Ratfor programs
@file{@var{n}.f} will be made automatically from @file{@var{n}.r} or
@file{@var{n}.F}.  This rule runs just the preprocessor to convert a
Ratfor or preprocessable Fortran program into a strict Fortran
program.  The precise command used is as follows:@refill

@table @samp
@item .F
@samp{$(FC) -F $(CPPFLAGS) $(FFLAGS)}.
@item .r
@samp{$(FC) -F $(FFLAGS) $(RFLAGS)}.
@end table

@item Compiling Modula-2 programs
@file{@var{n}.sym} will be made from @file{@var{n}.def} with a command
of the form @samp{$(M2C) $(M2FLAGS) $(DEFFLAGS)}.  @file{@var{n}.o}
will be made from @file{@var{n}.mod} with a command of the form
@samp{$(M2C) $(M2FLAGS) $(MODFLAGS)}.@refill

@item Assembling and preprocessing assembler programs
@file{@var{n}.o} will be made automatically from @file{@var{n}.s} by
running the assembler @code{as}.  The precise command used is
@samp{$(AS) $(ASFLAGS)}.@refill

@file{@var{n}.s} will be made automatically from @file{@var{n}.S} by
running the C preprocessor @code{cpp}.  The precise command used is
@samp{$(CPP) $(CPPFLAGS)}.@refill

@item Linking a single object file
@file{@var{n}} will be made automatically from @file{@var{n}.o} by
running the linker @code{ld} via the C compiler.  The precise command
used is @samp{$(CC) $(LDFLAGS) @var{n}.o $(LOADLIBES)}.@refill

This rule does the right thing for a simple program with only one
source file.  It will also do the right thing if there are multiple
object files (presumably coming from various other source files), the
first of which has a name matching that of the executable file.  Thus,

@example
x: y.o z.o
@end example

@noindent
when @file{x.c}, @file{y.c} and @file{z.c} all exist will execute:

@example
cc -c x.c -o x.o
cc -c y.c -o y.o
cc -c z.c -o z.o
cc x.o y.o z.o -o x
rm -f x.o
rm -f y.o
rm -f z.o
@end example

@noindent
In more complicated cases, such as when there is no object file whose
name derives from the executable file name, you must write an explicit
command for linking.

Each kind of file automatically made into @code{.o} object files will
be automatically linked by using the compiler (@samp{$(CC)},
@samp{$(FC)} or @samp{$(PC)}; the C compiler @samp{$(CC)} is used to
assemble @code{.s} files) without the @samp{-c} option.  This could be
done by using the @code{.o} object files as intermediates, but it is
faster to do the compiling and linking in one step, so that's how it's
done.@refill

@item Yacc for C programs
@file{@var{n}.c} will be made automatically from @file{@var{n}.y} by
running Yacc with the command @samp{$(YACC) $(YFLAGS)}.

@item Lex for C programs
@file{@var{n}.c} will be made automatically from @file{@var{n}.l} by
by running Lex.  The actual command is @samp{$(LEX) $(LFLAGS)}.

@item Lex for Ratfor programs
@file{@var{n}.r} will be made automatically from @file{@var{n}.l} by
by running Lex.  The actual command is @samp{$(LEX) $(LFLAGS)}.

The convention of using the same suffix @samp{.l} for all Lex files
regardless of whether they produce C code or Ratfor code makes it
impossible for @code{make} to determine automatically which of the two
languages you are using in any particular case.  If @code{make} is
called upon to remake an object file from a @samp{.l} file, it must
guess which compiler to use.  It will guess the C compiler, because
that is more common.  If you are using Ratfor, make sure @code{make}
knows this by mentioning @file{@var{n}.r} in the makefile.  Or, if you
are using Ratfor exclusively, with no C files, remove @code{.c} from
the list of implicit rule suffixes with:@refill

@example
.SUFFIXES:
.SUFFIXES: .r .f .l @dots{}
@end example

@item Making Lint Libraries from C, Yacc, or Lex programs
@file{@var{n}.ln} will be made from @file{@var{n}.c} with a command of
the form @samp{$(LINT) $(LINTFLAGS) $(CPPFLAGS) -i}.  The same command
will be used on the C code produced from @file{@var{n}.y} or
@file{@var{n}.l}.@refill

@item @TeX{} and Web
@file{@var{n}.dvi} will be made from @file{@var{n}.tex} with the
command @samp{$(TEX)}.  @file{@var{n}.tex} will be made from
@file{@var{n}.web} with @samp{$(WEAVE)}, or from @file{@var{n}.cweb}
with @samp{$(CWEAVE)}.  @file{@var{n}.p} will be made from
@file{@var{n}.web} with @samp{$(TANGLE)} and @file{@var{n}.c} will be
made from @file{@var{n}.cweb} with @samp{$(CTANGLE)}.@refill

@item Texinfo and Info
@file{@var{n}.dvi} will be made from @file{@var{n}.texinfo} using the
@samp{$(TEX)} and @samp{$(TEXINDEX)} commands.  The actual command
sequence contains many shell conditionals to avoid unnecessarily
running @TeX{} twice and to create the proper sorted index files.
@file{@var{n}.info} will be made from @file{@var{n}.texinfo} with the
command @samp{$(MAKEINFO)}.@refill

@item RCS
Any file @file{@var{n}} will be extracted if necessary from an RCS file
named either @file{@var{n},v} or @file{RCS/@var{n},v}.  The precise
command used is @samp{$(CO) $(COFLAGS)}.  @file{@var{n}} will not be
extracted from RCS if it already exists, even if the RCS file is
newer.@refill

@item SCCS
Any file @file{@var{n}} will be extracted if necessary from an SCCS
file named either @file{s.@var{n}} or @file{SCCS/s.@var{n}}.  The
precise command used is @samp{$(GET) $(GFLAGS)}.@refill

For the benefit of SCCS, a file @file{@var{n}} will be copied from
@file{@var{n}.sh} and made executable (by everyone).  This is for
shell scripts that are checked into SCCS.  Since RCS preserves the
execution permission of a file, you don't need to use this feature
with RCS.@refill

We recommend that you avoid the use of SCCS.  RCS is widely held to be
superior, and is also free.  By choosing free software in place of
comparable (or inferior) proprietary software, you support the free
software movement.
@end table

@node Implicit Variables, Chained Rules, Catalogue of Rules, Implicit
@section Variables Used by Implicit Rules
@cindex flags for compilers

The commands in built-in implicit rules make liberal use of certain
predefined variables.  You can alter these variables, either in the
makefile or with arguments to @code{make}, to alter how the implicit rules
work without redefining the rules themselves.

For example, the command used to compile a C source file actually says
@samp{$(CC) -c $(CFLAGS) $(CPPFLAGS)}.  The default values of the variables
used are @samp{cc} and nothing, resulting in the command @samp{cc -c}.  By
redefining @samp{$(CC)} to @samp{ncc}, you could cause @samp{ncc} to be
used for all C compilations performed by the implicit rule.  By redefining
@samp{$(CFLAGS)} to be @samp{-g}, you could pass the @samp{-g} option to
each compilation.  @emph{All} implicit rules that do C compilation use
@samp{$(CC)} to get the program name for the compiler and @emph{all}
include @samp{$(CFLAGS)} among the arguments given to the compiler.@refill

The variables used in implicit rules fall into two classes: those that are
names of programs (like @code{CC}) and those that contain arguments for the
programs (like @code{CFLAGS}).  (The ``name of a program'' may also contain
some command arguments, but it must start with an actual executable program
name.)  If a variable value contains more than one argument, separate them
with spaces.

Here is a table of variables used as names of programs:

@table @code
@item AR
@vindex AR
Archive-maintaing program; default @samp{ar}.

@item AS
@vindex AS
Program for doing assembly; default @samp{as}.

@item CC
@vindex CC
Program for compiling C programs; default @samp{cc}.

@item C++
@vindex C++
Program for compiling C++ programs; default @samp{g++}.

@item CO
@vindex CO
Program for extracting a file from RCS; default @samp{co}.

@item CPP
@vindex CPP
Program for running the C preprocessor, with results to standard output;
default @samp{$(CC) -E}.

@item FC
@vindex FC
Program for compiling or preprocessing Fortran, Ratfor,
and EFL programs; default @samp{f77}.

@item GET
@vindex GET
Program for extracting a file from SCCS; default @samp{get}.

@item LEX
@vindex LEX
Program to use to turn Lex grammars into C programs or Ratfor programs;
default @samp{lex}.

@item PC
@vindex PC
Program for compiling Pascal programs; default @samp{pc}.

@item FC
@itemx EC
@itemx RC
@vindex FC
@vindex EC
@vindex RC
Programs for compiling Fortran, EFL, and Ratfor programs,
respectively; these all default to @samp{f77}.

@item YACC
@vindex YACC
Program to use to turn Yacc grammars into C programs; default @samp{yacc}.

@item YACCR
@vindex YACCR
Program to use to turn Yacc grammars into Ratfor
programs; default @samp{yacc -r}.

@item YACCE
@vindex YACCE
Program to use to turn Yacc grammars into EFL
programs; default @samp{yacc -e}.

@item MAKEINFO
@vindex MAKEINFO
Program to make Info files from Texinfo source; default @samp{makeinfo}.

@item TEX
@vindex TEX
Program to make @TeX{} DVI files from @TeX{} or Texinfo source;
default @samp{tex}.

@item TEXINDEX
@vindex TEXINDEX
The @code{texindex} program distributed with Emacs.
This is used in the process to make @TeX{} DVI files from Texinfo source.

@item WEAVE
@vindex WEAVE
Program to translate Web into @TeX{}; default @samp{weave}.

@item CWEAVE
@vindex CWEAVE
Program to translate C Web into @TeX{}; default @samp{cweave}.

@item TANGLE
@vindex TANGLE
Program to translate Web into Pascal; default @samp{tangle}.

@item CTANGLE
@vindex CTANGLE
Program to translate C Web into C; default @samp{ctangle}.

@item RM
@vindex RM
Command to remove a file; default @samp{rm -f}.
@end table

Here is a table of variables whose values are additional arguments for the
programs above.  The default values for all of these is the empty
string, unless otherwise noted.

@table @code
@item ARFLAGS
@vindex ARFLAGS
Flags to give the archive- maintaining program; default @samp{rv}.

@item ASFLAGS
@vindex ASFLAGS
Extra flags to give to the assembler (when explicitly
invoked on a @samp{.s} file).

@item CFLAGS
@vindex CFLAGS
Extra flags to give to the C compiler.

@item C++FLAGS
@vindex C++FLAGS
Extra flags to give to the C++ compiler.

@item COFLAGS
@vindex COFLAGS
Extra flags to give to the RCS @code{co} program.

@item CPPFLAGS
@vindex CPPFLAGS
Extra flags to give to the C preprocessor and programs
that use it (the C and Fortran compilers).

@item EFLAGS
@vindex EFLAGS
Extra flags to give to the Fortran compiler for EFL programs.

@item FFLAGS
@vindex FFLAGS
Extra flags to give to the Fortran compiler.

@item GFLAGS
@vindex GFLAGS
Extra flags to give to the SCCS @code{get} program.

@item LDFLAGS
@vindex LDFLAGS
Extra flags to give to compilers when they are
supposed to invoke the linker, @samp{ld}.

@item LFLAGS
@vindex LFLAGS
Extra flags to give to Lex.

@item PFLAGS
@vindex PFLAGS
Extra flags to give to the Pascal compiler.

@item RFLAGS
@vindex RFLAGS
Extra flags to give to the Fortran compiler for Ratfor programs.

@item YFLAGS
@vindex YFLAGS
Extra flags to give to Yacc.
@end table

@node Chained Rules, Pattern Rules, Implicit Variables, Implicit
@section Chains of Implicit Rules

@cindex chains of rules
Sometimes a file can be made by a sequence of implicit rules.  For example,
a file @file{@var{n}.o} could be made from @file{@var{n}.y} by running
first Yacc and then @code{cc}.  Such a sequence is called a @dfn{chain}.

If the file @file{@var{n}.c} exists, or is mentioned in the makefile, no
special searching is required: @code{make} finds that the object file can
be made by C compilation from @file{@var{n}.c}; later on, when considering
how to make @file{@var{n}.c}, the rule for running Yacc will be
used.  Ultimately both @file{@var{n}.c} and @file{@var{n}.o} are
updated.@refill

@cindex intermediate file
However, even if @file{@var{n}.c} does not exist and is not mentioned,
@code{make} knows how to envision it as the missing link between
@file{@var{n}.o} and @file{@var{n}.y}!  In this case, @file{@var{n}.c} is
called an @dfn{intermediate file}.  Once @code{make} has decided to use the
intermediate file, it is entered in the data base as if it had been
mentioned in the makefile, along with the implicit rule that says how to
create it.@refill

Intermediate files are remade using their rules just like all other
files.  The difference is that the intermediate file is deleted when
@code{make} is finished.  Therefore, the intermediate file which did
not exist before @code{make} also does not exist after @code{make}.
The deletion is reported to you by printing a @samp{rm -f} command
that shows what @code{make} is doing.  (You can optionally define an
implicit rule so as to preserve certain intermediate files.  You can also
list the target pattern of an implicit rule (such as @samp{%.o}) as a
dependency file of the special target @code{.PRECIOUS} to preserve intermediate
files whose target patterns match that file's name.)@refill
@cindex preserving intermediate files with .PRECIOUS

A chain can involve more than two implicit rules.  For example, it is
possible to make a file @file{foo} from @file{RCS/foo.y,v} by running RCS,
Yacc and @code{cc}.  Then both @file{foo.y} and @file{foo.c} are
intermediate files that are deleted at the end.@refill

No single implicit rule can appear more than once in a chain.  This means
that @code{make} will not even consider such a ridiculous thing as making
@file{foo} from @file{foo.o.o} by running the linker twice.  This
constraint has the added benefit of preventing any infinite loop in the
search for an implicit rule chain.

There are some special implicit rules to optimize certain cases that would
otherwise by handled by rule chains.  For example, making @file{foo} from
@file{foo.c} could be handled by compiling and linking with separate
chained rules, using @file{foo.o} as an intermediate file.  But what
actually happens is that a special rule for this case does the compilation
and linking with a single @code{cc} command.  The optimized rule is used in
preference to the step-by-step chain because it comes earlier in the
ordering of rules.

@node Pattern Rules, Last Resort, Chained Rules, Implicit
@section Defining and Redefining Pattern Rules

You define an implicit rule by writing a @dfn{pattern rule}.  A pattern
rule looks like an ordinary rule, except that its target contains the
character @samp{%} (exactly one of them).  The target is considered a
pattern for matching file names; the @samp{%} can match any nonempty
substring, while other characters match only themselves.  The dependencies
likewise use @samp{%} to show how their names relate to the target name.

Thus, a pattern rule @samp{%.o : %.c} says how to make any file
@file{@var{stem}.o} from another file @file{@var{stem}.c}.@refill

@menu
* Intro: Pattern Intro.        Basics of writing pattern rules.
* Examples: Pattern Examples.  Real examples of pattern rule definitions.

* Vars: Automatic.             The automatic variables enable the commands
                                in pattern rules to act on the right files.

* Matching: Pattern Match.     Details of how patterns match.

* Match-Anything Rules::       Precautions in defining a rules that can
                                match any target file whatever.

* Canceling Rules::            Overriding or canceling built-in rules.

* Last Resort::                How to define a last-resort rule
                                that applies to any target that no other
                                rule applies to.

* Suffix Rules::               The old-fashioned way to define implicit rules.
@end menu

@node Pattern Intro, Pattern Examples, Pattern Rules, Pattern Rules
@subsection Introduction to Pattern Rules

@cindex pattern rule
You define an implicit rule by writing a @dfn{pattern rule}.  A pattern
rule looks like an ordinary rule, except that its target contains the
character @samp{%} (exactly one of them).  The target is considered a
pattern for matching file names; the @samp{%} can match any nonempty
substring, while other characters match only themselves.

For example, @samp{%.c} as a pattern matches any file name that ends in
@samp{.c}.  @samp{s.%.c} as a pattern matches any file name that starts
with @samp{s.}, ends in @samp{.c} and is at least five characters long.
(There must be at least one character to match the @samp{%}.)  The substring
that the @samp{%} matches is called the @dfn{stem}.@refill

@samp{%} in a dependency of a pattern rule stands for the same stem
that was matched by the @samp{%} in the target.  In order for
the pattern rule to apply, its target pattern must match the file name
under consideration, and its dependency patterns must name files that
exist or can be made.  These files become dependencies of the target.

Thus, a rule of the form

@example
%.o : %.c
@end example

@noindent
would specify how to make any file @file{@var{n}.o}, with another file
@file{@var{n}.c} as its dependency, provided that the other file exists or
can be made.

There may also be dependencies that do not use @samp{%}; such a dependency
attaches to every file made by this pattern rule.  These unvarying
dependencies are useful occasionally.

It is allowed for a pattern rule to have no dependencies that contain
@samp{%} or to have no dependencies at all.  This is effectively a general
wildcard.  It provides a way to make any file that matches the target pattern.

Pattern rules may have more than one target.  Unlike normal rules, this
does not act as many different rules with the same dependencies and
commands.  If a pattern rule has multiple targets, @code{make} knows that
the rule's commands are responsible for making all of the targets.  The
commands are executed only once to make all of the targets.  When searching
for a pattern rule to match a target, the target patterns of a rule other
than the one that matches the target in need of a rule are incidental:
@code{make} worries only about giving commands and dependencies to the file
presently in question.  However, when this file's commands are run, the
other targets are marked as having been updated themselves.

The order in which pattern rules appear in the makefile is important
because the rules are considered in that order.  Of equally applicable
rules, the first one found is used.  The rules you write take precedence
over those that are built in.  Note, however, that a rule whose
dependencies actually exist or are mentioned always takes priority over a
rule with dependencies that must be made by chaining other implicit rules.

@node Pattern Examples, Automatic, Pattern Intro, Pattern Rules
@subsection Pattern Rule Examples

Here are some examples of pattern rules actually predefined in
@code{make}.  First, the rule that compiles @samp{.c} files into @samp{.o}
files:@refill

@example
%.o : %.c
        $(CC) -c $(CFLAGS) $(CPPFLAGS) $< -o $@@
@end example

@noindent
defines a rule that can make any file @file{@var{x}.o} from
@file{@var{x}.c}.  The command uses the automatic variables @samp{$@@} and
@samp{$<} to substitute the names of the target file and the source file
in each case where the rule applies (@pxref{Automatic}).@refill

Here is a second built-in rule:

@example
% :: RCS/%,v
        $(CO) $(COFLAGS) $<
@end example

@noindent
defines a rule that can make any file @file{@var{x}} whatever from a
corresponding file @file{@var{x},v} in the subdirectory @file{RCS}.  Since
the target is @samp{%}, this rule will apply to any file whatever, provided
the appropriate dependency file exists.  The double colon makes the rule
@dfn{terminal}, which means that its dependency may not be an intermediate
file (@pxref{Match-Anything Rules}).@refill

This pattern rule has two targets:

@example
%.tab.c %.tab.h: %.y
        bison -d $<
@end example

@noindent
This tells @code{make} that the command @samp{bison -d @var{x}.y}
will make both @file{@var{x}.tab.c} and @file{@var{x}.tab.h}.  If the file
@file{foo} depends on the files @file{parse.tab.o} and @file{scan.o} and
@file{scan.o} depends on @file{parse.tab.h}, when @file{parse.y} is
changed, the command @samp{bison -d parse.y} will be executed only once,
and the dependencies of both @file{parse.tab.o} and @file{scan.o} will be
satisfied.  (Presumably, @file{parse.tab.o} will be recompiled from
@file{parse.tab.c} and @file{scan.o} from @file{scan.c}, and @file{foo}
will be linked from @file{parse.tab.o}, @file{scan.o}, and its other
dependencies, and it will execute happily ever after.)@refill

@node Automatic, Pattern Match, Pattern Examples, Pattern Rules
@subsection Automatic Variables
@cindex automatic variables

Suppose you are writing a pattern rule to compile a @samp{.c} file into a
@samp{.o} file: how do you write the @samp{cc} command so that it operates
on the right source file name?  You can't write the name in the command,
because the name is different each time the implicit rule is applied.

What you do is use a special feature of @code{make}, the @dfn{automatic
variables}.  These variables have values computed afresh for each rule that
is executed, based on the target and dependencies of the rule.  In this
example, you would use @samp{$@@} for the object file name and @samp{$<}
for the source file name.

Here is a table of automatic variables:

@table @code
@item $@@
The file name of the target of the rule.  If the target is an archive
member, then @samp{$@@} is the name of the archive file.

@item $%
The target member name, when the target is an archive member.  For
example, if the target is @file{foo.a(bar.o)} then @samp{$%} is
@file{bar.o} and @samp{$@@} is @file{foo.a}.  @samp{$%} is empty
when the target is not an archive member.

@item $<
The name of the first dependency.

@item $?
The names of all the dependencies that are
newer than the target, with spaces between them.

@item $^
The names of all the dependencies, with spaces between them.

@item $*
The stem with which an implicit rule matches (@pxref{Pattern Match}).
If the target is @file{dir/a.foo.b} and the target pattern is
@file{a.%.b} then the stem is @file{dir/foo}.  The stem is useful for
constructing names of related files.@refill

In an explicit rule, there is no stem; so @samp{$*} cannot be
determined in that way.  Instead, if the target name ends with a
recognized suffix (@pxref{Suffix Rules}), @samp{$*} is set to the
target name minus the suffix.  For example, if the target name is
@samp{foo.c}, then @samp{$*} is set to @samp{foo}, since @samp{.c} is
a suffix.@refill

If the target name in an explicit rule does not end with a recognized
suffix, @samp{$*} is set to the empty string for that rule.
@end table

@samp{$?} is useful even in explicit rules when you wish to operate on only
the dependencies that have changed.  For example, suppose that an archive
named @file{lib} is supposed to contain copies of several object files.
This rule copies just the changed object files into the archive:

@example
lib: foo.o bar.o lose.o win.o
        ar r lib $?
@end example

Of the variables listed above, four have values that are single file
names, and two have values that are lists of file names.  These six have
variants that get just the file's directory name or just the file name
within the directory.  The variant variables' names are formed by
appending @samp{D} or @samp{F}, respectively.  These variants are
semi-obsolete in GNU @code{make} since the functions @code{dir} and
@code{notdir} can be used to get an equivalent effect (@pxref{Filename
Functions}).  Here is a table of the variants:@refill

@table @samp
@item $(@@D)
The directory part of the file name of the target.  If the value of
@samp{$@@} is @file{dir/foo.o} then @samp{$(@@D)} is @file{dir/}.
This value is @file{./} if @samp{$@@} does not contain a slash.
@samp{$(@@D)} is equivalent to @samp{$(dir $@@)}.@refill

@item $(@@F)
The file-within-directory part of the file name of the target.  If the
value of @samp{$@@} is @file{dir/foo.o} then @samp{$(@@F)} is
@file{foo.o}.  @samp{$(@@F)} is equivalent to @samp{$(notdir $@@)}.

@item $(*D)
@itemx $(*F)
The directory part and the file-within-directory
part of the stem; @file{dir/} and @file{foo} in this example.

@item $(%D)
@itemx $(%F)
The directory part and the file-within-directory part of the target
archive member name.  This makes sense only for archive member
targets of the form @file{@var{archive}(@var{member})}
and useful only when @var{member} may contain a directory name.
(@xref{Archive Members}.)

@item $(<D)
@itemx $(<F)
The directory part and the file-within-directory
part of the first dependency.

@item $(^D)
@itemx $(^F)
Lists of the directory parts and the file-within-directory
parts of all dependencies.

@item $(?D)
@itemx $(?F)
Lists of the directory parts and the file-within-directory
parts of all dependencies that are out of date with
respect to the target.
@end table

Note that we use a special stylistic convention when we talk about these
automatic variables; we write ``the value of @samp{$<}'', rather than ``the
variable @code{<}'' as we would write for ordinary variables such as
@code{objects} and @code{CFLAGS}.  We think this convention looks more
natural in this special case.  Please don't assume it has a deep
significance; @samp{$<} refers to the variable named @code{<} just as
@samp{$(CFLAGS)} refers to the variable named @code{CFLAGS}.@refill

@node Pattern Match, Match-Anything Rules, Automatic, Pattern Rules
@subsection How Patterns Match

@cindex stem
A target pattern is composed of a @samp{%} between a prefix and a suffix,
either or both of which may be empty.  The pattern matches a file name only
if the file name starts with the prefix and ends with the suffix, without
overlap.  The text between the prefix and the suffix is called the
@dfn{stem}.  Thus, when the pattern @samp{%.o} matches the file name
@file{test.o}, the stem is @samp{test}.  The pattern rule dependencies are
turned into actual file names by substituting the stem for the character
@samp{%}.  Thus, if in the same example one of the dependencies is written
as @samp{%.c}, it expands to @samp{test.c}.@refill

When the target pattern does not contain a slash (and usually it does not),
directory names in the file names are removed from the file name before it
is compared with the target prefix and suffix.  The directory names, along
with the slash that ends them, are added back to the stem.  Thus,
@samp{e%t} does match the file name @file{src/eat}, with @samp{src/a} as
the stem.  When dependencies are turned into file names, the directories
from the stem are added at the front, while the rest of the stem is
substituted for the @samp{%}.  The stem @samp{src/a} with a dependency
pattern @samp{c%r} gives the file name @file{src/car}.@refill

@node Match-Anything Rules, Canceling Rules, Pattern Match, Pattern Rules
@subsection Match-Anything Pattern Rules

@cindex match-anything rule
@cindex terminal rule
When a pattern rule's target is just @samp{%}, it matches any filename
whatever.  We call these rules @dfn{match-anything} rules.  They are very
useful, but it can take a lot of time for @code{make} to think about them,
because it must consider every such rule for each file name listed either
as a target or as a dependency.

Suppose the makefile mentions @file{foo.c}.  For this target, @code{make}
would have to consider making it by linking an object file @file{foo.c.o},
or by C compilation-and-linking in one step from @file{foo.c.c}, or by
Pascal compilation-and-linking from @file{foo.c.p}, and many other
possibilities.

We know these possibilities are ridiculous since @file{foo.c} is a C source
file, not an executable.  If @code{make} did consider these possibilities,
it would ultimately reject them, because files such as @file{foo.c.o},
@file{foo.c.p}, etc. would not exist.  But these possibilities are so
numerous that @code{make} would run very slowly if it had to consider
them.@refill

To gain speed, we have put various constraints on the way @code{make}
considers match-anything rules.  There are two different constraints that
can be applied, and each time you define a match-anything rule you must
choose one or the other for that rule.

One choice is to mark the match-anything rule as @dfn{terminal} by defining
it with a double colon.  When a rule is terminal, it does not apply unless
its dependencies actually exist.  Dependencies that could be made with
other implicit rules are not good enough.  In other words, no further
chaining is allowed beyond a terminal rule.

For example, the built-in implicit rules for extracting sources from RCS
and SCCS files are terminal; as a result, if the file @file{foo.c,v} does
not exist, @code{make} will not even consider trying to make it as an
intermediate file from @file{foo.c,v.o} or from @file{RCS/SCCS/s.foo.c,v}.
RCS and SCCS files are generally ultimate source files, which should not be
remade from any other files; therefore, @code{make} can save time by not
looking for ways to remake them.@refill

If you do not mark the match-anything rule as terminal, then it is
nonterminal.  A nonterminal match-anything rule cannot apply to a file name
that indicates a specific type of data.  A file name indicates a specific
type of data if some non-match-anything implicit rule target matches it.

For example, the file name @file{foo.c} matches the target for the pattern
rule @samp{%.c : %.y} (the rule to run Yacc).  Regardless of whether this
rule is actually applicable (which happens only if there is a file
@file{foo.y}), the fact that its target matches is enough to prevent
consideration of any nonterminal match-anything rules for the file
@file{foo.c}.  Thus, @code{make} will not even consider trying to make
@file{foo.c} as an executable file from @file{foo.c.o}, @file{foo.c.c},
@file{foo.c.p}, etc.@refill

The motivation for this constraint is that nonterminal match-anything
rules are used for making files containing specific types of data (such as
executable files) and a file name with a recognized suffix indicates some
other specific type of data (such as a C source file).

Special built-in dummy pattern rules are provided solely to recognize
certain file names so that nonterminal match-anything rules won't be
considered.  These dummy rules have no dependencies and no commands, and
they are ignored for all other purposes.  For example, the built-in
implicit rule

@example
%.p :
@end example

@noindent
exists to make sure that Pascal source files such as @file{foo.p} match a
specific target pattern and thereby prevent time from being wasted looking
for @file{foo.p.o} or @file{foo.p.c}.

Dummy pattern rules such as the one for @samp{%.p} are made for every
suffix listed as valid for use in suffix rules.  @xref{Suffix Rules}.

@node Canceling Rules,  , Match-Anything Rules, Pattern Rules
@subsection Canceling Implicit Rules

You can override a built-in implicit rule by defining a new pattern rule
with the same target and dependencies, but different commands.  When the
new rule is defined, the built-in one is replaced.  The new rule's position
in the sequence of implicit rules is determined by where you write the new
rule.

You can cancel a built-in implicit rule by defining a pattern rule with the
same target and dependencies, but no commands.  For example, the following
would cancel the rule that runs the assembler:

@example
%.o : %.s
@end example

@node Last Resort, Suffix Rules, Pattern Rules, Implicit
@section Defining Last-Resort Default Rules

@findex .DEFAULT
You can define a last-resort implicit rule by writing a rule for the target
@code{.DEFAULT}.  Such a rule's commands are used for all targets and
dependencies that have no commands of their own and for which no other
implicit rule applies.  Naturally, there is no @code{.DEFAULT} rule unless
you write one.

For example, when testing a makefile, you might not care if the source
files contain real data, only that they exist.  Then you might do this:

@example
.DEFAULT:
        touch $@@
@end example

@noindent
to cause all the source files needed (as dependencies) to be created
automatically.

If you give @code{.DEFAULT} with no commands or dependencies:

@example
.DEFAULT:
@end example

@noindent
the commands previously stored for @code{.DEFAULT} are cleared.
Then @code{make} acts as if you had never defined @code{.DEFAULT} at all.

If you want a target not to get the commands from @code{.DEFAULT}, but nor
do you want any commands to be run for the target, you can give it empty
commands.  @xref{Empty Commands}.

@node Suffix Rules, Search Algorithm, Last Resort, Implicit
@section Old-Fashioned Suffix Rules
@cindex suffix rules

@dfn{Suffix rules} are the old-fashioned way of defining implicit rules for
@code{make}.  Suffix rules are obsolete because pattern rules are more
general and clearer.  They are supported in GNU @code{make} for
compatibility with old makefiles.  They come in two kinds:
@dfn{double-suffix} and @dfn{single-suffix}.@refill

A double-suffix rule is defined by a pair of suffixes: the target suffix
and the source suffix.  It matches any file whose name ends with the target
suffix.  The corresponding implicit dependency is to the file name made by
replacing the target suffix with the source suffix.  A two-suffix rule
whose target and source suffixes are @samp{.o} and @samp{.c} is equivalent
to the pattern rule @samp{%.o : %.c}.

A single-suffix rule is defined by a single suffix, which is the source
suffix.  It matches any file name, and the corresponding implicit
dependency name is made by appending the source suffix.  A single-suffix
rule whose source suffix is @samp{.c} is equivalent to the pattern rule
@samp{% : %.c}.

Suffix rule definitions are recognized by comparing each rule's target
against a defined list of known suffixes.  When @code{make} sees a rule
whose target is a known suffix, this rule is considered a single-suffix
rule.  When @code{make} sees a rule whose target is two known suffixes
concatenated, this rule is taken as a double-suffix rule.

For example, @samp{.c} and @samp{.o} are both on the default list of known
suffixes.  Therefore, if you define a rule whose target is @samp{.c.o},
@code{make} takes it to be a double-suffix rule with source suffix
@samp{.c} and target suffix @samp{.o}.  For example, here is the old
fashioned way to define the rule for compiling a C source:@refill

@example
.c.o:
        $(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@@ $<
@end example

Suffix rules cannot have any dependencies of their own.  If they have any,
they are treated as normal files with funny names, not as suffix rules.
Thus, the rule:

@example
.c.o: foo.h
        $(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@@ $<
@end example

@noindent
tells how to make the file @file{.c.o} from the dependency file
@file{foo.h}, and is not at all like the pattern rule:

@example
%.o: %.c foo.h
        $(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@@ $<
@end example

@noindent
which tells how to make @code{.o} files from @code{.c} files, and makes all
@code{.o} files using this pattern rule also depend on @file{foo.h}.

Suffix rules with no commands are also meaningless.  They do not remove
previous rules as do pattern rules with no commands (@pxref{Canceling
Rules}).  They simply enter the suffix or pair of suffixes concatenated as
a target in the data base.@refill

@findex .SUFFIXES
The known suffixes are simply the names of the dependencies of the special
target @code{.SUFFIXES}.  You can add your own suffixes by writing a rule
for @code{.SUFFIXES} that adds more dependencies, as in:

@example
.SUFFIXES: .hack .win
@end example

@noindent
which adds @samp{.hack} and @samp{.win} to the end of the list of suffixes.

If you wish to eliminate the default known suffixes instead of just adding
to them, write a rule for @code{.SUFFIXES} with no dependencies.  By
special dispensation, this eliminates all existing dependencies of
@code{.SUFFIXES}.  You can then write another rule to add the suffixes you
want.  For example,

@example
.SUFFIXES:    # @r{Delete the default suffixes}
.SUFFIXES: .c .o .h   # @r{Define our suffix list}
@end example

The @samp{-r} flag causes the default list of suffixes to be empty.

@vindex SUFFIXES
The variable @code{SUFFIXES} is defined to the default list of suffixes
before @code{make} reads any makefiles.  You can change the list of suffixes
with a rule for the special target @code{.SUFFIXES}, but that does not alter
this variable.

@node Search Algorithm,  , Suffix Rules, Implicit
@section Implicit Rule Search Algorithm

Here is the procedure @code{make} uses for searching for an implicit rule
for a target @var{t}.  This procedure is followed for each double-colon
rule with no commands, for each target of ordinary rules none of which have
commands, and for each dependency that is not the target of any rule.  It
is also followed recursively for dependencies that come from implicit
rules, in the search for a chain of rules.

Suffix rules are not mentioned in this algorithm because suffix rules are
converted to equivalent pattern rules once the makefiles have been read in.

For an archive member target of the form
@samp{@var{archive}(@var{member})}, the following algorithm is run twice,
first using @samp{(@var{member})} as the target @var{t}, and second using
the entire target if the first run found no rule.@refill

@enumerate
@item
Split @var{t} into a directory part, called @var{d}, and the rest,
called @var{n}.  For example, if @var{t} is @samp{src/foo.o}, then
@var{d} is @samp{src/} and @var{n} is @samp{foo.o}.@refill

@item
Make a list of all the pattern rules one of whose targets matches
@var{t} or @var{n}.  If the target pattern contains a slash, it is
matched against @var{t}; otherwise, against @var{n}.

@item
If any rule in that list is @emph{not} a match-anything rule, then
remove all nonterminal match-anything rules from the list.

@item
Remove from the list all rules with no commands.

@item
For each pattern rule in the list:

@enumerate
@item
Find the stem @var{s}, which is the nonempty part of @var{t} or @var{n}
matched by the @samp{%} in the target pattern.@refill

@item
Compute the dependency names by substituting @var{s} for @samp{%}; if
the target pattern does not contain a slash, append @var{d} to
the front of each dependency name.@refill

@item
Test whether all the dependencies exist or ought to exist.  (If a
file name is mentioned in the makefile as a target or as an explicit
dependency then we say it ought to exist.)

If all dependencies exist or ought to exist, or there are no dependencies,
then this rule applies.
@end enumerate

@item
If no pattern rule has been found so far, try harder.
For each pattern rule in the list:

@enumerate
@item
If the rule is terminal, ignore it and go on to the next rule.

@item
Compute the dependency names as before.

@item
Test whether all the dependencies exist or ought to exist.

@item
For each dependency that does not exist, follow this algorithm
recursively to see if the dependency can be made by an implicit
rule.

@item
If all dependencies exist, ought to exist, or can be
made by implicit rules, then this rule applies.
@end enumerate

@item
If no implicit rule applies, the rule for @code{.DEFAULT}, if any,
applies.  In that case, give @var{t} the same commands that
@code{.DEFAULT} has.  Otherwise, there are no commands for @var{t}.
@end enumerate

Once a rule that applies has been found, for each target pattern of the
rule other than the one that matched @var{t} or @var{n}, the @samp{%} in
the pattern is replaced with @var{s} and the resultant file name is stored
until the commands to remake the target file @var{t} are executed.  After
these commands are executed, each of these stored file names are entered
into the data base and marked as having been updated and having the same
update status as the file @var{t}.

When the commands of a pattern rule are executed for @var{t}, the automatic
variables are set corresponding to the target and dependencies.
@xref{Automatic}.

@node Archives, Features, Implicit, Top
@chapter Using @code{make} to Update Archive Files
@cindex archive

@dfn{Archive files} are files containing named subfiles called
@dfn{members}; they are maintained with the program @code{ar} and their
main use is as subroutine libraries for linking.

@menu
* Members: Archive Members.    How to name an archive member
                                as a target or dependency.
* Update: Archive Update.      An implicit rule can update
                                most archive member targets just right.
* Symbols: Archive Symbols.    Special things to do for library archives.
@end menu

@node Archive Members, Archive Update, Archives, Archives
@section Archive Members as Targets
@cindex archive member targets

An individual member of an archive file can be used as a target or
dependency in @code{make}.  The archive file must already exist, but the
member need not exist.  You specify the member named @var{member} in
archive file @var{archive} as follows:

@example
@var{archive}(@var{member})
@end example

@noindent
This construct is available only in targets and dependencies, not in
commands!  Most programs that you might use in commands do not support this
syntax and cannot act directly on archive members.  Only @code{ar} and
other programs specifically designed to operate on archives can do so.
Therefore, valid commands to update an archive member target probably must
use @code{ar}.  For example, this rule says to create a member
@file{hack.o} in archive @file{foolib} by copying the file @file{hack.o}:

@example
foolib(hack.o) : hack.o
        ar r foolib hack.o
@end example

In fact, nearly all archive member targets are updated in just this way
and there is an implicit rule to do it for you.

@node Archive Update, Archive Symbols, Archive Members, Archives
@section Implicit Rule for Archive Member Targets

Recall that a target that looks like @file{@var{a}(@var{m})} stands for the
member named @var{m} in the archive file @var{a}.

When @code{make} looks for an implicit rule for such a target, as a special
feature it considers implicit rules that match @file{(@var{m})}, as well as
those that match the actual target @file{@var{a}(@var{m})}.

This causes one special rule whose target is @file{(%)} to match.  This
rule updates the target @file{@var{a}(@var{m})} by copying the file @var{m}
into the archive.  For example, it will update the archive member target
@file{foo.a(bar.o)} by copying the @emph{file} @file{bar.o} into the
archive @file{foo.a} as a @emph{member} named @file{bar.o}.

When this rule is chained with others, the result is very powerful.  Thus,
@samp{make "foo.a(bar.o)"} in the presence of a file @file{bar.c} is enough
to cause the following commands to be run, even without a makefile:

@example
cc -c bar.c -o bar.o
ar r foo.a bar.o
rm -f bar.o
@end example

@noindent
Here @code{make} has envisioned the file @file{bar.o} as an intermediate
file.

Implicit rules such as this one are written using the automatic variable
@samp{$%}.  @xref{Automatic}.

An archive member name in an archive cannot contain a directory name, but
it may be useful in a makefile to pretend that it does.  If you write an
archive member target @file{foo.a(dir/file.o)}, @code{make} will perform
automatic updating with this command:

@example
ar r foo.a dir/file.o
@end example

@noindent
which has the effect of copying the file @file{dir/foo.o} into a member
named @file{foo.o}.  In connection with such usage, the automatic variables
@code{%D} and @code{%F} may be useful.

@node Archive Symbols, , Archive Update, Archives
@subsection Updating Archive Symbol Directories
@cindex __.SYMDEF

An archive file that is used as a library usually contains a special member
named @file{__.SYMDEF} that contains a directory of the external symbol
names defined by all the other members.  After you update any other
members, you need to update @file{__.SYMDEF} so that it will summarize the
other members properly.  This is done by running the @code{ranlib} program:

@example
ranlib @var{archivefile}
@end example

Normally you would put this command in the rule for the archive file,
and make all the members of the archive file dependents of that rule.
For example,

@example
libfoo.a: libfoo.a(x.o) libfoo.a(y.o) @dots{}
        ranlib libfoo.a
@end example

@noindent
The effect of this is to update archive members @file{x.o}, @file{y.o},
etc., and then update the symbol directory member @file{__.SYMDEF} by
running @code{ranlib}.  The rules for updating the members are not shown
here; most likely you can omit them and use the implicit rule which copies
files into the archive, as described in the preceding section.

This is not necessary when using the GNU @code{ar} program, which
updates the @file{__.SYMDEF} member automatically.

@node Features, Missing, Archives, Top
@chapter Features of GNU @code{make}

Here is a summary of the features of GNU @code{make}, for comparison
with and credit to other versions of @code{make}.  We consider the features
of @code{make} in BSD 4.2 systems as a baseline.

Many features come from the version of @code{make} in System V.

@itemize @bullet
@item
The @code{VPATH} variable and its special meaning.  @xref{Directory
Search}.  This feature exists in System V @code{make}, but is undocumented.
It is documented in 4.3 BSD @code{make} (which says it mimics System V's
@code{VPATH} feature).@refill

@item
Included makefiles.  @xref{Include}.

@item
Variables are read from and communicated via the environment.
@xref{Environment}.

@item
Options passed through the variable @code{MAKEFLAGS} to recursive
invocations of @code{make}.  @xref{Options/Recursion}.

@item
The automatic variable @code{$%} is set to the member name
in an archive reference.  @xref{Automatic}.

@item
The automatic variables @code{$@@}, @code{$*}, @code{$<} and @code{$%}
have corresponding forms like @code{$(@@F)} and @code{$(@@D)}.
@xref{Automatic}.@refill

@item
Substitution variable references.  @xref{Reference}.

@item
The command-line options @samp{-b} and @samp{-m}, accepted and ignored.

@item
Execution of recursive commands to run @code{make} via the variable
@code{MAKE} even if @samp{-n}, @samp{-q} or @samp{-t} is specified.
@xref{Recursion}.

@item
Support for suffix @samp{.a} in suffix rules.  In GNU @code{make},
this is actually implemented by chaining with one pattern rule for
installing members in an archive.  @xref{Chained Rules}.@refill

@item
The arrangement of lines and backslash-newline combinations in
commands is retained when the commands are printed, so they appear as
they do in the makefile, except for the stripping of initial
whitespace.
@end itemize

The following features were inspired by various other versions of
@code{make}.  In some cases it is unclear exactly which versions inspired
which others.

@itemize @bullet
@item
Pattern rules using @samp{%}.
This has been implemented in several versions of @code{make}.
We're not sure who invented it first, but it's been spread around a bit.
@xref{Pattern Rules}.@refill

@item
Rule chaining and implicit intermediate files.
This was implemented by Stu Feldman in his version of @code{make}
for AT&T Eighth Edition Research Unix, and later by Andrew Hume of
AT&T Bell Labs in his @code{mk} program.  We don't really know if
we got this from either of them or thought it up ourselves at the
same time.  @xref{Chained Rules}.

@item
The automatic variable @code{$^} containing a list of all dependencies
of the current target.  We didn't invent this, but we have no idea who did.
@xref{Automatic}.

@item
The ``what if'' flag (@samp{-W} in GNU @code{make}) was (as far as we know)
invented by Andrew Hume in @code{mk}.  @xref{Instead of Execution}.

@item
The concept of doing several things at once (parallelism) exists in
many incarnations of @code{make} and similar programs, though not in the
System V or BSD implementations.  @xref{Execution}.

@item
Modified variable references using pattern substitution come from
SunOS 4.0.  @xref{Reference}.  This functionality was provided in GNU
@code{make} by the @code{patsubst} function before the alternate syntax
was implemented for compatibility with SunOS 4.0.  It is not altogether
clear who inspired whom, since GNU @code{make} had @code{patsubst}
before SunOS 4.0 was released.@refill

@item
The special significance of @samp{+} characters preceding command lines
(@pxref{Instead of Execution}) is mandated by draft 8 of IEEE Std 1003.2
(POSIX).@refill
@end itemize

The remaining features are inventions new in GNU @code{make}:

@itemize @bullet
@item
The @samp{-v} option to print version and copyright information.

@item
Simply-expanded variables.  @xref{Flavors}.

@item
Passing command-line variable assignments automatically through the
variable @code{MAKE} to recursive @code{make} invocations.
@xref{Recursion}.

@item
The @samp{-C} command option to change directory.  @xref{Options}.

@item
Verbatim variable definitions made with @code{define}.  @xref{Defining}.

@item
Phony targets declared with the special target @code{.PHONY}.
A similar feature with a different syntax was implemented by
Andrew Hume of AT&T Bell Labs in his @code{mk} program.  This
seems to be a case of parallel discovery.  @xref{Phony Targets}.

@item
Text manipulation by calling functions.  @xref{Functions}.

@item
The @samp{-o} option to pretend a file's modification-time is old.
@xref{Avoiding Compilation}.

@item
Conditional execution.
This has been implemented numerous times in various versions of
@code{make}; it seems a natural extension derived from the features
of the C preprocessor and similar macro languages and is not a
revolutionary concept.  @xref{Conditionals}.

@item
The included makefile search path.  @xref{Include}.

@item
Specifying extra makefiles to read.  @xref{MAKEFILES Variable}.

@item
Stripping leading sequences of @samp{./} from file names, so that
@file{./@var{file}} and @file{@var{file}} are considered to be the
same file.@refill

@item
Special search method for library dependencies written in the form
@samp{-l@var{name}}.  @xref{Libraries/Search}.

@item
Allowing suffixes for suffix rules (@pxref{Suffix Rules}) to contain
any characters.  In other version of @code{make}, they must begin with
@samp{.} and not contain any @samp{/} characters.

@item
The variable @code{MAKELEVEL} which keeps track of the current level
of @code{make} recursion.  @xref{Recursion}.

@item
Static pattern rules.  @xref{Static Pattern}.

@item
Selective @code{vpath} search.  @xref{Directory Search}.

@item
Recursive variable references.  @xref{Reference}.

@item
Updated makefiles.  @xref{Remaking Makefiles}.
System V @code{make} has a very, very limited form of this
functionality in that it will check out SCCS files for makefiles.

@item
Several new built-in implicit rules.  @xref{Catalogue of Rules}.
@end itemize

@node Missing, Concept Index, Features, Top
@chapter Missing Features in GNU @code{make}

The @code{make} programs in various other systems support a few features
that are not implemented in GNU @code{make}.  Draft 11.1 of the POSIX.2
standard which specifies @code{make} does not require any of these
features.@refill

@itemize @bullet
@item
A target of the form @samp{@var{file}((@var{entry}))} stands for a member
of archive file @var{file}.  The member is chosen, not by name, but by
being an object file which defines the linker symbol @var{entry}.@refill

This feature was not put into GNU @code{make} because of the
nonmodularity of putting knowledge into @code{make} of the internal
format of archive file symbol directories.  @xref{Archive Symbols}.

@item
Suffixes (used in suffix rules) that end with the character @samp{~}
have a special meaning; they refer to the SCCS file that corresponds
to the file one would get without the @samp{~}.  For example, the
suffix rule @samp{.c~.o} would make the file @file{@var{n}.o} file from
the SCCS file @file{s.@var{n}.c}.  For complete coverage, a whole
series of such suffix rules is required.  @xref{Suffix Rules}.@refill

In GNU @code{make}, this entire series of cases is handled by two
pattern rules for extraction from SCCS, in combination with the
general feature of rule chaining.  @xref{Chained Rules}.

@item
In System V @code{make}, the string @samp{$$@@} has the strange meaning
that, in the dependencies of a rule with multiple targets, it stands
for the particular target that is being processed.

This is not defined in GNU @code{make} because @samp{$$} should always
stand for an ordinary @samp{$}.

It is possible to get this functionality through the use of static pattern
rules (@pxref{Static Pattern}).  The System V @code{make} rule:

@example
$(targets): $$@@.o lib.a
@end example

@noindent
can be replaced with the GNU @code{make} static pattern rule:

@example
$(targets): %: %.o lib.a
@end example

@item
In System V and 4.3 BSD @code{make}, files found by @code{VPATH} search
(@pxref{Directory Search}) have their names changed inside command
strings.  We feel it is much cleaner to always use automatic variables
and thus make this feature obsolete.@refill

@item
In some Unix @code{make}s, implicit rule search (@pxref{Implicit}) is
apparently done for @emph{all} targets, not just those without commands.
This means you can do:@refill

@example
foo.o:
        cc -c foo.c
@end example

@noindent
and Unix @code{make} will intuit that @file{foo.o} depends on
@file{foo.c}.@refill

We feel that such usage is broken.  The dependency properties of
@code{make} are well-defined (for GNU @code{make}, at least),
and doing such a thing simply does not fit the model.@refill
@end itemize

@node Concept Index, Name Index, Missing, Top
@unnumbered Index of Concepts

@printindex cp

@node Name Index,  , Concept Index, Top
@unnumbered Index of Functions, Variables, and Directives

@printindex fn

@summarycontents
@contents
@bye