This is Info file make.info, produced by Makeinfo version 1.67 from the
input file make.texinfo.

INFO-DIR-SECTION The GNU make utility
START-INFO-DIR-ENTRY
* Make: (make.info).               The GNU make utility.
END-INFO-DIR-ENTRY

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

   This is Edition 0.52, last updated 20 May 1998, of `The GNU Make
Manual', for `make', Version 3.77.

   Copyright (C) 1988, '89, '90, '91, '92, '93, '94, '95, '96, '97, '98
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.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, 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 this permission notice may be stated in a
translation approved by the Free Software Foundation.


File: make.info,  Node: Origin Function,  Next: Shell Function,  Prev: Foreach Function,  Up: Functions

The `origin' Function
=====================

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

   The syntax of the `origin' function is:

     $(origin VARIABLE)

   Note that VARIABLE is the *name* of a variable to inquire about; not
a *reference* to that variable.  Therefore you would not normally use a
`$' 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
VARIABLE was defined:

`undefined'
     if VARIABLE was never defined.

`default'
     if VARIABLE has a default definition, as is usual with `CC' and so
     on.  *Note Variables Used by Implicit Rules: Implicit Variables.
     Note that if you have redefined a default variable, the `origin'
     function will return the origin of the later definition.

`environment'
     if VARIABLE was defined as an environment variable and the `-e'
     option is *not* turned on (*note Summary of Options: Options
     Summary.).

`environment override'
     if VARIABLE was defined as an environment variable and the `-e'
     option *is* turned on (*note Summary of Options: Options Summary.).

`file'
     if VARIABLE was defined in a makefile.

`command line'
     if VARIABLE was defined on the command line.

`override'
     if VARIABLE was defined with an `override' directive in a makefile
     (*note The `override' Directive: Override Directive.).

`automatic'
     if VARIABLE is an automatic variable defined for the execution of
     the commands for each rule (*note Automatic Variables: Automatic.).

   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 `foo' that includes another
makefile `bar'.  You want a variable `bletch' to be defined in `bar' if
you run the command `make -f bar', even if the environment contains a
definition of `bletch'.  However, if `foo' defined `bletch' before
including `bar', you do not want to override that definition.  This
could be done by using an `override' directive in `foo', giving that
definition precedence over the later definition in `bar';
unfortunately, the `override' directive would also override any command
line definitions.  So, `bar' could include:

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

If `bletch' has been defined from the environment, this will redefine
it.

   If you want to override a previous definition of `bletch' if it came
from the environment, even under `-e', you could instead write:

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

   Here the redefinition takes place if `$(origin bletch)' returns
either `environment' or `environment override'.  *Note Functions for
String Substitution and Analysis: Text Functions.


File: make.info,  Node: Shell Function,  Prev: Origin Function,  Up: Functions

The `shell' Function
====================

   The `shell' function is unlike any other function except the
`wildcard' function (*note The Function `wildcard': Wildcard Function.)
in that it communicates with the world outside of `make'.

   The `shell' function performs the same function that backquotes
(``') perform in most shells: it does "command expansion".  This means
that it takes an argument that is a shell command and returns the
output of the command.  The only processing `make' does on the result,
before substituting it into the surrounding text, is to convert each
newline or carriage-return / newline pair to a single space.  It also
removes the trailing (carriage-return and) newline, if it's the last
thing in the result.

   The commands run by calls to the `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
`shell' function calls like all others.

   Here are some examples of the use of the `shell' function:

     contents := $(shell cat foo)

sets `contents' to the contents of the file `foo', with a space (rather
than a newline) separating each line.

     files := $(shell echo *.c)

sets `files' to the expansion of `*.c'.  Unless `make' is using a very
strange shell, this has the same result as `$(wildcard *.c)'.


File: make.info,  Node: Running,  Next: Implicit Rules,  Prev: Functions,  Up: Top

How to Run `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.  Usually, makefiles are written so that if you run `make' with
no arguments, it does just that.

   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 giving arguments when you run `make', you can do any of these
things and many others.

   The exit status of `make' is always one of three values:
`0'
     The exit status is zero if `make' is successful.

`2'
     The exit status is two if `make' encounters any errors.  It will
     print messages describing the particular errors.

`1'
     The exit status is one if you use the `-q' flag and `make'
     determines that some target is not already up to date.  *Note
     Instead of Executing the Commands: Instead of Execution.

* Menu:

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


File: make.info,  Node: Makefile Arguments,  Next: Goals,  Up: Running

Arguments to Specify the Makefile
=================================

   The way to specify the name of the makefile is with the `-f' or
`--file' option (`--makefile' also works).  For example, `-f altmake'
says to use the file `altmake' as the makefile.

   If you use the `-f' flag several times and follow each `-f' with an
argument, all the specified files are used jointly as makefiles.

   If you do not use the `-f' or `--file' flag, the default is to try
`GNUmakefile', `makefile', and `Makefile', in that order, and use the
first of these three which exists or can be made (*note Writing
Makefiles: Makefiles.).


File: make.info,  Node: Goals,  Next: Instead of Execution,  Prev: Makefile Arguments,  Up: Running

Arguments to Specify the Goals
==============================

   The "goals" are the targets that `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.  If the first rule in the makefile
has several targets, only the first target in the rule becomes the
default goal, not the whole list.

   You can specify a different goal or goals with arguments to `make'.
Use the name of the goal as an argument.  If you specify several goals,
`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 `-' or contains an `=', in which case it will be parsed as
a switch or variable definition, respectively).  Even targets not in
the makefile may be specified, if `make' can find implicit rules that
say how to make them.

   `Make' will set the special variable `MAKECMDGOALS' to the list of
goals you specified on the command line.  If no goals were given on the
command line, this variable is empty.  Note that this variable should
be used only in special circumstances.

   An example of appropriate use is to avoid including `.d' files
during `clean' rules (*note Automatic Dependencies::.), so `make' won't
create them only to immediately remove them again:

     sources = foo.c bar.c
     
     ifneq ($(MAKECMDGOALS),clean)
     include $(sources:.c=.d)
     endif

   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:

     .PHONY: all
     all: size nm ld ar as

   If you are working on the program `size', you might want to say
`make size' so that only the files of that program are recompiled.

   Another use of specifying a goal is to make files that are not
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 is not a dependency of the default
goal.

   Another use of specifying a goal is to run the commands associated
with a phony target (*note Phony Targets::.) or empty target (*note
Empty Target Files to Record Events: Empty Targets.).  Many makefiles
contain a phony target named `clean' which deletes everything except
source files.  Naturally, this is done only if you request it
explicitly with `make clean'.  Following is a list of typical phony and
empty target names.  *Note Standard Targets::, for a detailed list of
all the standard target names which GNU software packages use.

`all'
     Make all the top-level targets the makefile knows about.

`clean'
     Delete all files that are normally created by running `make'.

`mostlyclean'
     Like `clean', but may refrain from deleting a few files that people
     normally don't want to recompile.  For example, the `mostlyclean'
     target for GCC does not delete `libgcc.a', because recompiling it
     is rarely necessary and takes a lot of time.

`distclean'
`realclean'
`clobber'
     Any of these targets might be defined to delete *more* files than
     `clean' does.  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.

`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.

`print'
     Print listings of the source files that have changed.

`tar'
     Create a tar file of the source files.

`shar'
     Create a shell archive (shar file) of the source files.

`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.

`TAGS'
     Update a tags table for this program.

`check'
`test'
     Perform self tests on the program this makefile builds.


File: make.info,  Node: Instead of Execution,  Next: Avoiding Compilation,  Prev: Goals,  Up: Running

Instead of Executing the Commands
=================================

   The makefile tells `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 `make'.

`-n'
`--just-print'
`--dry-run'
`--recon'
     "No-op".  The activity is to print what commands would be used to
     make the targets up to date, but not actually execute them.

`-t'
`--touch'
     "Touch".  The activity is to mark the targets as up to date without
     actually changing them.  In other words, `make' pretends to compile
     the targets but does not really change their contents.

`-q'
`--question'
     "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.

`-W FILE'
`--what-if=FILE'
`--assume-new=FILE'
`--new-file=FILE'
     "What if".  Each `-W' flag is followed by a file name.  The given
     files' modification times are recorded by `make' as being the
     present time, although the actual modification times remain the
     same.  You can use the `-W' flag in conjunction with the `-n' flag
     to see what would happen if you were to modify specific files.

   With the `-n' flag, `make' prints the commands that it would
normally execute but does not execute them.

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

   With the `-q' flag, `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.  If the exit status is one,
then some updating needs to be done.  If `make' encounters an error,
the exit status is two, so you can distinguish an error from a target
that is not up to date.

   It is an error to use more than one of these three flags in the same
invocation of `make'.

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

   The `-W' flag provides two features:

   * If you also use the `-n' or `-q' flag, you can see what `make'
     would do if you were to modify some files.

   * Without the `-n' or `-q' flag, when `make' is actually executing
     commands, the `-W' flag can direct `make' to act as if some files
     had been modified, without actually modifying the files.

   Note that the options `-p' and `-v' allow you to obtain other
information about `make' or about the makefiles in use (*note Summary
of Options: Options Summary.).


File: make.info,  Node: Avoiding Compilation,  Next: Overriding,  Prev: Instead of Execution,  Up: Running

Avoiding Recompilation of Some Files
====================================

   Sometimes you may have changed a source file but you do not 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, `make' assumes that any change in the
header file requires recompilation of all dependent files, but you know
that they do not 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 `-t' flag.  This flag tells `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:

  1. Use the command `make' to recompile the source files that really
     need recompilation.

  2. Make the changes in the header files.

  3. Use the command `make -t' to mark all the object files as up to
     date.  The next time you run `make', the changes in the header
     files will not cause any recompilation.

   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 `-o FILE' flag, which marks a specified file as "old" (*note
Summary of Options: Options Summary.).  This means that the file itself
will not be remade, and nothing else will be remade on its account.
Follow this procedure:

  1. Recompile the source files that need compilation for reasons
     independent of the particular header file, with `make -o
     HEADERFILE'.  If several header files are involved, use a separate
     `-o' option for each header file.

  2. Touch all the object files with `make -t'.


File: make.info,  Node: Overriding,  Next: Testing,  Prev: Avoiding Compilation,  Up: Running

Overriding Variables
====================

   An argument that contains `=' specifies the value of a variable:
`V=X' sets the value of the variable V to 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 "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
`CFLAGS' is included in each command that runs the C compiler, so a
file `foo.c' would be compiled something like this:

     cc -c $(CFLAGS) foo.c

   Thus, whatever value you set for `CFLAGS' affects each compilation
that occurs.  The makefile probably specifies the usual value for
`CFLAGS', like this:

     CFLAGS=-g

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

   The variable `CFLAGS' is only one of many standard variables that
exist just so that you can change them this way.  *Note Variables Used
by Implicit Rules: 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 `:=' instead of `='.  But, unless you
want to include a variable reference or function call in the *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 `override' directive, which is a
line that looks like this: `override VARIABLE = VALUE' (*note The
`override' Directive: Override Directive.).


File: make.info,  Node: Testing,  Next: Options Summary,  Prev: Overriding,  Up: Running

Testing the Compilation of a Program
====================================

   Normally, when an error happens in executing a shell command, `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 `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 `make' try compiling
every file that can be tried, to show you as many compilation errors as
possible.

   On these occasions, you should use the `-k' or `--keep-going' flag.
This tells `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, `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, `make -k' will
continue as much as possible after discovering that it does not know
how to make a target or dependency file.  This will always cause an
error message, but without `-k', it is a fatal error (*note Summary of
Options: Options Summary.).

   The usual behavior of `make' assumes that your purpose is to get the
goals up to date; once `make' learns that this is impossible, it might
as well report the failure immediately.  The `-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'
`M-x compile' command passes the `-k' flag by default.


File: make.info,  Node: Options Summary,  Prev: Testing,  Up: Running

Summary of Options
==================

   Here is a table of all the options `make' understands:

`-b'
`-m'
     These options are ignored for compatibility with other versions of
     `make'.

`-C DIR'
`--directory=DIR'
     Change to directory DIR before reading the makefiles.  If multiple
     `-C' options are specified, each is interpreted relative to the
     previous one: `-C / -C etc' is equivalent to `-C /etc'.  This is
     typically used with recursive invocations of `make' (*note
     Recursive Use of `make': Recursion.).

`-d'
`--debug'
     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 `make' decides what to do.

`-e'
`--environment-overrides'
     Give variables taken from the environment precedence over
     variables from makefiles.  *Note Variables from the Environment:
     Environment.

`-f FILE'
`--file=FILE'
`--makefile=FILE'
     Read the file named FILE as a makefile.  *Note Writing Makefiles:
     Makefiles.

`-h'
`--help'
     Remind you of the options that `make' understands and then exit.

`-i'
`--ignore-errors'
     Ignore all errors in commands executed to remake files.  *Note
     Errors in Commands: Errors.

`-I DIR'
`--include-dir=DIR'
     Specifies a directory DIR to search for included makefiles.  *Note
     Including Other Makefiles: Include.  If several `-I' options are
     used to specify several directories, the directories are searched
     in the order specified.

`-j [JOBS]'
`--jobs=[JOBS]'
     Specifies the number of jobs (commands) to run simultaneously.
     With no argument, `make' runs as many jobs simultaneously as
     possible.  If there is more than one `-j' option, the last one is
     effective.  *Note Parallel Execution: Parallel, for more
     information on how commands are run.  Note that this option is
     ignored on MS-DOS.

`-k'
`--keep-going'
     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.
     *Note Testing the Compilation of a Program: Testing.

`-l [LOAD]'
`--load-average[=LOAD]'
`--max-load[=LOAD]'
     Specifies that no new jobs (commands) should be started if there
     are other jobs running and the load average is at least LOAD (a
     floating-point number).  With no argument, removes a previous load
     limit.  *Note Parallel Execution: Parallel.

`-n'
`--just-print'
`--dry-run'
`--recon'
     Print the commands that would be executed, but do not execute them.
     *Note Instead of Executing the Commands: Instead of Execution.

`-o FILE'
`--old-file=FILE'
`--assume-old=FILE'
     Do not remake the file FILE even if it is older than its
     dependencies, and do not remake anything on account of changes in
     FILE.  Essentially the file is treated as very old and its rules
     are ignored.  *Note Avoiding Recompilation of Some Files: Avoiding
     Compilation.

`-p'
`--print-data-base'
     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
     `-v' switch (see below).  To print the data base without trying to
     remake any files, use `make -p -f /dev/null'.

`-q'
`--question'
     "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, one if any remaking is required, or two if an
     error is encountered.  *Note Instead of Executing the Commands:
     Instead of Execution.

`-r'
`--no-builtin-rules'
     Eliminate use of the built-in implicit rules (*note Using Implicit
     Rules: Implicit Rules.).  You can still define your own by writing
     pattern rules (*note Defining and Redefining Pattern Rules:
     Pattern Rules.).  The `-r' option also clears out the default list
     of suffixes for suffix rules (*note Old-Fashioned Suffix Rules:
     Suffix Rules.).  But you can still define your own suffixes with a
     rule for `.SUFFIXES', and then define your own suffix rules.  Note
     that only *rules* are affected by the `-r' option; default
     variables remain in effect (*note Variables Used by Implicit
     Rules: Implicit Variables.).

`-s'
`--silent'
`--quiet'
     Silent operation; do not print the commands as they are executed.
     *Note Command Echoing: Echoing.

`-S'
`--no-keep-going'
`--stop'
     Cancel the effect of the `-k' option.  This is never necessary
     except in a recursive `make' where `-k' might be inherited from
     the top-level `make' via `MAKEFLAGS' (*note Recursive Use of
     `make': Recursion.) or if you set `-k' in `MAKEFLAGS' in your
     environment.

`-t'
`--touch'
     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
     `make'.  *Note Instead of Executing the Commands: Instead of
     Execution.

`-v'
`--version'
     Print the version of the `make' program plus a copyright, a list
     of authors, and a notice that there is no warranty; then exit.

`-w'
`--print-directory'
     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 `make' commands.
     *Note Recursive Use of `make': Recursion.  (In practice, you
     rarely need to specify this option since `make' does it for you;
     see *Note The `--print-directory' Option: -w Option.)

`--no-print-directory'
     Disable printing of the working directory under `-w'.  This option
     is useful when `-w' is turned on automatically, but you do not
     want to see the extra messages.  *Note The `--print-directory'
     Option: -w Option.

`-W FILE'
`--what-if=FILE'
`--new-file=FILE'
`--assume-new=FILE'
     Pretend that the target FILE has just been modified.  When used
     with the `-n' flag, this shows you what would happen if you were
     to modify that file.  Without `-n', it is almost the same as
     running a `touch' command on the given file before running `make',
     except that the modification time is changed only in the
     imagination of `make'.  *Note Instead of Executing the Commands:
     Instead of Execution.

`--warn-undefined-variables'
     Issue a warning message whenever `make' sees a reference to an
     undefined variable.  This can be helpful when you are trying to
     debug makefiles which use variables in complex ways.


File: make.info,  Node: Implicit Rules,  Next: Archives,  Prev: Running,  Up: Top

Using Implicit Rules
********************

   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, `cc'.

   "Implicit rules" tell `make' how to use customary techniques so that
you do not have to specify them in detail when you want to use them.
For example, there is an implicit rule for C compilation.  File names
determine which implicit rules are run.  For example, C compilation
typically takes a `.c' file and makes a `.o' file.  So `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, `make'
will remake a `.o' file from a `.y' file by way of a `.c' file.

   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 `CFLAGS'
controls the flags given to the C compiler by the implicit rule for C
compilation.

   You can define your own implicit rules by writing "pattern rules".

   "Suffix rules" are a more limited way to define implicit rules.
Pattern rules are more general and clearer, but suffix rules are
retained for compatibility.

* Menu:

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


File: make.info,  Node: Using Implicit,  Next: Catalogue of Rules,  Up: Implicit Rules

Using Implicit Rules
====================

   To allow `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 `make' will figure out which implicit rule to use based on
which kind of source file exists or can be made.

   For example, suppose the makefile looks like this:

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

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

   If an implicit rule is found, it can supply both commands and one or
more dependencies (the source files).  You would want to write a rule
for `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 `.o' files: one, from a `.c' file with the
C compiler; another, from a `.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 `foo.c', `make' will run the C
compiler; otherwise, if you have a file `foo.p', `make' will run the
Pascal compiler; and so on.

   Of course, when you write the makefile, you know which implicit rule
you want `make' to use, and you know it will choose that one because you
know which possible dependency files are supposed to exist.  *Note
Catalogue of Implicit Rules: 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
"chaining" is occurring.  *Note Chains of Implicit Rules: Chained Rules.

   In general, `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.  *Note
Implicit Rule Search Algorithm: Implicit Rule Search, for the details
of how the search is done.

   Note that explicit dependencies do not influence implicit rule
search.  For example, consider this explicit rule:

     foo.o: foo.p

The dependency on `foo.p' does not necessarily mean that `make' will
remake `foo.o' according to the implicit rule to make an object file, a
`.o' file, from a Pascal source file, a `.p' file.  For example, if
`foo.c' also exists, the implicit rule to make an object file from a C
source file is used instead, because it appears before the Pascal rule
in the list of predefined implicit rules (*note Catalogue of Implicit
Rules: Catalogue of Rules.).

   If you do not 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 (*note Defining Empty Commands: Empty Commands.).


File: make.info,  Node: Catalogue of Rules,  Next: Implicit Variables,  Prev: Using Implicit,  Up: Implicit Rules

Catalogue of Implicit Rules
===========================

   Here is a catalogue of predefined implicit rules which are always
available unless the makefile explicitly overrides or cancels them.
*Note Canceling Implicit Rules: Canceling Rules, for information on
canceling or overriding an implicit rule.  The `-r' or
`--no-builtin-rules' option cancels all predefined rules.

   Not all of these rules will always be defined, even when the `-r'
option is not given.  Many of the predefined implicit rules are
implemented in `make' as suffix rules, so which ones will be defined
depends on the "suffix list" (the list of dependencies of the special
target `.SUFFIXES').  The default suffix list is: `.out', `.a', `.ln',
`.o', `.c', `.cc', `.C', `.p', `.f', `.F', `.r', `.y', `.l', `.s',
`.S', `.mod', `.sym', `.def', `.h', `.info', `.dvi', `.tex', `.texinfo',
`.texi', `.txinfo', `.w', `.ch' `.web', `.sh', `.elc', `.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.  *Note Old-Fashioned
Suffix Rules: Suffix Rules, for full details on suffix rules.

Compiling C programs
     `N.o' is made automatically from `N.c' with a command of the form
     `$(CC) -c $(CPPFLAGS) $(CFLAGS)'.

Compiling C++ programs
     `N.o' is made automatically from `N.cc' or `N.C' with a command of
     the form `$(CXX) -c $(CPPFLAGS) $(CXXFLAGS)'.  We encourage you to
     use the suffix `.cc' for C++ source files instead of `.C'.

Compiling Pascal programs
     `N.o' is made automatically from `N.p' with the command `$(PC) -c
     $(PFLAGS)'.

Compiling Fortran and Ratfor programs
     `N.o' is made automatically from `N.r', `N.F' or `N.f' by running
     the Fortran compiler.  The precise command used is as follows:

    `.f'
          `$(FC) -c $(FFLAGS)'.

    `.F'
          `$(FC) -c $(FFLAGS) $(CPPFLAGS)'.

    `.r'
          `$(FC) -c $(FFLAGS) $(RFLAGS)'.

Preprocessing Fortran and Ratfor programs
     `N.f' is made automatically from `N.r' or `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:

    `.F'
          `$(FC) -F $(CPPFLAGS) $(FFLAGS)'.

    `.r'
          `$(FC) -F $(FFLAGS) $(RFLAGS)'.

Compiling Modula-2 programs
     `N.sym' is made from `N.def' with a command of the form `$(M2C)
     $(M2FLAGS) $(DEFFLAGS)'.  `N.o' is made from `N.mod'; the form is:
     `$(M2C) $(M2FLAGS) $(MODFLAGS)'.

Assembling and preprocessing assembler programs
     `N.o' is made automatically from `N.s' by running the assembler,
     `as'.  The precise command is `$(AS) $(ASFLAGS)'.

     `N.s' is made automatically from `N.S' by running the C
     preprocessor, `cpp'.  The precise command is `$(CPP) $(CPPFLAGS)'.

Linking a single object file
     `N' is made automatically from `N.o' by running the linker
     (usually called `ld') via the C compiler.  The precise command
     used is `$(CC) $(LDFLAGS) N.o $(LOADLIBES)'.

     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),
     one of which has a name matching that of the executable file.
     Thus,

          x: y.o z.o

     when `x.c', `y.c' and `z.c' all exist will execute:

          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

     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 `.o' object files will
     be automatically linked by using the compiler (`$(CC)', `$(FC)' or
     `$(PC)'; the C compiler `$(CC)' is used to assemble `.s' files)
     without the `-c' option.  This could be done by using the `.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.

Yacc for C programs
     `N.c' is made automatically from `N.y' by running Yacc with the
     command `$(YACC) $(YFLAGS)'.

Lex for C programs
     `N.c' is made automatically from `N.l' by by running Lex.  The
     actual command is `$(LEX) $(LFLAGS)'.

Lex for Ratfor programs
     `N.r' is made automatically from `N.l' by by running Lex.  The
     actual command is `$(LEX) $(LFLAGS)'.

     The convention of using the same suffix `.l' for all Lex files
     regardless of whether they produce C code or Ratfor code makes it
     impossible for `make' to determine automatically which of the two
     languages you are using in any particular case.  If `make' is
     called upon to remake an object file from a `.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 `make'
     knows this by mentioning `N.r' in the makefile.  Or, if you are
     using Ratfor exclusively, with no C files, remove `.c' from the
     list of implicit rule suffixes with:

          .SUFFIXES:
          .SUFFIXES: .o .r .f .l ...

Making Lint Libraries from C, Yacc, or Lex programs
     `N.ln' is made from `N.c' by running `lint'.  The precise command
     is `$(LINT) $(LINTFLAGS) $(CPPFLAGS) -i'.  The same command is
     used on the C code produced from `N.y' or `N.l'.

TeX and Web
     `N.dvi' is made from `N.tex' with the command `$(TEX)'.  `N.tex'
     is made from `N.web' with `$(WEAVE)', or from `N.w' (and from
     `N.ch' if it exists or can be made) with `$(CWEAVE)'.  `N.p' is
     made from `N.web' with `$(TANGLE)' and `N.c' is made from `N.w'
     (and from `N.ch' if it exists or can be made) with `$(CTANGLE)'.

Texinfo and Info
     `N.dvi' is made from `N.texinfo', `N.texi', or `N.txinfo', with
     the command `$(TEXI2DVI) $(TEXI2DVI_FLAGS)'.  `N.info' is made from
     `N.texinfo', `N.texi', or `N.txinfo', with the command
     `$(MAKEINFO) $(MAKEINFO_FLAGS)'.

RCS
     Any file `N' is extracted if necessary from an RCS file named
     either `N,v' or `RCS/N,v'.  The precise command used is
     `$(CO) $(COFLAGS)'.  `N' will not be extracted from RCS if it
     already exists, even if the RCS file is newer.  The rules for RCS
     are terminal (*note Match-Anything Pattern Rules: Match-Anything
     Rules.), so RCS files cannot be generated from another source;
     they must actually exist.

SCCS
     Any file `N' is extracted if necessary from an SCCS file named
     either `s.N' or `SCCS/s.N'.  The precise command used is
     `$(GET) $(GFLAGS)'.  The rules for SCCS are terminal (*note
     Match-Anything Pattern Rules: Match-Anything Rules.), so SCCS
     files cannot be generated from another source; they must actually
     exist.

     For the benefit of SCCS, a file `N' is copied from `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 do not need to use this feature with RCS.

     We recommend that you avoid using 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.

   Usually, you want to change only the variables listed in the table
above, which are documented in the following section.

   However, the commands in built-in implicit rules actually use
variables such as `COMPILE.c', `LINK.p', and `PREPROCESS.S', whose
values contain the commands listed above.

   `make' follows the convention that the rule to compile a `.X' source
file uses the variable `COMPILE.X'.  Similarly, the rule to produce an
executable from a `.X' file uses `LINK.X'; and the rule to preprocess a
`.X' file uses `PREPROCESS.X'.

   Every rule that produces an object file uses the variable
`OUTPUT_OPTION'.  `make' defines this variable either to contain `-o
$@', or to be empty, depending on a compile-time option.  You need the
`-o' option to ensure that the output goes into the right file when the
source file is in a different directory, as when using `VPATH' (*note
Directory Search::.).  However, compilers on some systems do not accept
a `-o' switch for object files.  If you use such a system, and use
`VPATH', some compilations will put their output in the wrong place.  A
possible workaround for this problem is to give `OUTPUT_OPTION' the
value `; mv $*.o $@'.


File: make.info,  Node: Implicit Variables,  Next: Chained Rules,  Prev: Catalogue of Rules,  Up: Implicit Rules

Variables Used by Implicit Rules
================================

   The commands in built-in implicit rules make liberal use of certain
predefined variables.  You can alter these variables in the makefile,
with arguments to `make', or in the environment 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 `$(CC) -c $(CFLAGS) $(CPPFLAGS)'.  The default values of the
variables used are `cc' and nothing, resulting in the command `cc -c'.
By redefining `CC' to `ncc', you could cause `ncc' to be used for all C
compilations performed by the implicit rule.  By redefining `CFLAGS' to
be `-g', you could pass the `-g' option to each compilation.  *All*
implicit rules that do C compilation use `$(CC)' to get the program
name for the compiler and *all* include `$(CFLAGS)' among the arguments
given to the compiler.

   The variables used in implicit rules fall into two classes: those
that are names of programs (like `CC') and those that contain arguments
for the programs (like `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 in built-in
rules:

`AR'
     Archive-maintaining program; default `ar'.

`AS'
     Program for doing assembly; default `as'.

`CC'
     Program for compiling C programs; default `cc'.

`CXX'
     Program for compiling C++ programs; default `g++'.

`CO'
     Program for extracting a file from RCS; default `co'.

`CPP'
     Program for running the C preprocessor, with results to standard
     output; default `$(CC) -E'.

`FC'
     Program for compiling or preprocessing Fortran and Ratfor programs;
     default `f77'.

`GET'
     Program for extracting a file from SCCS; default `get'.

`LEX'
     Program to use to turn Lex grammars into C programs or Ratfor
     programs; default `lex'.

`PC'
     Program for compiling Pascal programs; default `pc'.

`YACC'
     Program to use to turn Yacc grammars into C programs; default
     `yacc'.

`YACCR'
     Program to use to turn Yacc grammars into Ratfor programs; default
     `yacc -r'.

`MAKEINFO'
     Program to convert a Texinfo source file into an Info file; default
     `makeinfo'.

`TEX'
     Program to make TeX DVI files from TeX source; default `tex'.

`TEXI2DVI'
     Program to make TeX DVI files from Texinfo source; default
     `texi2dvi'.

`WEAVE'
     Program to translate Web into TeX; default `weave'.

`CWEAVE'
     Program to translate C Web into TeX; default `cweave'.

`TANGLE'
     Program to translate Web into Pascal; default `tangle'.

`CTANGLE'
     Program to translate C Web into C; default `ctangle'.

`RM'
     Command to remove a file; default `rm -f'.

   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.

`ARFLAGS'
     Flags to give the archive-maintaining program; default `rv'.

`ASFLAGS'
     Extra flags to give to the assembler (when explicitly invoked on a
     `.s' or `.S' file).

`CFLAGS'
     Extra flags to give to the C compiler.

`CXXFLAGS'
     Extra flags to give to the C++ compiler.

`COFLAGS'
     Extra flags to give to the RCS `co' program.

`CPPFLAGS'
     Extra flags to give to the C preprocessor and programs that use it
     (the C and Fortran compilers).

`FFLAGS'
     Extra flags to give to the Fortran compiler.

`GFLAGS'
     Extra flags to give to the SCCS `get' program.

`LDFLAGS'
     Extra flags to give to compilers when they are supposed to invoke
     the linker, `ld'.

`LFLAGS'
     Extra flags to give to Lex.

`PFLAGS'
     Extra flags to give to the Pascal compiler.

`RFLAGS'
     Extra flags to give to the Fortran compiler for Ratfor programs.

`YFLAGS'
     Extra flags to give to Yacc.

