Languguage OS 2/Languguage OS II Version 10-94 (Knowledge Media)(1994).ISO/gnu/make-367.lha/make-3.67/make.info-3

GNU Info File | 1993-05-16 | 51KB | 910 lines

This is Info file make.info, produced by Makeinfo-1.54 from the input
file make.texinfo.
   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 0.42, last updated 14 May 1993, of `The GNU Make
Manual', for `make', Version 3.66 Beta.
   Copyright (C) 1988, '89, '90, '91, '92, '93 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 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.
File: make.info,  Node: Static Usage,  Next: Static versus Implicit,  Up: Static Pattern
Syntax of Static Pattern Rules
------------------------------
   Here is the syntax of a static pattern rule:
     TARGETS ...: TARGET-PATTERN: DEP-PATTERNS ...
             COMMANDS
             ...
The TARGETS list specifies the targets that the rule applies to.  The
targets can contain wildcard characters, just like the targets of
ordinary rules (*note Using Wildcard Characters in File Names:
Wildcards.).
   The TARGET-PATTERN and DEP-PATTERNS say how to compute the
dependencies of each target.  Each target is matched against the
TARGET-PATTERN to extract a part of the target name, called the "stem".
This stem is substituted into each of the DEP-PATTERNS to make the
dependency names (one from each DEP-PATTERN).
   Each pattern normally contains the character `%' just once.  When the
TARGET-PATTERN matches a target, the `%' can match any part of the
target name; this part is called the "stem".  The rest of the pattern
must match exactly.  For example, the target `foo.o' matches the
pattern `%.o', with `foo' as the stem.  The targets `foo.c' and
`foo.out' do not match that pattern.
   The dependency names for each target are made by substituting the
stem for the `%' in each dependency pattern.  For example, if one
dependency pattern is `%.c', then substitution of the stem `foo' gives
the dependency name `foo.c'.  It is legitimate to write a dependency
pattern that does not contain `%'; then this dependency is the same for
all targets.
   `%' characters in pattern rules can be quoted with preceding
backslashes (`\').  Backslashes that would otherwise quote `%'
characters can be quoted with more backslashes.  Backslashes that quote
`%' characters or other backslashes are removed from the pattern before
it is compared to file names or has a stem substituted into it.
Backslashes that are not in danger of quoting `%' characters go
unmolested.  For example, the pattern `the\%weird\\%pattern\\' has
`the%weird\' preceding the operative `%' character, and `pattern\\'
following it.  The final two backslashes are left alone because they
cannot affect any `%' character.
   Here is an example, which compiles each of `foo.o' and `bar.o' from
the corresponding `.c' file:
     objects = foo.o bar.o

     $(objects): %.o: %.c
             $(CC) -c $(CFLAGS) $< -o $@
Here `$<' is the automatic variable that holds the name of the
dependency and `$@' is the automatic variable that holds the name of
the target; see *Note Automatic Variables: Automatic.
   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 `filter'
function to remove nonmatching file names (*note Functions for String
Substitution and Analysis: Text Functions.):
     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 $<
Here the result of `$(filter %.o,$(files))' is `bar.o lose.o', and the
first static pattern rule causes each of these object files to be
updated by compiling the corresponding C source file.  The result of
`$(filter %.elc,$(files))' is `foo.elc', so that file is made from
`foo.el'.
   Another example shows how to use `$*' in static pattern rules:
     bigoutput littleoutput : %output : text.g
             generate text.g -$* > $@
When the `generate' command is run, `$*' will expand to the stem,
either `big' or `little'.
File: make.info,  Node: Static versus Implicit,  Prev: Static Usage,  Up: Static Pattern
Static Pattern Rules versus Implicit Rules
------------------------------------------
   A static pattern rule has much in common with an implicit rule
defined as a pattern rule (*note Defining and Redefining Pattern Rules:
Pattern Rules.).  Both have a pattern for the target and patterns for
constructing the names of dependencies.  The difference is in how
`make' decides *when* the rule applies.
   An implicit rule *can* apply to any target that matches its pattern,
but it *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:
   * 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.
   * 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 `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.
File: make.info,  Node: Double-Colon,  Next: Automatic Dependencies,  Prev: Static Pattern,  Up: Rules
Double-Colon Rules
==================
   "Double-colon" rules are rules written with `::' instead of `:'
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.  *Note Using Implicit Rules:
Implicit Rules.
File: make.info,  Node: Automatic Dependencies,  Prev: Double-Colon,  Up: Rules
Generating Dependencies Automatically
=====================================
   In the makefile for a program, many of the rules you need to write
often say only that some object file depends on some header file.  For
example, if `main.c' uses `defs.h' via an `#include', you would write:
     main.o: defs.h
You need this rule so that `make' knows that it must remake `main.o'
whenever `defs.h' changes.  You can see that for a large program you
would have to write dozens of such rules in your makefile.  And, you
must always be very careful to update the makefile every time you add
or remove an `#include'.
   To avoid this hassle, most modern C compilers can write these rules
for you, by looking at the `#include' lines in the source files.
Usually this is done with the `-M' option to the compiler.  For
example, the command:
     cc -M main.c
generates the output:
     main.o : main.c defs.h
Thus you no longer have to write all those rules yourself.  The
compiler will do it for you.
   With old `make' programs, it was traditional practice to use this
compiler feature to generate dependencies on demand with a command like
`make depend'.  That command would create a file `depend' containing
all the automatically-generated dependencies; then the makefile could
use `include' to read them in (*note Include::.).
   In GNU `make', the feature of remaking makefiles makes this practice
obsolete--you need never tell `make' explicitly to regenerate the
dependencies, because it always regenerates any makefile that is out of
date.  *Note Remaking Makefiles::.
   The practice we recommend for automatic dependency generation is to
have one makefile corresponding to each source file.  For each source
file `NAME.c' there is a makefile `NAME.d' which lists what files the
object file `NAME.o' depends on.  That way only the source files that
have changed need to be rescanned to produce the new dependencies.
   Here is the pattern rule to generate a file of dependencies (i.e., a
makefile) called `NAME.d' from a C source file called `NAME.c':
     %.d: %.c
         $(CC) -M $(CPPFLAGS) $< | sed 's/$*.o/& $@/g' > $@
*Note Pattern Rules::, for information on defining pattern rules.  The
purpose of the `sed' command is to translate (for example):
     main.o : main.c defs.h
into:
     main.o main.d : main.c defs.h
This makes each `.d' file depend on all the source and header files
that the corresponding `.o' file depends on.  `make' then knows it must
regenerate the dependencies whenever any of the source or header files
changes.
   Once you've defined the rule to remake the `.d' files, you then use
the `include' directive to read them all in.  *Note Include::.  For
example:
     sources = foo.c bar.c

     include $(sources:.c=.d)
(This example uses a substitution variable reference to translate the
list of source files `foo.c bar.c' into a list of dependency makefiles,
`foo.d bar.d'.  *Note Substitution Refs::, for full information on
substitution references.)  Since the `.d' files are makefiles like any
others, `make' will remake them as necessary with no further work from
you.  *Note Remaking Makefiles::.
File: make.info,  Node: Commands,  Next: Using Variables,  Prev: Rules,  Up: Top
Writing the Commands in Rules
*****************************
   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 `/bin/sh' unless the makefile specifies
otherwise.  *Note Command Execution: Execution.
   The shell that is in use determines whether comments can be written
on command lines, and what syntax they use.  When the shell is
`/bin/sh', a `#' starts a comment that extends to the end of the line.
The `#' does not have to be at the beginning of a line.  Text on a line
before a `#' is not part of the comment.
* Menu:
* Echoing::                     How to control when commands are echoed.
* Execution::                   How commands are executed.
* Parallel::                    How commands can be executed in parallel.
* Errors::                      What happens after a command execution error.
* Interrupts::                  What happens when a command is interrupted.
* Recursion::                   Invoking `make' from makefiles.
* Sequences::                   Defining canned sequences of commands.
* Empty Commands::              Defining useful, do-nothing commands.
File: make.info,  Node: Echoing,  Next: Execution,  Up: Commands
Command Echoing
===============
   Normally `make' prints each command line before it is executed.  We
call this "echoing" because it gives the appearance that you are typing
the commands yourself.
   When a line starts with `@', the echoing of that line is suppressed.
The `@' 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 `echo' command to indicate progress through the
makefile:
     @echo About to make distribution files
   When `make' is given the flag `-n' or `--just-print', echoing is all
that happens, no execution.  *Note Summary of Options: Options Summary.
In this case and only this case, even the commands starting with `@'
are printed.  This flag is useful for finding out which commands `make'
thinks are necessary without actually doing them.
   The `-s' or `--silent' flag to `make' prevents all echoing, as if
all commands started with `@'.  A rule in the makefile for the special
target `.SILENT' has the same effect (*note Special Built-in Target
Names: Special Targets.).  `.SILENT' is essentially obsolete since `@'
is more flexible.
File: make.info,  Node: Execution,  Next: Parallel,  Prev: Echoing,  Up: Commands
Command Execution
=================
   When it is time to execute commands to update a target, they are
executed by making a new subshell for each line.  (In practice, `make'
may take shortcuts that do not affect the results.)
   *Please note:* this implies that shell commands such as `cd' that
set variables local to each process will not affect the following
command lines.  If you want to use `cd' to affect the next command, put
the two on a single line with a semicolon between them.  Then `make'
will consider them a single command and pass them, together, to a shell
which will execute them in sequence.  For example:
     foo : bar/lose
             cd bar; gobble lose > ../foo
   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:
     foo : bar/lose
             cd bar;  \
             gobble lose > ../foo
   The program used as the shell is taken from the variable `SHELL'.
By default, the program `/bin/sh' is used.
   Unlike most variables, the variable `SHELL' is never set from the
environment.  This is because the `SHELL' environment variable 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.  *Note Variables from the Environment:
Environment.
File: make.info,  Node: Parallel,  Next: Errors,  Prev: Execution,  Up: Commands
Parallel Execution
==================
   GNU `make' knows how to execute several commands at once.  Normally,
`make' will execute only one command at a time, waiting for it to
finish before executing the next.  However, the `-j' or `--jobs' option
tells `make' to execute many commands simultaneously.
   If the `-j' option is followed by an integer, this is the number of
commands to execute at once; this is called the number of "job slots".
If there is nothing looking like an integer after the `-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, `make' will invalidate the standard input
streams of all but one running command.  This means that attempting to
read from standard input will usually be a fatal error (a `Broken pipe'
signal) for most child processes if there are several.
   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 `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 `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 (*note Errors in
Commands: Errors.), the remaining command lines to remake the same
target will not be run.  If a command fails and the `-k' or
`--keep-going' option was not given (*note Summary of Options: Options
Summary.), `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.
   When the system is heavily loaded, you will probably want to run
fewer jobs than when it is lightly loaded.  You can use the `-l' option
to tell `make' to limit the number of jobs to run at once, based on the
load average.  The `-l' or `--max-load' option is followed by a
floating-point number.  For example,
     -l 2.5
will not let `make' start more than one job if the load average is
above 2.5.  The `-l' option with no following number removes the load
limit, if one was given with a previous `-l' option.
   More precisely, when `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 `-l', `make' waits until the load
average goes below that limit, or until all the other jobs finish.
   By default, there is no load limit.
File: make.info,  Node: Errors,  Next: Interrupts,  Prev: Parallel,  Up: Commands
Errors in Commands
==================
   After each shell command returns, `make' looks at its exit status.
If the command completed successfully, the next command line is executed
in a new shell; after the last command line is finished, the rule is
finished.
   If there is an error (the exit status is nonzero), `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 `mkdir' command to ensure that a
directory exists.  If the directory already exists, `mkdir' will report
an error, but you probably want `make' to continue regardless.
   To ignore errors in a command line, write a `-' at the beginning of
the line's text (after the initial tab).  The `-' is discarded before
the command is passed to the shell for execution.
   For example,
     clean:
             -rm -f *.o
This causes `rm' to continue even if it is unable to remove a file.
   When you run `make' with the `-i' or `--ignore-errors' flag, errors
are ignored in all commands of all rules.  A rule in the makefile for
the special target `.IGNORE' has the same effect.  These ways of
ignoring errors are obsolete because `-' is more flexible.
   When errors are to be ignored, because of either a `-' or the `-i'
flag, `make' treats an error return just like success, except that it
prints out a message that tells you the status code the command exited
with, and says that the error has been ignored.
   When an error happens that `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 `make' gives up immediately in this circumstance, returning
a nonzero status.  However, if the `-k' or `--keep-going' flag is
specified, `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, `make -k' will continue compiling other object files even
though it already knows that linking them will be impossible.  *Note
Summary of Options: Options Summary.
   The usual behavior assumes that your purpose is to get the specified
targets up to date; once `make' learns that this is impossible, it
might as well report the failure immediately.  The `-k' option says
that the real purpose is to test as many of the changes made in the
program as possible, perhaps to find several independent problems so
that you can correct them all before the next attempt to compile.  This
is why Emacs' `compile' command passes the `-k' flag by default.
File: make.info,  Node: Interrupts,  Next: Recursion,  Prev: Errors,  Up: Commands
Interrupting or Killing `make'
==============================
   If `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
`make' first checked it.
   The purpose of deleting the target is to make sure that it is remade
from scratch when `make' is next run.  Why is this?  Suppose you type
`Ctrl-c' while a compiler is running, and it has begun to write an
object file `foo.o'.  The `Ctrl-c' kills the compiler, resulting in an
incomplete file whose last-modification time is newer than the source
file `foo.c'.  But `make' also receives the `Ctrl-c' signal and deletes
this incomplete file.  If `make' did not do this, the next invocation
of `make' would think that `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.
   You can prevent the deletion of a target file in this way by making
the special target `.PRECIOUS' depend on it.  Before remaking a target,
`make' checks to see whether it appears on the dependencies of
`.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.
File: make.info,  Node: Recursion,  Next: Sequences,  Prev: Interrupts,  Up: Commands
Recursive Use of `make'
=======================
   Recursive use of `make' means using `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 `subdir' which has its own makefile, and you
would like the containing directory's makefile to run `make' on the
subdirectory.  You can do it by writing this:
     subsystem:
             cd subdir; $(MAKE)
or, equivalently, this (*note Summary of Options: Options Summary.):
     subsystem:
             $(MAKE) -C subdir
   You can write recursive `make' commands just by copying this example,
but there are many things to know about how they work and why, and about
how the sub-`make' relates to the top-level `make'.
* Menu:
* MAKE Variable::               The special effects of using `$(MAKE)'.
* Variables/Recursion::         How to communicate variables to a sub-`make'.
* Options/Recursion::           How to communicate options to a sub-`make'.
* -w Option::                   How the `-w' or `--print-directory' option
                                 helps debug use of recursive `make' commands.
File: make.info,  Node: MAKE Variable,  Next: Variables/Recursion,  Up: Recursion
How the `MAKE' Variable Works
-----------------------------
   Recursive `make' commands should always use the variable `MAKE', not
the explicit command name `make', as shown here:
     subsystem:
             cd subdir; $(MAKE)
   The value of this variable is the file name with which `make' was
invoked.  If this file name was `/bin/make', then the command executed
is `cd subdir; /bin/make'.  If you use a special version of `make' to
run the top-level makefile, the same special version will be executed
for recursive invocations.
   Also, any arguments that define variable values are added to `MAKE',
so the sub-`make' gets them too.  Thus, if you do `make CFLAGS=-O', so
that all C compilations will be optimized, the sub-`make' is run with
`cd subdir; /bin/make CFLAGS=-O'.
   The `MAKE' variable actually just refers to two other variables
which contain these special values.  In fact, `MAKE' is always defined
as `$(MAKE_COMMAND) $(MAKEOVERRIDES)'.  The variable `MAKE_COMMAND' is
the file name with which `make' was invoked (such as `/bin/make',
above).  The variable `MAKEOVERRIDES' contains definitions for the
variables defined on the command line; in the above example, its value
is `CFLAGS=-O'.  If you *do not* want these variable definitions done
in all recursive `make' invocations, you can redefine the
`MAKEOVERRIDES' variable to remove them.  You do this in any of the
normal ways for defining variables: in a makefile (*note Setting
Variables: Setting.); on the command line with an argument like
`MAKEOVERRIDES=' (*note Overriding Variables: Overriding.); or with an
environment variable (*note Variables from the Environment:
Environment.).
   As a special feature, using the variable `MAKE' in the commands of a
rule alters the effects of the `-t' (`--touch'), `-n' (`--just-print'),
or `-q' (`--question') option.  Using the `MAKE' variable has the same
effect as using a `+' character at the beginning of the command line.
*Note Instead of Executing the Commands: Instead of Execution.
   Consider the command `make -t' in the above example.  (The `-t'
option marks targets as up to date without actually running any
commands; see *Note Instead of Execution::.)  Following the usual
definition of `-t', a `make -t' command in the example would create a
file named `subsystem' and do nothing else.  What you really want it to
do is run `cd subdir; make -t'; but that would require executing the
command, and `-t' says not to execute commands.
   The special feature makes this do what you want: whenever a command
line of a rule contains the variable `MAKE', the flags `-t', `-n' and
`-q' do not apply to that line.  Command lines containing `MAKE' are
executed normally despite the presence of a flag that causes most
commands not to be run.  The usual `MAKEFLAGS' mechanism passes the
flags to the sub-`make' (*note Communicating Options to a Sub-`make':
Options/Recursion.), so your request to touch the files, or print the
commands, is propagated to the subsystem.
File: make.info,  Node: Variables/Recursion,  Next: Options/Recursion,  Prev: MAKE Variable,  Up: Recursion
Communicating Variables to a Sub-`make'
---------------------------------------
   Variable values of the top-level `make' can be passed to the
sub-`make' through the environment by explicit request.  These
variables are defined in the sub-`make' as defaults, but do not
override what is specified in the sub-`make''s makefile unless you use
the `-e' switch (*note Summary of Options: Options Summary.).
   To pass down, or "export", a variable, `make' adds the variable and
its value to the environment for running each command.  The sub-`make',
in turn, uses the environment to initialize its table of variable
values.  *Note Variables from the Environment: Environment.
   Except by explicit request, `make' exports a variable only if it is
either defined in the environment initially or set on the command line,
and if its name consists only of letters, numbers, and underscores.
Some shells cannot cope with environment variable names consisting of
characters other than letters, numbers, and underscores.
   The special variables `SHELL' and `MAKEFLAGS' are always exported.
`MAKEFILES' is exported if you set it to anything.
   Variables are *not* normally passed down if they were created by
default by `make' (*note Variables Used by Implicit Rules: Implicit
Variables.).  The sub-`make' will define these for itself.
   If you want to export specific variables to a sub-`make', use the
`export' directive, like this:
     export VARIABLE ...
If you want to *prevent* a variable from being exported, use the
`unexport' directive, like this:
     unexport VARIABLE ...
As a convenience, you can define a variable and export it at the same
time by doing:
     export VARIABLE = value
has the same result as:
     VARIABLE = value
     export VARIABLE
     export VARIABLE := value
has the same result as:
     VARIABLE := value
     export VARIABLE
   Likewise,
     export VARIABLE += value
is just like:
     VARIABLE += value
     export VARIABLE
*Note Appending More Text to Variables: Appending.
   You may notice that the `export' and `unexport' directives work in
`make' in the same way they work in the shell, `sh'.
   If you want all variables to be exported by default, you can use
`export' by itself:
     export
This tells `make' that variables which are not explicitly mentioned in
an `export' or `unexport' directive should be exported.  Any variable
given in an `unexport' directive will still *not* be exported.  If you
use `export' by itself to export variables by default, variables whose
names contain characters other than alphanumerics and underscores will
not be exported unless specifically mentioned in an `export' directive.
   The behavior elicited by an `export' directive by itself was the
default in older versions of GNU `make'.  If your makefiles depend on
this behavior and you want to be compatible with old versions of
`make', you can write a rule for the special target
`.EXPORT_ALL_VARIABLES' instead of using the `export' directive.  This
will be ignored by old `make's, while the `export' directive will cause
a syntax error.
   Likewise, you can use `unexport' by itself to tell `make' *not* to
export variables by default.  Since this is the default behavior, you
would only need to do this if `export' had been used by itself earlier
(in an included makefile, perhaps).  You *cannot* use `export' and
`unexport' by themselves to have variables exported for some commands
and not for others.  The last `export' or `unexport' directive that
appears by itself determines the behavior for the entire run of `make'.
   As a special feature, the variable `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 `0'
for the top-level `make'; `1' for a sub-`make', `2' for a
sub-sub-`make', and so on.  The incrementation happens when `make' sets
up the environment for a command.
   The main use of `MAKELEVEL' is to test it in a conditional directive
(*note Conditional Parts of Makefiles: Conditionals.); this way you can
write a makefile that behaves one way if run recursively and another
way if run directly by you.
   You can use the variable `MAKEFILES' to cause all sub-`make'
commands to use additional makefiles.  The value of `MAKEFILES' is a
whitespace-separated list of file names.  This variable, if defined in
the outer-level makefile, is passed down through the environment; then
it serves as a list of extra makefiles for the sub-`make' to read
before the usual or specified ones.  *Note The Variable `MAKEFILES':
MAKEFILES Variable.
File: make.info,  Node: Options/Recursion,  Next: -w Option,  Prev: Variables/Recursion,  Up: Recursion
Communicating Options to a Sub-`make'
-------------------------------------
   Flags such as `-s' and `-k' are passed automatically to the
sub-`make' through the variable `MAKEFLAGS'.  This variable is set up
automatically by `make' to contain the flag letters that `make'
received.  Thus, if you do `make -ks' then `MAKEFLAGS' gets the value
`ks'.
   As a consequence, every sub-`make' gets a value for `MAKEFLAGS' in
its environment.  In response, it takes the flags from that value and
processes them as if they had been given as arguments.  *Note Summary
of Options: Options Summary.
   The options `-C', `-f', `-I', `-o', and `-W' are not put into
`MAKEFLAGS'; these options are not passed down.
   The `-j' option is a special case (*note Parallel Execution:
Parallel.).  If you set it to some numeric value, `-j 1' is always put
into `MAKEFLAGS' instead of the value you specified.  This is because if
the `-j' option were passed down to sub-`make's, you would get many
more jobs running in parallel than you asked for.  If you give `-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.
   If you do not want to pass the other flags down, you must change the
value of `MAKEFLAGS', like this:
     MAKEFLAGS=
     subsystem:
             cd subdir; $(MAKE)
   or like this:
     subsystem:
             cd subdir; $(MAKE) MAKEFLAGS=
   A similar variable `MFLAGS' exists also, for historical
compatibility.  It has the same value as `MAKEFLAGS' except that a
hyphen is added at the beginning if it is not empty.  `MFLAGS' was
traditionally used explicitly in the recursive `make' command, like
this:
     subsystem:
             cd subdir; $(MAKE) $(MFLAGS)
but now `MAKEFLAGS' makes this usage redundant.
   The `MAKEFLAGS' and `MFLAGS' variables can also be useful if you
want to have certain options, such as `-k' (*note Summary of Options:
Options Summary.) set each time you run `make'.  Just put `MAKEFLAGS=k'
or `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.
   If you do put `MAKEFLAGS' or `MFLAGS' in your environment, you
should be sure not to include any options that will drastically affect
the actions of `make' and undermine the purpose of makefiles and of
`make' itself.  For instance, the `-t', `-n', and `-q' options, if put
in one of these variables, could have disastrous consequences and would
certainly have at least surprising and probably annoying effects.
File: make.info,  Node: -w Option,  Prev: Options/Recursion,  Up: Recursion
The `--print-directory' Option
------------------------------
   If you use several levels of recursive `make' invocations, the `-w'
or `--print-directory' option can make the output a lot easier to
understand by showing each directory as `make' starts processing it and
as `make' finishes processing it.  For example, if `make -w' is run in
the directory `/u/gnu/make', `make' will print a line of the form:
     make: Entering directory `/u/gnu/make'.
before doing anything else, and a line of the form:
     make: Leaving directory `/u/gnu/make'.
when processing is completed.
   Normally, you do not need to specify this option because `make' does
it for you: `-w' is turned on automatically when you use the `-C'
option, and in sub-`make's.  `make' will not automatically turn on `-w'
if you also use `-s', which says to be silent, or if you use
`--no-print-directory' to explicitly disable it.
File: make.info,  Node: Sequences,  Next: Empty Commands,  Prev: Recursion,  Up: Commands
Defining Canned Command Sequences
=================================
   When the same sequence of commands is useful in making various
targets, you can define it as a canned sequence with the `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:
     define run-yacc
     yacc $(firstword $^)
     mv y.tab.c $@
     endef
Here `run-yacc' is the name of the variable being defined; `endef'
marks the end of the definition; the lines in between are the commands.
The `define' directive does not expand variable references and
function calls in the canned sequence; the `$' characters, parentheses,
variable names, and so on, all become part of the value of the variable
you are defining.  *Note Defining Variables Verbatim: Defining, for a
complete explanation of `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 `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
(*note Basics of Variable References: Reference.).  Because variables
defined by `define' are recursively expanded variables, all the
variable references you wrote inside the `define' are expanded now.
For example:
     foo.c : foo.y
             $(run-yacc)
`foo.y' will be substituted for the variable `$^' when it occurs in
`run-yacc''s value, and `foo.c' for `$@'.
   This is a realistic example, but this particular one is not needed in
practice because `make' has an implicit rule to figure out these
commands based on the file names involved (*note Using Implicit Rules:
Implicit Rules.).
   In command execution, each line of a canned sequence is treated just
as if the line appeared on its own in the rule, preceded by a tab.  In
particular, `make' invokes a separate subshell for each line.  You can
use the special prefix characters that affect command lines (`@', `-',
and `+') on each line of a canned sequence.  *Note Writing the Commands
in Rules: Commands.  For example, using this canned sequence:
     define frobnicate
     @echo "frobnicating target $@"
     frob-step-1 $< -o $@-step-1
     frob-step-2 $@-step-1 -o $@
     endef
`make' will not echo the first line, the `echo' command.  But it *will*
echo the following two command lines.
   On the other hand, prefix characters on the command line that refers
to a canned sequence apply to every line in the sequence.  So the rule:
     frob.out: frob.in
         @$(frobnicate)
does not echo *any* commands.  (*Note Command Echoing: Echoing, for a
full explanation of `@'.)
File: make.info,  Node: Empty Commands,  Prev: Sequences,  Up: Commands
Using 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:
     target: ;
defines an empty command string for `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
`.DEFAULT' special target; *note Implicit Rules::. and *note Defining
Last-Resort Default Rules: 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 the
dependencies may not be remade properly if the target file actually
does exist.  *Note Phony Targets: Phony Targets, for a better way to do
this.
File: make.info,  Node: Using Variables,  Next: Conditionals,  Prev: Commands,  Up: Top
How to Use Variables
********************
   A "variable" is a name defined in a makefile to represent a string
of text, called the variable's "value".  These values are substituted
by explicit request into targets, dependencies, commands, and other
parts of the makefile.  (In some other versions of `make', variables
are called "macros".)
   Variables and functions in all parts of a makefile are expanded when
read, except for the shell commands in rules, the right-hand sides of
variable definitions using `=', and the bodies of variable definitions
using the `define' directive.
   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 of characters not containing `:',
`#', `=', 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 with some shells they cannot be passed through the environment to a
sub-`make' (*note Communicating Variables to a Sub-`make':
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 (*note Overriding Variables:
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.
* Appending::                   How to append more text to the old value
                                  of a variable.
* Override Directive::          How to set a variable in the makefile even if
                                  the user has set it with a command argument.
* Defining::                    An alternate way to set a variable
                                  to a verbatim string.
* Environment::                 Variable values can come from the environment.
File: make.info,  Node: Reference,  Next: Flavors,  Up: Using Variables
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 `$(foo)' or
`${foo}' is a valid reference to the variable `foo'.  This special
significance of `$' is why you must write `$$' 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 an example of a common case, where a variable holds the names of all
the object files in a program:
     objects = program.o foo.o utils.o
     program : $(objects)
             cc -o program $(objects)

     $(objects) : defs.h
   Variable references work by strict textual substitution.  Thus, the
     foo = c
     prog.o : prog.$(foo)
             $(foo)$(foo) -$(foo) prog.$(foo)
could be used to compile a C program `prog.c'.  Since spaces before the
variable value are ignored in variable assignments, the value of `foo'
is precisely `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 `x' with `$x'.
However, this practice is strongly discouraged, except in the case of
the automatic variables (*note Automatic Variables: Automatic.).
File: make.info,  Node: Flavors,  Next: Advanced,  Prev: Reference,  Up: Using Variables
The Two Flavors of Variables
============================
   There are two ways that a variable in GNU `make' can have a value;
we call them the two "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 "recursively expanded" variable.
Variables of this sort are defined by lines using `=' (*note Setting
Variables: Setting.) or by the `define' directive (*note Defining
Variables Verbatim: Defining.).  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 "recursive expansion".
   For example,
     foo = $(bar)
     bar = $(ugh)
     ugh = Huh?

     all:;echo $(foo)
will echo `Huh?': `$(foo)' expands to `$(bar)' which expands to
`$(ugh)' which finally expands to `Huh?'.
   This flavor of variable is the only sort supported by other versions
of `make'.  It has its advantages and its disadvantages.  An advantage
(most would say) is that:
     CFLAGS = $(include_dirs) -O
     include_dirs = -Ifoo -Ibar
will do what was intended: when `CFLAGS' is expanded in a command, it
will expand to `-Ifoo -Ibar -O'.  A major disadvantage is that you
cannot append something on the end of a variable, as in
     CFLAGS = $(CFLAGS) -O
because it will cause an infinite loop in the variable expansion.
(Actually `make' detects the infinite loop and reports an error.)
   Another disadvantage is that any functions (*note Functions for
Transforming Text: Functions.) referenced in the definition will be
executed every time the variable is expanded.  This makes `make' run
slower; worse, it causes the `wildcard' and `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: simply expanded variables.
   "Simply expanded variables" are defined by lines using `:=' (*note
Setting Variables: 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 *as of the time this variable was
defined*.  Therefore,
     x := foo
     y := $(x) bar
     x := later
is equivalent to
     y := foo bar
     x := later
   When a simply expanded variable is referenced, its value is
substituted verbatim.
   Here is a somewhat more complicated example, illustrating the use of
`:=' in conjunction with the `shell' function.  (*Note The `shell'
Function: Shell Function.)  This example also shows use of the variable
`MAKELEVEL', which is changed when it is passed down from level to
level.  (*Note Communicating Variables to a Sub-`make':
Variables/Recursion, for information about `MAKELEVEL'.)
     ifeq (0,${MAKELEVEL})
     cur-dir   := $(shell pwd)
     whoami    := $(shell whoami)
     host-type := $(shell arch)
     MAKE := ${MAKE} host-type=${host-type} whoami=${whoami}
     endif
An advantage of this use of `:=' is that a typical `descend into a
directory' command then looks like this:
     ${subdirs}:
           ${MAKE} cur-dir=${cur-dir}/$@ -C $@ all
   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
(*note Functions for Transforming Text: 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:
     nullstring :=
     space := $(nullstring) $(nullstring)
Here the value of the variable `space' is precisely one space.
File: make.info,  Node: Advanced,  Next: Values,  Prev: Flavors,  Up: Using Variables
Advanced Features for Reference to Variables
============================================
   This section describes some advanced features you can use to
reference variables in more flexible ways.
* Menu:
* Substitution Refs::           Referencing a variable with
                                  substitutions on the value.
* Computed Names::              Computing the name of the variable to refer to.
File: make.info,  Node: Substitution Refs,  Next: Computed Names,  Up: Advanced
Substitution References
-----------------------
   A "substitution reference" substitutes the value of a variable with
alterations that you specify.  It has the form `$(VAR:A=B)' (or
`${VAR:A=B}') and its meaning is to take the value of the variable VAR,
replace every A at the end of a word with B in that value, and
substitute the resulting string.
   When we say "at the end of a word", we mean that A must appear
either followed by whitespace or at the end of the value in order to be
replaced; other occurrences of A in the value are unaltered.  For
example:
     foo := a.o b.o c.o
     bar := $(foo:.o=.c)
sets `bar' to `a.c b.c c.c'.  *Note Setting Variables: Setting.
   A substitution reference is actually an abbreviation for use of the
`patsubst' expansion function (*note Functions for String Substitution
and Analysis: Text Functions.).  We provide substitution references as
well as `patsubst' for compatibility with other implementations of
`make'.
   Another type of substitution reference lets you use the full power of
the `patsubst' function.  It has the same form `$(VAR:A=B)' described
above, except that now A must contain a single `%' character.  This
case is equivalent to `$(patsubst A,B,$(VAR))'.  *Note Functions for
String Substitution and Analysis: Text Functions, for a description of
the `patsubst' function.
For example:
     foo := a.o b.o c.o
     bar := $(foo:%.o=%.c)
sets `bar' to `a.c b.c c.c'.